Some other circuits and devices have a large current for short periods and a normal (smaller) current most of the time. An electric motor, for instance, will draw a large current when the motor starts, but normal operating current for the motor will be much smaller. A fuse used to protect a motor would have to allow for this large temporary current, but would open if the large current were to continue.
Fuses are time delay rated to indicate the relationship between the current through the fuse and the time it takes for the fuse to open. The three time delay ratings are delay, standard, and fast. Delay
A delay, or slow-blowing, fuse has a built-in delay that is activated when the current through the fuse is greater than the current rating of the fuse. This fuse will allow temporary increases in current (surge) without opening. Some delay fuses have two elements; this allows a very long time delay. If the over- current condition continues, a delay fuse will open, but it will take longer to open than a standard or a fast fuse.
Delay fuses are used for circuits with high surge or starting currents, such as motors, solenoids, and transformers.
2-9
Standard Standard fuses have no built-in time delay. Also, they are not designed to be very fast acting.
Standard fuses are sometimes used to protect against direct shorts only. They may be wired in series with a delay fuse to provide faster direct short protection. For example, in a circuit with a 1-ampere delay fuse, a 5-ampere standard fuse may be used in addition to the delay fuse to provide faster protection against a direct short.
A standard fuse can be used in any circuit where surge currents are not expected and a very fast opening of the fuse is not needed. A standard fuse opens faster than a delay fuse, but slower than a fast rated fuse.
Standard fuses can be used for automobiles, lighting circuits, or electrical power circuits. Fast
Fast fuses are designed to open very quickly when the current through the fuse exceeds the current rating of the fuse. Fast fuses are used to protect devices that are very sensitive to increased current. A fast fuse will open faster than a delay or standard fuse.
Fast fuses can be used to protect delicate instruments or semiconductor devices.
Figure 2-7 will help you understand the differences between delay, standard, and fast fuses. Figure 2-7 shows that, if a 1-ampere rated fuse had 2 amperes of current through it, (200% of the rated value), a fast fuse would open in about .7 second, a standard rated fuse would open in about 1.5 seconds, and a delay rated fuse would open in about 10 seconds. Notice that in each of the fuses, the time required to open the fuse decreases as the rated current increases.
Q11. In what three ways are fuses rated? Q12. What does the current rating of a fuse indicate? Q13. What does the voltage rating of a fuse indicate? Q14. What are the three time delay ratings of fuses? Q15. Give an example of a device you could protect with each type of time delay fuse.
2-10
Fuses have identifications printed on them. The printing on the fuse will identify the physical size, the type of fuse, and the fuse ratings. There are four different systems used to identify fuses. The systems are the old military designation, the new military designation, the old commercial designation, and the new commercial designation. All four systems are presented here, so you will be able to identify a fuse no matter which designation is printed on the fuse.
You may have to replace an open fuse that is identified by one system with a good fuse that is identified by another system. The designation systems are fairly simple to understand and cross-reference once you are familiar with them.
Figure 2-8 shows a fuse with the old military designation. The tables in the lower part of the figure show the voltage and current codes used in this system. The upper portion of the figure is the explanation of the old military designation. The numbers and letters in parentheses are the coding for the fuse shown in figure 2-8.
The old military designation always starts with "F," which stands for fuse. Next, the set of numbers (02) indicates the style. Style means the construction and dimensions (size) of the fuse. Following the style is a letter that represents the voltage rating of the fuse (G). The voltage code table in figure 2-8 shows each voltage rating letter and its meaning in volts. In the example shown, the voltage ratings is G,
2-11
which means the fuse should be used in a circuit where the voltage is 250 volts or less. After this is a set of three numbers and the letter "R," which represent the current rating of the fuse. The "R" indicates the decimal point. In the example shown, the current rating is 1R00 or 1.00 ampere. Some other examples of the current rating are shown in the current code table of figure 2-8. The final letter in the old military designation (A) indicates the time delay rating of the fuse.
While the old military designation is still found on some fuses, the voltage and current ratings must be "translated," since they use letters to represent numerical values. The military developed the new military designations to make fuse identification easier.
Figure 2-9 is an example of a fuse coded in the new military designation. The fuse identified in the example in figure 2-9 is the same type as the fuse used as an example in figure 2-8.
The new military designation always start with the letter "F," which stands for fuse. The set of numbers (02) next to this indicates the style. The style numbers are identical to the ones used in the old military designation and indicate the construction and dimensions of the fuse. Following the style designation is a single letter (A) that indicates the time delay rating of the fuse. This is the same time delay rating code as indicated in the old military designation, but the position of this letter in the coding is changed to avoid confusing the "A" for standard time delay with the "A" for ampere. Following the time delay rating is the voltage rating of the fuse (250) V. In the old military designation, a letter was used to indicate the voltage rating. In the new military designation, the voltage is indicated by numbers followed by a "V," which stands for volts or less. After the voltage rating, the current rating is given by numbers followed by the letter "A." The current rating may be a whole number (1A), a fraction (1/500 A), a whole number and a fraction (1 1/2A), a decimal (0.250A), or a whole number and a decimal (1.50A). If the ferrules of the fuse are silver-plated, the current rating will be followed by the letter "S." If any other plating is used, the current rating will be the last part of the fuse identification.
2-12
As you can see, the new military designation is much easier to understand than the old military designation.
You may find a fuse coded in one of the commercial designations. The commercial designations are fairly easy to understand and figure 2-10 shows the old and new commercial designations for the same type of fuse that was used in figures 2-8 and 2-9.
Figure 2-10, view A, shows the old commercial designation for a fuse. The first part of the designation is a combination of letters and numbers (three in all) that indicates the style and time delay characteristics. This part of the designation (3AG) is the information contained in the style and time delay rating portions of military designations.
In the example shown, the code 3AG represents the same information as the underlined portions of F02 G 1R00 A from figure 2-8 (Old Military Designation) and F02A 250VIAS from figure 2-9 (New Military Designation). The only way to know the time delay rating of this fuse is to look it up in the manufacturer’s catalog or in a cross-reference listing to find the military designation. The catalog will tell you the physical size, the material from which the fuse is constructed, and the time delay rating of the fuse. A 3AG fuse is a glass-bodied fuse, 1/4 inch · 1 1/4 inches (6.35 millimeters · 31.8 millimeters) and has a standard time delay rating.
2-13
Following the style designation is a number that is the current rating of the fuse (1). This could be a whole number, a fraction, a whole number and a fraction, a decimal, or a whole number and a decimal. Following the current rating is the voltage rating; which, in turn, is followed by the letter "V," which stands for volts or less (250V). NEW COMMERCIAL DESIGNATION
Figure 2-10, view B, shows the new commercial designation for fuses. It is the same as the old commercial designation except for the style portion of the coding. In the old commercial system, the style was a combination of letters and numbers. In the new commercial system, only letters are used. In the example shown, 3AG in the old system becomes AGC in the new system. Since "C" is the third letter of the alphabet, it is used instead of the "3" used in the old system. Once again, the only way to find out the time delay rating is to look up this coding in the manufacturer’s catalog or to use a cross-reference listing. The remainder of the new commercial designation is exactly the same as the old commercial designation.
Q16. What are the voltage, current, and time delay ratings for a fuse with the designation
Q17. What are the voltage and current ratings for a fuse designated
Q18. What is the new military designation for a fuse with the old military designation F05A20ROB? FUSEHOLDERS
For a fuse to be useful, it must be connected to the circuit it will protect. Some fuses are "wired in" or soldered to the wiring of circuits, but most circuits make use of FUSEHOLDERS. A fuseholder is a device that is wired into the circuit and allows easy replacement of the fuse.
Fuseholders are made in many shapes and sizes, but most fuseholders are basically either clip-type or post-type. Figure 2-11 shows a typical clip-type and post-type fuseholder.
2-14
The clip-type fuseholder is used for cartridge fuses. The ferrules or knife blade of the fuse are held by the spring tension of the clips. These clips provide the electrical connection between the fuse and the circuit. If a glass-bodied fuse is used, the fuse can be inspected visually for an open without removing the fuse from the fuse holder. Clip-type fuseholders are made in several sizes to hold the many styles of fuses. The clips maybe made for ferrules or knife blade cartridge fuses. While the base of a clip-type fuseholder is made from insulating material, the clips themselves are conductors. The current through the fuse goes through the clips and care must be taken to not touch the clips when there is power applied. If the clips are touched, with power applied, a severe shock or a short circuit will occur.
Post-type fuseholders are made for cartridge fuses. The post-type fuseholder is much safer because the fuse and fuse connections are covered with insulating material. The disadvantage of the post-type fuseholder is that the fuse must be removed to visually check for an open. The post-type fuseholder has a cap that screws onto the body of the fuseholder. The fuse is held in this cap by a spring-type connector and, as the cap is screwed on, the fuse makes contact with the body of the fuseholder. When the cap and fuse are removed from the body of the fuseholder, the fuse is removed from the circuit and there is no danger of shock or short circuit from touching the fuse.
Post-type fuseholders are usually mounted on the chassis of the equipment in which they are used. After wires are connected to the fuseholder, insulating sleeves are placed over the connections to reduce the possibility of a short circuit. Notice the two connections on the post-type fuseholder of figure 2-11. The connection on the right is called the center connector. The other connector is the outside connector. The outside connector will be closer to the equipment chassis. (The threads and nut shown are used to fasten the fuseholder to the chassis.) The possibility of the outside connector coming in contact with the chassis (causing a short circuit) is much higher than the possibility of the center conductor contacting the chassis. The power source should always be connected to the center connector so the fuse will open if the outside connector contacts the chassis. If the power source were connected to the outside connector, and the outside connector contacted the chassis, there would be a direct short, but the fuse would not open.
2-15
Q19. Label the fuseholders in figure 2-12. Q20. Which connector should you use to connect the (a) power source and (b) load to the fuseholder
shown in figure 2-12(A)?
A fuse, if properly used, should not open unless something is wrong in the circuit the fuse is protecting. When a fuse is found to be open, you must determine the reason the fuse is open. Replacing the fuse is not enough.
Before you look for the cause of an open fuse, you must be able to determine if the fuse is open.
There are several ways of checking for an open fuse. Some fuses and fuseholders have indicators built in to help you find an open fuse; also, a multimeter can be used to check fuses. The simplest way to check glass-bodied fuses, and the method you should use first, is visual inspection. Visual Inspection
An open glass-bodied fuse can usually be found by visual inspection. Earlier in this chapter, figures 2-4 and 2-5 showed you how an open plug-type and an open glass-bodied cartridge-type fuse would look. If the fuse element is not complete, or if the element has been melted onto the glass tube, the fuse is open.
It is not always possible to tell if a fuse is open by visual inspection. Fuses with low current ratings have elements that are so small, it is sometimes not possible to know if the fuse link is complete simply by looking at it. If the fuse is not glass-bodied, it will not be possible to check the fuse visually. Also, sometimes a fuse will look good, but will, in fact, be open. Therefore, while it is sometimes possible to know if a fuse is open by visual inspection, it is not possible to be sure a fuse is good just by looking at it. Fuse Indicators
Some fuses and fuseholders have built-in indicators to show when a fuse is open. Examples of these open-fuse indicators are shown in figure 2-13. Figure 2-13, view A, shows a cartridge-type fuse with an open-fuse indicator. The indicator is spring loaded and held by the fuse link. If the fuse link opens, the spring forces the indicator out. Some manufacturers color the indicator so it is easier to see in the open-fuse position.
2-16
Figure 2-13, view B, shows a plug-type fuseholder with an indicating lamp in the fuse cap. If the fuse opens, the lamp in the fuse cap will light. Figure 2-13, view C, shows a clip-type fuseholder with an indicating lamp.
Just as in visual checking, the indicator can show an open fuse. Since the indicator may not always work, you cannot be sure a fuse is good just because there is no open-fuse indication. Checking Fuses with a Meter
The only sure method of determining if a fuse is open is to use a meter. An ohmmeter can be used to check for an open fuse by removing the fuse from the circuit and checking for continuity through the fuse (0 ohms). If the fuse is not removed from the circuit, and the fuse is open, the ohmmeter may measure the circuit resistance. This resistance reading might lead you to think the fuse is good. You must be careful when you use an ohmmeter to check fuses with small current ratings (such as 1/32 ampere or less), because the current from the ohmmeter may be larger than the current rating of the fuse. For most practical uses, a small current capacity fuse can be checked out of the circuit through the use of a resistor. The ohmic value of the resistor is first measured and then placed in series with the fuse. The continuity reading on the ohmmeter should be of the same value, or close to it, as the original value of the resistor. This method provides protection for the fuse by dropping the voltage across the resistor. This in turn decreases the power in the form of heat at the fuse. Remember, it is heat which melts the fuse element.
A voltmeter can also be used to check for an open fuse. The measurement is taken between each end of the fuse and the common or ground side of the line. If voltage is present on both sides of the fuse (from the voltage source and to the load), the fuse is not open. Another method commonly used, is to measure across the fuse with the voltmeter. If NO voltage is indicated on the meter, the fuse is good, (not open).
2-17
Remember there is no voltage drop across a straight piece of wire. Some plug-type fuseholders have test points built in to allow you to check the voltage. To check for voltage on a clip-type fuseholder, check each of the clips. The advantage of using a voltmeter to check for an open fuse is that the circuit does not have to be deenergized and the fuse does not have to be removed.
avoid being shocked or damaging the circuit. The following safety precautions will protect you and the equipment you are using.
• Turn the power off and discharge the circuit before removing a fuse.
• Use a fusepuller (an insulated tool) when you remove a fuse from a clip-type fuseholder.
• When you check a fuse with a voltmeter, be careful to avoid shocks and short circuits.
• When you use an ohmmeter to check fuses with low current ratings, be careful to avoid opening the fuse by excessive current from the ohmmeter.
Q21. What are three methods for determining if a fuse is open? Q22. You have just checked a fuse with an ohmmeter and find that the fuse is shorted. What should you
do?
Q23. You have just checked a 1/500-ampere fuse with an ohmmeter and find it is open. Checking the replacement fuse shows the replacement fuse is open also. Why would the replacement fuse indicate open?
Q24. How could you check a 1/500-ampere fuse with an ohmmeter? Q25. List the safety precautions to be observed when checking fuses.
After an open fuse is found and the trouble that caused the fuse to open has been corrected, the fuse must be replaced. Before you replace the fuse, you must be certain the replacement fuse is the proper type and fits correctly. Proper Type of Replacement Fuse
To be certain a fuse is the proper type, check the technical manual for the equipment. The parts list will show you the proper fuse identification for a replacement fuse. Obtain the exact fuse specified, if possible, and check the identification number of the replacement fuse against the parts list.
If you cannot obtain a direct replacement, use the following guidelines:
• Never use a fuse with a higher current rating, a lower voltage rating, or a slower time delay rating than the specified fuse.
2-18
• The best substitution fuse is a fuse with the same current and time delay ratings and a higher voltage rating.
• If a lower current rating or a faster time delay rating is used, the fuse may open under normal circuit conditions.
• Substitute fuses must have the same style (physical dimensions) as the specified fuse. Proper Fit of Replacement Fuses
When you have obtained a proper replacement fuse, you must make certain it will fit correctly in the fuseholder. If the fuseholder is corroded, the fuse will not fit properly. In addition, the corrosion can cause increased resistance or heating. Clean corroded terminals with fine sandpaper so that all corrosion is removed. Do NOT lubricate the terminals. If the terminals are badly pitted, replace the fuseholder. Be certain the replacement fuseholder is the correct size and type by checking the parts list in the technical manual for the equipment.
After you check for and correct any corrosion problems, be certain the fuse fits tightly in the fuseholder. When you insert the fuse in the cap of a plug-type fuseholder, the fuse should fit tightly. A small amount of pressure should be needed to insert the fuse and cap into the fuseholder body.
In clip-type fuseholders, the clips can be easily bent out of shape. This causes an incorrect fit, which in time could cause an equipment malfunction. Figure 2-14 shows examples of correct and incorrect fuse contacts for clip-type fuseholders used with knifeblade and ferrule cartridge fuses. The clips shown in the left picture of each row have the correct contact. The three pictures on the right of each row show incorrect contact. Notice how the clips are not contacting completely with the knifeblade or ferrules. This incomplete contact can. cause corrosion at the contacts, which in turn can create a high resistance and drop some of the circuit voltage at this point.
If the fuse clips do not make complete contact with the fuse, try to bend the clips back into shape. If the clips cannot be repaired by bending, replace the fuseholder or use clip clamps. Clip clamps are shown in figure 2-15.
2-19
Safety Precautions When Replacing Fuses The following safety precautions will prevent injury to personnel and damage to equipment. These
are the MINIMUM safety precautions for replacing fuses.
• Be sure the power is off in the circuit and the circuit is discharged before replacing a fuse.
• Use an identical replacement fuse if possible.
• Remove any corrosion from the fuseholder before replacing the fuse.
• Be certain the fuse properly fits the fuseholder.
Preventive maintenance of fuses consists of checking for the following conditions and correcting any discrepancies.
1. IMPROPER FUSE. Check the fuse installed against that recommended in the technical manual for the equipment. If an incorrect fuse is installed, replace it with the correct fuse.
2. CORROSION. Check for corrosion on the fuseholder terminals or the fuse itself. If corrosion is present, remove it with fine sandpaper.
3. IMPROPER FIT. Check for contact between the fuse and fuseholder. If a piece of paper will fit between the fuse and the clips on a clip-type fuseholder, there is improper contact. If the fuse is not held in the cap of a plug-type fuseholder, the contacts are too loose.
4. OPEN FUSES. Check fuses for opens. If any fuse is open, repair the trouble that caused the open fuse and replace the fuse.
2-20
Q26. You have removed an open fuse from a fuseholder and repaired the cause of the fuse opening. The parts list specifies a fuse coded F02BI25VñA. There are no fuses available with that identification. In the following list, indicate if the fuse is a direct replacement, a good substitute, or not acceptable. For the fuses that are good substitutes, number them in order of preference and explain why they are numbered that way. If the fuse is not acceptable, explain why.
Q27. What two things should you check before replacing a fuse? Q28. List the safety precautions to be observed when replacing a fuse. Q29. What conditions should you check for when conducting preventive maintenance on fuses?
A circuit breaker is a circuit protection device that, like a fuse, will stop current in the circuit if there is a direct short, excessive current, or excessive heat. Unlike a fuse, a circuit breaker is reusable. The circuit breaker does not have to be replaced after it has opened or broken the circuit. Instead of replacing the circuit breaker, you reset it.
Circuit breakers can also be used as circuit control devices.  Circuit control devices will be covered in more detail in the next chapter.
Circuit breakers are available in a great variety of sizes and types. It would not be possible to describe every type of circuit breaker in use today, but this chapter will describe the basic types of circuit breakers and their operational principles.
Circuit breakers have five main components, as shown in figure 2-16. The components are the frame, the operating mechanism, the arc extinguishers and contacts, the terminal connectors, and the trip elements.
2-21
The FRAME provides an insulated housing and is used to mount the circuit breaker components (fig. 2-17). The frame determines the physical size of the circuit breaker and the maximum allowable voltage and current.
The OPERATING MECHANISM provides a means of opening and closing the breaker contacts (turning, the circuit ON and OFF). The toggle mechanism shown in figure 2-17 is the quick-make, quick-break type, which means the contacts snap open or closed quickly, regardless of how fast the handle is moved. In addition to indicating whether the breaker is ON or OFF, the operating mechanism handle indicates when the breaker has opened automatically (tripped) by moving to a position between ON and OFF. To reset the circuit breaker, the handle must first be moved to the OFF position, and then to the ON position.
2-22
The ARC EXTINGUISHER confines, divides, and extinguishes the arc drawn between contacts each time the circuit breaker interrupts current. The arc extinguisher is actually a series of contacts that open gradually, dividing the arc and making it easier to confine and extinguish. This is shown in figure 2-18. Arc extinguishers are generally used in circuit breakers that control a large amount of power, such as those found in power distribution panels. Small power circuit breakers (such as those found in lighting panels) may not have arc extinguishers.
TERMINAL CONNECTORS are used to connect the circuit breaker to the power source and the load. They are electrically connected to the contacts of the circuit breaker and provide the means of connecting the circuit breaker to the circuit.
The TRIP ELEMENT is the part of the circuit breaker that senses the overload condition and causes the circuit breaker to trip or break the circuit. This chapter will cover the thermal, magnetic, and thermal-
2-23
magnetic trip units used by most circuit breakers. (Some circuit breakers make use of solid-state trip units using current transformers and solid-state circuitry.) THERMAL TRIP ELEMENT
A thermal trip element circuit breaker uses a bimetallic element that is heated by the load current. The bimetallic element is made from strips of two different metals bonded together. The metals expand at different rates as they are heated. This causes the bimetallic element to bend as it is heated by the current going to the load. Figure 2-19 shows how this can be used to trip the circuit breaker.
Figure 2-19, view A, shows the trip element with normal current. The bimetallic element is not heated excessively and does not bend. If the current increases (or the temperature around the circuit breaker increases), the bimetallic element bends, pushes against the trip bar, and releases the latch. Then, the contacts open, as shown in figure 2-19, view B.
The amount of time it takes for the bimetallic element to bend and trip the circuit breaker depends on the amount the element is heated. A large overload will heat the element quickly. A small overload will require a longer time to trip the circuit breaker.
A magnetic trip element circuit breaker uses an electromagnet in series with the circuit load as in figure 2-20. With normal current, the electromagnet will not have enough attraction to the trip bar to move it, and the contacts will remain closed as shown in figure 2-20, view A. The strength of the magnetic field of the electromagnet increases as current through the coil increases. As soon as the current in the circuit becomes large enough, the trip bar is pulled toward the magnetic element (electromagnet), the contacts are opened, and the current stops, as shown in figure 2-20, view B.
2-24
The amount of current needed to trip the circuit breaker depends on the size of the gap between the trip bar and the magnetic element. On some circuit breakers, this gap (and therefore the trip current) is adjustable. THERMAL-MAGNETIC TRIP ELEMENT
The thermal trip element circuit breaker, like a delay fuse, will protect a circuit against a small overload that continues for a long time. The larger the overload, the faster the circuit breaker will trip. The thermal element will also protect the circuit against temperature increases. A magnetic circuit breaker will trip instantly when the preset current is present. In some applications, both types of protection are desired. Rather than use two separate circuit breakers, a single trip element combining thermal and magnetic trip elements is used. A thermal-magnetic trip element is shown in figure 2-21.
2-25
In the thermal-magnetic trip element circuit breaker, a magnetic element (electromagnet) is connected in series with the circuit load, and a bimetallic element is heated by the load current. With normal circuit current, the bimetallic element does not bend, and the magnetic element does not attract the trip bar, as shown in figure 2-21, view A.
If the temperature or current increases over a sustained period of time, the bimetallic element will bend, push the trip bar and release the latch. The circuit breaker will trip as shown in figure 2-21, view B.
If the current suddenly or rapidly increases enough, the magnetic element will attract the trip bar, release the latch, and the circuit breaker will trip, as shown in figure 2-21, view C. (This circuit breaker has tripped even though the thermal element has not had time to react to the increased current.)
Q30. What are the five main components of a circuit breaker? Q31. What are the three types of circuit breaker trip elements? Q32. How does each type of trip element react to an overload?
Circuit breakers are classified as being trip free or nontrip free. A trip-free circuit breaker is a circuit breaker that will trip (open) even if the operating mechanism (ON-OFF switch) is held in the ON position. A nontrip-free circuit breaker can be reset and/or held ON even if an overload or excessive heat condition is present. In other words, a nontrip-free circuit breaker can be bypassed by holding the operating mechanism ON.
Trip-free circuit breakers are used on circuits that cannot tolerate overloads and on nonemergency circuits. Examples of these are precision or current sensitive circuits, nonemergency lighting circuits, and nonessential equipment circuits.Nontrip-free circuit breakers are used for circuits that are essential for operations. Examples of these circuits are emergency lighting, required control circuits, and essential equipment circuits.
2-26
Circuit breakers, like fuses, are rated by the amount of time delay. In circuit breakers the ratings are instantaneous, short time delay, and longtime delay. The delay times of circuit breakers can be used to provide for SELECTIVE TRIPPING.
Selective tripping is used to cause the circuit breaker closest to the faulty circuit to trip. This will remove power from the faulty circuit without affecting other, nonfaulty circuits. Figure 2-22 should help you understand selective tripping.
Figure 2-22 shows a power distribution system using circuit breakers for protection. Circuit breaker 1 (CB1) has the entire current for all seven loads through it. CB2 feeds loads 1, 2, 3, and 4 (through CB4, CB5, CB6, and CB7), and CB3 feeds loads 5, 6, and 7 (through CB8, CB9, and CB10). If all the circuit breakers were rated with the same time delay, an overload on load 5 could cause CB1, CB3, and CB8 to trip. This would remove power from all seven loads, even though load 5 was the only circuit with an overload.
Selective tripping would have CB1 rated as long time delay, CB2 and CB3 rated as short time delay, and CB4 through CB10 rated as instantaneous. With this arrangement, if load 5 had an overload, only CB8 would trip. CB8 would remove the power from load 5 before CB1 or CB3 could react to the overload. In this way, only load 5 would be affected and the other circuits would continue to operate.
 Not all circuit breakers are of this type. The circuit breaker in figure 2-23 is physically large and controls large amounts of power; but the operating mechanism is not a toggle. Except for the difference in the operating mechanism, this circuit breaker is identical to the circuit breakers already presented.
2-27
Circuit breakers used for low power protection, such as 28-volt dc, 30 amperes, can be physically small. With low power use, arc extinguishers are not required, and so are not used in the construction of these circuit breakers. Figure 2-24 shows a low power circuit breaker of the push-button or push-pull type. This circuit breaker has a thermal trip element (the bimetallic disk) and is nontrip-free. The push button is the operating mechanism of this circuit breaker.
2-28
You will find other physical types of circuit breakers as you work with electrical circuits. They are found in power distribution systems, lighting panels, and even on individual pieces of equipment. Regardless of the physical size and the amount of power through the circuit breaker, the basic operating principles of circuit breakers apply.
Q33. What is a trip-free circuit breaker? Q34. What is a nontrip-free circuit breaker? Q35. Where should you use a trip-free circuit breaker? Q36. Where should you use a nontrip-free circuit breaker? The magnetic trip element makes use of a magnetic element (electromagnet). If current reaches a
preset quantity, the magnetic element attracts the trip bar and releases the latch.
The thermal-magnetic trip element combines the actions of the bimetallic and magnetic elements in a single trip element. If either the bimetal element or the magnetic element reacts, the circuit breaker will trip.
Q37. What are the three time delay ratings for circuit breakers? Q38. What is selective tripping and why is it used? Q39. If the power distribution system shown in figure 2-22 uses selective tripping, what is the time delay
rating for each of the circuit breakers shown? Q40. What factors are used to select a circuit breaker? Q41. What type of circuit breaker is used on a multimeter?
Circuit breakers require careful inspection and periodic cleaning. Before you attempt to work on circuit breakers, check the applicable technical manual carefully. When you work on shipboard circuit breakers, the approval of the electrical or engineering officer must be obtained before starting work. Be certain to remove all power to the circuit breaker before you work on it. Tag the switch that removes the power to the circuit breaker to ensure that power is not applied while you are working.
Once approval has been obtained, the incoming power has been removed, the switch tagged, and you have checked the technical manual, you may begin to check the circuit breaker. Manually operate the circuit breaker several times to be sure the operating mechanism works smoothly. Inspect the contacts for
2-29
pitting caused by arcing or corrosion. If pitting is present, smooth the contacts with a fine file or number 00 sandpaper. Be certain the contacts make proper contact when the operating mechanism is ON.
Check the connections at the terminals to be certain the terminals and wiring are tight and free from corrosion. Check all mounting hardware for tightness and wear. Check all components for wear. Clean the circuit breaker completely.
When you have finished working on the circuit breaker, restore power and remove the tag from the switch that applies power to the circuit.
Q42. What steps are to be taken before beginning work on a circuit breaker? Q43. What items are you to check when working on a circuit breaker?
protection devices. The following is a summary of the main points in this chapter.
CIRCUIT PROTECTION DEVICES are needed to protect personnel and circuits from hazardous conditions. The hazardous conditions can be caused by a direct short, excessive current, or excessive heat. Circuit protection devices are always connected in series with the circuit being protected.
A DIRECT SHORT is a condition in which some point in the circuit, where full system voltage is present, comes in direct contact with the ground or return side of the circuit.
EXCESSIVE CURRENT describes a condition that is not a direct short but in which circuit current increases beyond the designed current carrying ability of the circuit.
EXCESSIVE HEAT describes a condition in which the heat in or around a circuit increases to a higher than normal level.
FUSES and CIRCUIT BREAKERS are the two types of circuit protection devices discussed in this chapter.
2-30
PLUG-TYPE FUSES are used in low-voltage, low-current circuits. This type fuse is rapidly being replaced by the circuit breaker.
CARTRIDGE FUSES are available in a wide range of physical sizes and voltage and current ratings. This type fuse is the most commonly used fuse.
2-31
The CURRENT RATING of a fuse is a value expressed in amperes that represents the amount of current the fuse will allow to flow without opening.
The VOLTAGE RATING of a fuse indicates the ability of the fuse to quickly extinguish the arc after the fuse element melts and the maximum voltage the open fuse will block.
The TIME DELAY RATING of a fuse indicates the relationship between the current through the fuse and the time it takes for the fuse to open. The three time delay ratings for fuses are DELAY, STANDARD, and FAST.
DELAY FUSES allow surge currents without opening. They are used to protect motors, solenoids, and transformers.
STANDARD FUSES have neither a time delay nor a fast acting characteristic. They are used in automobiles, lighting circuits and electrical power circuits.
FAST FUSES open very quickly with any current above the current rating of the fuse. They are used to protect delicate instruments or semiconductor devices.
The OLD MILITARY FUSE DESIGNATION is a system of fuse identification that uses coding to represent the current, voltage, and time-delay rating of the fuse. New fuses purchased by the Navy will no longer use this designation.
2-32
The NEW MILITARY FUSE DESIGNATION is the system used to identify fuses purchased by the Navy at the present time. The coding of current and voltage ratings has been replaced with direct printing of these ratings.
2-33
The OLD COMMERCIAL FUSE DESIGNATION was used by the fuse manufacturers to identify fuses. The current and voltage ratings are printed on the fuse, but the time delay rating is contained in the style coding of the fuse.
2-34
The NEW COMMERCIAL FUSE DESIGNATION is currently used by fuse manufacturers to identify fuses. It is similar to the old commercial fuse designation with the difference being in the style coding portion of the designation.
FUSEHOLDERS are used to allow easy replacement of fuses in a circuit.
2-35
The CLIP-TYPE has clips to connect the ferrules or knifeblades of the fuse to the circuit. The POST-TYPE is an enclosed fuseholder. The center connection of the post type should be connected to the power source and the outside connector should be connected to the load.
OPEN FUSES can be found by VISUAL INSPECTION, FUSE INDICATORS, or by the use of a METER. The following SAFETY PRECAUTIONS should be observed when checking a fuse:
• Turn the power off and discharge the circuit before removing a FUSE.
• Use a fusepuller when you remove a fuse from clip-type fuseholders.
• When you check a fuse with a voltmeter, be careful to avoid shocks and short circuits.
• When you use an ohmmeter to check fuses with low current ratings, be careful to avoid opening the fuse by excessive current.
REPLACEMENT FUSES must be of the proper type. Check the technical manual parts list to find the identification of the proper fuse. If a substitute fuse must be used, the following guidelines should be followed:
• Never use a fuse with a higher current rating, a lower voltage rating, or a slower time delay rating than the specified fuse.
• The best substitution fuse is a fuse with the same current and time delay ratings and a higher voltage rating.
2-36
• If a lower current rating, or a lower time delay rating is used, the fuse may open under normal circuit conditions. Substitute fuses must have the same style (physical dimensions) as the specified fuse.
PROPER FIT between the fuse and fuseholder is essential. If the clips on clip-type fuseholders are sprung, the clips should be reformed, or clip clamps should be used. Any corrosion on fuses or fuseholders must be removed with fine sandpaper.
PREVENTIVE MAINTENANCE of fuses involves checking for the proper fuse, corrosion, proper fit, and open fuses; and correcting any discrepancies.
CIRCUIT BREAKERS have five main components: The frame, the operating mechanism, the arc extinguisher, the terminal connectors, and the trip element.
2-37
A THERMAL TRIP ELEMENT uses a bimetallic element that is heated by load current and bends due to this heating. If current (or temperature) increases above normal, the bimetallic element bends to push against a trip bar and opens the circuit.
A MAGNETIC TRIP ELEMENT uses an electromagnet in series with the load current to attract the trip bar and open the circuit if excessive current is present.
2-38
A THERMAL-MAGNETIC TRIP ELEMENT combines the thermal and magnetic trip elements into a single unit. A TRIP-FREE circuit breaker will trip (open) even if the operating mechanism is held in the ON position. A TRIP-FREE circuit breaker would be used on non-essential circuits.
A NONTRIP-FREE circuit breaker can be bypassed by holding the operating mechanism ON. A NONTRIP-FREE circuit breaker would be used for emergency or essential equipment circuits.
2-39
The TIME DELAY RATINGS of circuit breakers are INSTANTANEOUS, SHORT TIME DELAY, and LONG TIME DELAY.
SELECTIVE TRIPPING is used to cause the circuit breaker closest to the faulty circuit to trip, isolating the faulty circuit without affecting other nonfaulty circuits. This is accomplished by using an instantaneous circuit breaker close to the load, a short time delay circuit breaker at the next junction, and a long time delay circuit breaker at the main junction box.
The FACTORS used to select a circuit breaker are the power requirements of the circuit and the physical space available.
When WORKING ON CIRCUIT BREAKERS, the following items should be done BEFORE working on the circuit breaker: Check the applicable technical manual, obtain the approval of the electrical or engineering officer (for shipboard circuit breakers), remove power from the circuit breaker, and tag the switch that removes power from the circuit breaker. The following items should be checked and discrepancies corrected when working on circuit breakers: Check the operating mechanism for smooth operation, check the contacts for pitting, check the terminals for tightness and corrosion, check the mounting hardware for tightness and wear, check all components for wear, and check the entire circuit breaker for cleanliness.
ANSWERS TO QUESTIONS Q1. THROUGH Q43. A1. To protect people and circuits from possible hazardous conditions. A2. A direct short, excessive current, and excessive heat.
A3. A condition in which some point in the circuit where full system voltage is present comes in contact with the ground or return side of the circuit.
A4. A condition that is not a direct short but in which circuit current increases beyond the designed current carrying ability of the circuit.
A5. A condition in which the heat in or around the circuit increases to a higher than normal level
2-40
A6. In series, so total current will be stopped when the device opens.
A7. Fuses and circuit breakers.
A8.
a. circuit breaker
b. fuse. A9.
a. cartridge
b. plug
c. plug
d. cartridge.
A10. A, C.
A11. Current, voltage, and time delay.
A12. The amount of current the fuse will allow without opening. A13. The ability of the fuse to quickly extinguish the arc after the fuse element melts and the maximum
voltage that cannot jump across the gap of the fuse after the fuse opens. A14. Delay, standard, and fast. A15. Delay-Motors, solenoids, or transformers. Standard-Automobiles, lighting or electrical power
circuits. Fast-Delicate instruments or semiconductor devices.
A16.
a. 125 volts or less, 1.5 amperes, delay
b. 250 volts or less, 1/8 ampere standard
A17.
a. 125 volts or less, 1/16 ampere
b. 250 volts or less, .15 ampere
A18. F05B32V20A.
A19.
a. Post-type fuseholder b. Clip-type fuseholder
2-41
A20.
a. Center connector
b. Outside connector
A21. Visual inspection, indicators, and using a meter.
A22. Put it back in the circuit. A good fuse will have zero ohms of resistance. A23. The ohmmeter causes more than 1/500 ampere through the fuse when you check the fuse, thus it
opens the fuse. A24. Use a resistor in series with the fuse when you check it with the ohmmeter. A25. Turn the power off and discharge the circuit before you remove fuses. Use a fuse puller (an
insulated tool) when you remove fuses front clip-type fuse holders. When you check fuses with a voltmeter, be careful to avoid shocks and short circuits.
A26.
a. Not acceptable-wrong style
b. Substitute #3-smaller current rating
c. Substitute #1-identical, except higher voltage rating
d. Not acceptable-lower voltage rating
e. Direct replacement
f. Not acceptable-higher current rating g. Substitute #2-Faster time delay rating
A27. Check for the proper type of replacement fuse and proper fit. A28. Be sure the power is off in the circuit and the circuit is discharged before replacing a fuse. Use an
identical replacement fuse if possible. Remove any corrosion from the fuseholders before replacing the fuses.
A29. Improper fuse, corrosion, improper fit, and open fuse. A30. Frame, operating mechanism, arc extinguishers, terminal connectors, and trip element.
A31. Thermal, magnetic, and thermal-magnetic.
A32. The thermal trip element makes use of a bimetallic element that bends with an increase in temperature or current. The bending causes the trip bar to be moved releasing the latch.
A33. A circuit breaker that will trip even if the operating mechanism is held ON. A34. A circuit breaker that can be overridden if the operating mechanism is held ON. A35. In current sensitive or nonemergency systems.
2-42
A36. In emergency or essential circuits.
A37. Instantaneous, short time delay, and long time delay.
A38. It is the use of time delay ratings to cause the circuit breaker closest to the faulty circuit to trip. This isolates the faulty circuit without affecting other circuits.
A39. CB1-long time delay; CB2, CB3-short time delay; CB4 through CB10-instantaneous.
A40. The power requirements of the circuit and the physical space available. A41. A push button or push-pull circuit breaker (small size, low power). A42. Check the applicable technical manual, obtain the approval of the electrical or engineering officer
(for shipboard circuit breakers), remove power from the circuit breaker, and tag the switch that supplies power to the circuit breaker.
A43. Check the operating mechanism for smooth operation, check the contacts for pitting, check the terminals for tightness and corrosion, check the mounting hardware for tightness and wear, check all components for wear, and check the entire circuit breaker for cleanliness.
3-1
Upon completion of this chapter you will be able to:
1. State three reasons circuit control devices are used and list three general types of circuit control devices.
2. Identify the schematic symbols for a switch, a solenoid, and a relay.
3. State the difference between a manual and an automatic switch and give an example of each.
4. State the reason multicontact switches are used.
5. Identify the schematic symbols for the following switches:
6. State the characteristics of a switch described as a rocker switch.
7. State the possible number of positions for a single-pole, double-throw switch.
8. Identify a type of momentary switch.
9. State the type of switch used to prevent the accidental energizing or deenergizing of a circuit.
10. State the common name for an accurate snap-acting switch.
11. State the meaning of the current and voltage rating of a switch.
12. State the two types of meters you can use to check a switch.
13. Select the proper substitute switch from a list.
14. State the conditions checked for in preventive maintenance of switches.
3-2
15. State the operating principle and one example of a solenoid.
16. State the ways in which a solenoid can be checked for proper operation.
17. State the operating principle of a relay and how it differs from a solenoid.
18. State the two types of relays according to use.
19. State the ways in which a relay can be checked for proper operation and the procedure for servicing it.
Circuit control devices are used everywhere that electrical or electronic circuits are used. They are found in submarines, computers, aircraft, televisions, ships, space vehicles, medical instruments, and many other places. In this chapter you will learn what circuit control devices are, how they are used, and some of their characteristics.
INTRODUCTION Electricity existed well before the beginning of recorded history. Lightning was a known and feared
force to early man, but the practical uses of electricity were not recognized until the late 18th century. The early experimenters in electricity controlled power to their experiments by disconnecting a wire from a battery or by the use of a clutch between a generator and a steam engine. As practical uses were found for electricity, a convenient means for turning power on and off was needed.
Telegraph systems, tried as early as the late 1700s and perfected by Morse in the 1830s, used a mechanically operated contact lever for opening and closing the signal circuit. This was later replaced by the hand-operated contact lever or "key."
Early power switches were simple hinged beams, arranged to close or open a circuit. The blade-and- jaw knife switch with a wooden, slate, or porcelain base and an insulated handle, was developed a short time later. This was the beginning of circuit control devices.
Modern circuit control devices can change their resistance from a few milliohms (when closed) to well over 100,000 megaohms (when open) in a couple of milliseconds. In some circuit control devices, the movement necessary to cause the device to open or close is only .001 inch (.025 millimeters). NEED FOR CIRCUIT CONTROL
Circuit control, in its simplest form, is the application and removal of power. This can also be expressed as turning a circuit on and off or opening and closing a circuit. Before you learn about the types of circuit control devices, you should know why circuit control is needed.
If a circuit develops problems that could damage the equipment or endanger personnel, it should be possible to remove the power from that circuit. The circuit protection devices discussed in the last chapter will remove power automatically if current or temperature increase enough to cause the circuit protection device to act. Even with this protection, a manual means of control is needed to allow you to remove power from the circuit before the protection device acts.
When you work on a circuit, you often need to remove power from it to connect test equipment or to remove and replace components. When you remove power from a circuit so that you can work on it, be
3-3
sure to "tag out" the switch to ensure that power is not applied to the circuit while you are working. When work has been completed, power must be restored to the circuit. This will allow you to check the proper operation of the circuit and place it back in service. After the circuit has been checked for proper operation, remove the tag from the power switch.
Many electrical devices are used some of the time and not needed at other times. Circuit control devices allow you to turn the device on when it is needed and off when it is not needed.
Some devices, like multimeters or televisions, require the selection of a specific function or circuit. A circuit control device makes possible the selection of the particular circuit you wish to use.
Circuit control devices have many different shapes and sizes, but most circuit control devices are either SWITCHES, SOLENOIDS, or RELAYS.
Figure 3-1 shows an example of each of these types of circuit control devices and their schematic symbols.
Figure 3-1, view A, is a simple toggle switch and the schematic symbol for this switch is shown below it. Figure 3-1, view B, is a cutaway view of a solenoid. The schematic symbol below the solenoid
3-4
is one of the schematic symbols used for this solenoid. Figure 3-1, view C, shows a simple relay. One of the schematic symbols for this relay is shown next to the relay.
Q1. What are three reasons circuit control is needed? Q2. What are the three types of circuit control devices? Q3. Label the schematic symbols shown in figure 3-2.
There are thousands and thousands of switch applications found in home, industry, and the Navy. Hundreds of electrical switches work for you everyday to perform functions you take for granted. Some switches operate by the touch of a finger and many others are operated automatically.
Switches are used in the home to turn off the alarm clock, to control the stove, to turn on the refrigerator light, to turn on and control radios and televisions, hair dryers, dishwashers, garbage disposals, washers and dryers, as well as to control heating and air conditioning. A typical luxury automobile with power seats and windows might have as many as 45 switches.
Industry uses switches in a wide variety of ways. They are found in the business office on computers, copy machines, electric typewriters, and other equipment. A factory or shop may use thousands of switches and they are found on almost every piece of machinery. Switches are used on woodworking machinery, metal working machinery, conveyors, automation devices, elevators, hoists, and lift trucks.
The Navy uses switches in a number of ways. A typical aircraft could have over 250 switches to control lights, electronic systems, and to indicate whether the landing gear is up or down. Ships, fire control systems, and missile launchers are also controlled by electrical switches. In fact, almost all electrical or electronic devices will have at least one switch.
Switches are designed to work in many different environments from extreme high pressure, as in a submarine, to extreme low pressure, as in a spacecraft. Other environmental conditions to consider are high or low temperature, rapid temperature changes, humidity, liquid splashing or immersion, ice, corrosion, sand or dust, fungus, shock or vibration, and an explosive atmosphere.
3-5
It would not be possible to describe all the different switches used. This chapter will describe the most common types of switches.
A manual switch is a switch that is controlled by a person. In other words, a manual switch is a switch that you turn on or off. Examples of common manual switches are a light switch, the ignition switch on a motor vehicle, or the channel selector on a television. You may not think of the channel selector as a switch that you use to turn something on or off, but that is what it does. The channel selector is used to turn on the proper circuit and allows the television to receive the channel you have selected.
An automatic switch is a switch that is controlled by a mechanical or electrical device. You do not have to turn an automatic switch on or off. Two examples of automatic switches are a thermostat and the distributor in a motor vehicle. The thermostat will turn a furnace or air conditioner on or off by responding to the temperature in a room. The distributor electrically turns on the spark plug circuit at the proper time by responding to the mechanical rotation of a shaft. Even the switch that turns on the light in a refrigerator when the door is opened is an automatic switch.
Automatic switches are not always as simple as the examples given above. Limit switches, which sense some limit such as fluid level, mechanical movement, pressure (altitude or depth under water), or an electrical quantity, are automatic switches. Computers use and control automatic switches that are sometimes quite complicated.
Basically, any switch that will turn a circuit on or off without human action is an automatic switch.
Switches are sometimes used to control more than one circuit or to select one of several possible circuits. An example of a switch controlling more than one circuit is the AM/FM selector on a radio. This switch enables you to control either the AM or FM portion of the radio with a single switch. An example of a switch that selects one of several circuits is the channel selector of a television set. These switches are called MULTICONTACT switches because they have more than one contact or MULTI(ple) CONTACTS. Number of Poles and Number of Throws
Multicontact switches (other than rotary switches, which will be covered later) are usually classified by the number of POLES and number of THROWS. Poles are shown in schematics as those contacts through which current enters the switch; they are connected to the movable contacts. Each pole may be connected to another part of the circuit through the switch by "throwing" the switch (movable contacts) to another position. This action provides an individual conduction path through the switch for each pole connection. The number of THROWS indicates the number of different circuits that can be controlled by each pole. By counting the number of points where current enters the switch (from the schematic symbol or the switch itself), you can determine the number of poles. By counting the number of different points each pole can connect with, you can determine the number of throws.
Figure 3-3 will help you understand this concept by showing illustrations of various multicontact switches and their schematic symbols.
3-6
Figure 3-3(A) shows a single-pole, double-throw switch. The illustration shows three terminals (connections) on this switch. The schematic symbol for the switch is also shown.
The center connection of the schematic symbol represents the point at which current enters the switch. The left and right connections represent the two different points to which this current can be switched. From the schematic symbol, it is easy to determine that this is a single-pole, double-throw switch.
Now look at figure 3-3(B). The switch is shown with its schematic symbol. The schematic symbol has two points at which current can enter the switch, so this is a double-pole switch. Each of the poles is mechanically connected (still electrically separate) to one point, so this is a single-throw switch. Only one throw is required to route two separate circuit paths through the switch.
Figure 3-3(C) shows a double-pole, double-throw switch and its schematic symbol. Figure 3-3(D) shows a four-pole, double-throw switch and its schematic symbol.
It might help you to think of switches with more than one pole as several switches connected together mechanically. For example, the knife switch shown in figure 3-3(D) could be thought of as four single-pole, double-throw switches mechanically connected together.
Q4. What is the difference between a manual and an automatic switch? Q5. What is one example of a manual switch? Q6. What is one example of an automatic switch? Q7. Why are multicontact switches used?
3-7
Q8. Label the schematic symbols shown in figure 3-4 as to number of poles and number of throws.
Single-Break and Double-Break Switches Switches can also be classified as SINGLE-BREAK or DOUBLE-BREAK switches. This refers to
the number of places in which the switch opens or breaks the circuit. All of the switches shown so far have been single-break switches. A double-break switch is shown in figure 3-5. The schematic symbol shown in figure 3-5(A) shows that this switch breaks the circuit in two places (at both terminals). The upper part of the schematic symbol indicates that these contacts are in the open position and the circuit will close when the switch is acted upon (manually or automatically). The lower symbol shows closed contacts. These contacts will open the circuit when the switch is acted upon.
Figure 3-5(B) is a picture of the switch. This switch is called a pushbutton switch because it has a button that must be pushed to change the switch contact connections. Notice that the switch has four terminals. The schematic symbol in figure 3-5(A) shows that when one set of contacts is open, the other set of contacts is closed. This switch is a double-pole, single-throw, double-break switch.
3-8
The number of poles in a switch is independent of the number of throws and whether it is a single or double break switch. The number of throws in a switch is independent of the number of poles and whether it is a single or double break switch. In other words, each characteristic of a switch (poles, throws, break) is not determined by either of the other characteristics. Figure 3-6 shows the schematic symbols for several different switch configurations.
partial circle. Instead of a pushbutton or toggle, the mechanism used to select the contact moves in a circular motion and must be turned. Rotary switches can be manual or automatic switches. An automobile distributor, the ignition switch on a motor vehicle, and the channel selector on some television sets are rotary switches.
The automobile distributor cap and rotor are an example of the simplest form of an automatic rotary switch. Figure 3-7 shows a portion of an automobile ignition system with the distributor cap and rotor shown. The rotor is the portion of this switch that moves (rotates) and selects the circuit (spark plug). The rotor does not actually touch the contacts going to the spark plugs, but the signal (spark) jumps the gap between the rotor and the contacts. This switch has one input (the rotor) and six positions (one for each spark plug). The schematic diagram for this rotary switch is shown below the illustration of the distributor cap.
3-9
The rotor in the distributor rotates continually (when in use) in one direction and makes a complete circle. This is not true for all rotary switches. The ignition switch in an automobile is also a rotary switch. It usually has four positions (accessory, off, on, start). Unlike the rotor, it does not rotate continually when in use, can be turned in either direction, and does not move through a complete circle.
Some rotary switches are made with several layers or levels. The arrangement makes possible the control of several circuits with a single switch. Figure 3-8 is an illustration of a rotary switch with two layers. Each layer has a selector and 20 contacts. As this switch is rotated, both layers select a single circuit (contact) of the 20.
The channel selector on some television sets is a multilayer rotary switch. It is also called a WAFER SWITCH. In a wafer switch, each layer is known as a wafer.
The schematic of the wafer is always drawn to represent the wafer as it would look if viewed from opposite the operating handle or mechanism. If the wafer has contacts on both sides, two drawings are used to show the two sides of the wafer. The two drawings are labeled "front" and "rear." The drawing labeled "front" represents the side of the wafer closest to the operating mechanism.
3-10
Figure 3-9(A) shows one wafer of a wafer switch and its schematic symbol. Contact 1 is the point at which current enters the wafer. It is always connected to the movable portion of the wafer. With the wafer in the position shown, contact 1 is connected to both contact 5 and 6 through the movable portion. If the movable portion was rotated slightly clockwise, contact 1 would only be connected to contact 5. This arrangement is known as MAKE BEFORE BREAK because the switch makes a contact before breaking the old contact.
Figure 3-9(B) is an illustration of the entire switch and its schematic symbol. Since the switch has two wafers mechanically connected by the shaft of the switch, the shaft rotates the movable portion of both wafers at the same time. This is represented on the schematic symbol by the dotted line connecting the two wafers.
The upper wafer of the schematic symbol is the wafer closest to the control mechanism, and is identical to the wafer shown in figure 3-9(A). When switches have more than one wafer, the first wafer shown is always the wafer closest to the operating mechanism. The lower wafer on the schematic diagram is the wafer farthest away from the operating mechanism. Contact 9 of this wafer is connected to the movable portion and is the point at which current enters the wafer. In the position shown, contact 9 is connected to both contact 13 and 16. If the switch is rotated slightly clockwise, contact 9 would no longer be connected to contact 13. A further clockwise movement would connect contact 9 to contact 12. This arrangement is called BREAK BEFORE MAKE. Contact 9 will also be connected to contact 15 at the same time as it is connected to contact 12.
Q9. Label the switch schematics shown in figure 3-10A through 3-10G.
3-11
You have learned that switches are classified by the number of poles, throws, and breaks. There are other factors used to describe a switch such as the type of actuator and the number of positions. In addition, switches are classified by whether the switch has momentary contacts or is locked into or out of position and whether or not the switch is snap-acting. Type of Actuator
In addition to the pushbutton, toggle, and knife actuated switches already described, switches can have other actuators. There are rocker switches, paddle switches, keyboard switches and mercury switches (in which a small amount of mercury makes the electrical contact between two conductors). Number of Positions
Switches are also classified by the number of positions of the actuating device. Figure 3-11 shows three toggle switches, the toggle positions, and schematic diagrams of the switch. Figure 3-11(A) is a single-pole, single-throw, two-position switch. The switch is marked to indicate the ON position (when the switch is closed) and the OFF position (when the switch is open). Figure 3-11(B) is a single-pole, double-throw, three-position switch. The switch markings show two ON positions and an OFF position. When this switch is OFF, no connection is made between any of the terminals. In either of the ON positions, the center terminal is connected to one of the outside terminals. (The outside terminals are not connected together in any position of the switch.) Figure 3-11(C) is a single-pole, double-throw, two-position switch. There is no OFF position. In either position of this switch, the center terminal is connected to one of the outside terminals.
3-12
Momentary and Locked Position Switches In some switches, one or more of the switch positions are MOMENTARY. This means that the
switch will only remain in the momentary position as long as the actuator is held in that position. As soon as you let go of the actuator, the switch will return to a non-momentary position. The starter switch on an automobile is an example of a momentary switch. As soon as you release the switch, it no longer applies power to the starter.
Another type of switch can be LOCKED IN or OUT of some of the switch positions. This locking prevents the accidental movement of the switch. If a switch has locked-in positions, the switch cannot be moved from those positions accidentally (by the switch being bumped or mistaken for an unlocked switch). If the switch has locked-out positions, the switch cannot be moved into those positions accidentally. Figure 3-12 shows a three-position, locking switch.
3-13
relatively independent of the activating mechanism movement. In other words, in a toggle switch, no matter how fast or slow you move the toggle, the actual switching of the circuit takes place at a fixed speed. The snap-acting switch is constructed by making the switch mechanism a leaf spring so that it "snaps" between positions. A snap-acting switch will always be in one of the positions designed for that switch. The switch cannot be "between" positions. A two-position, single-pole, double-throw, snap-acting switch could not be left in an OFF position. Accurate Snap-Acting Switches
An ACCURATE SNAP-ACTING SWITCH is a snap-acting switch in which the operating point is pre-set and very accurately known. The operating point is the point at which the plunger causes the switch to "switch." The accurate snap-acting switch is commonly called a MICROSWITCH. A microswitch is shown in figure 3-13.
The full description of the microswitch shown in figure 3-13 is a two-position, single-pole, double-throw, single-break, momentary-contact, accurate, snap-acting switch. Notice the terminals
3-14
marked C, NO, and NC. These letters stand for common, normally open, and normally closed. The common terminal is connected to the normally closed terminal until the plunger is depressed. When the plunger is depressed, the spring will "snap" into the momentary position and the common terminal will be connected to the normally open terminal. As soon as the plunger is released, the spring will "snap" back to the original condition.
This basic accurate snap-acting switch is used in many applications as an automatic switch. Several different methods are used to actuate this type of switch. Some of the more common actuators and their uses are shown in figure 3-14.
Q10. What classification of a switch is used when you describe it as a rocker switch? Q11. In describing a switch by the number of positions of the actuator, what are the two possible
configurations for a single-pole, double-throw switch? Q12. What type of switch should be used to control a circuit that requires a temporary actuation
signal?
Q13. What type of switch is used if it is necessary to guard against a circuit being accidentally turned on or off?
Q14. What is the common name used for an accurate snap-acting switch?
3-15
SWITCH RATING Switches are rated according to their electrical characteristics. The rating of a switch is determined
by such factors as contact size, contact material, and contact spacing. There are two basic parts to a switch rating-the current and voltage rating. For example, a switch may be rated at 250 volts dc, 10 amperes. Some switches have more than one rating. For example, a single switch may be rated at 250 volts dc, 10 amperes; 500 volts ac, 10 amperes; and 28 volts dc, 20 amperes. This rating indicates a current rating that depends upon the voltage applied.
The current rating of a switch refers to the maximum current the switch is designed to carry. This rating is dependent on the voltage of the circuit in which the switch is used. This is shown in the example given above. The current rating of a switch should never be exceeded. If the current rating of a switch is exceeded, the contacts may "weld" together making it impossible to open the circuit.
The voltage rating of a switch refers to the maximum voltage allowable in the circuit in which the switch is used. The voltage rating may be given as an ac voltage, a dc voltage, or both. The voltage rating of a switch should never be exceeded. If a voltage higher than the voltage rating of the switch is applied to the switch, the voltage may be able to "jump" the open contacts of the switch. This would make it impossible to control the circuit in which the switch was used.
Q15. What is the current rating of a switch? Q16. What is the voltage rating of a switch?
MAINTENANCE AND REPLACEMENT OF SWITCHES Switches are usually a very reliable electrical component. This means, they don’t fail very often.
Most switches are designed to operate 100,000 times or more without failure if the voltage and current ratings are not exceeded. Even so, switches do fail. The following information will help you in maintaining and changing switches.
There are two basic methods used to check a switch. You can use an ohmmeter or a voltmeter. Each of these methods will be explained using a single-pole, double-throw, single-break, three-position, snap-acting, toggle switch.
Figure 3-15 is used to explain the method of using an ohmmeter to check a switch. Figure 3-15(A) shows the toggle positions and schematic diagrams for the three switch positions. Figure 3-15(B) shows the ohmmeter connections used to check the switch while the toggle is in position 1. Figure 3-15(C) is a table showing the switch position, ohmmeter connection, and correct ohmmeter reading for those conditions.
3-16
With the switch in position 1 and the ohmmeter connected to terminals 1 and 2 of the switch, the ohmmeter should indicate ( ¥ ). When the ohmmeter is moved to terminals 2 and 3, the ohmmeter should indicate zero ohms. With the ohmmeter connected to terminals 1 and 3, the indication should be (¥ ).
As you remember from chapter 1, before the ohmmeter is used, power must be removed from the circuit and the component being checked should be isolated from the circuit. The best way to isolate the switch is to remove it from the circuit completely. This is not always practical, and it is sometimes necessary to check a switch while there is power applied to it. In these cases, you would not be able to use an ohmmeter to check the switch, but you can check the switch by the use of a voltmeter.
Figure 3-16(A) shows a switch connected between a power source (battery) and two loads. In figure 3-16(B), a voltmeter is shown connected between ground and each of the three switch terminals while the switch is in position 1. Figure 3-16(C) is a table showing the switch position, voltmeter connection, and the correct voltmeter reading.
3-17
With the switch in position 1 and the voltmeter connected between ground and terminal 1, the voltmeter should indicate no voltage (OV). When the voltmeter is connected to terminal 2, the voltmeter should indicate the source voltage. With the voltmeter connected to terminal 3, the source voltage should also be indicated. The table in figure 3-16(C) will show you the correct readings with the switch in position 2 or 3.
When a switch is faulty, it must be replaced. The technical manual for the equipment will specify the exact replacement switch. If it is necessary to use a substitute switch, the following guidelines should be used. The substitute switch must have all of the following characteristics.
• At least the same number of poles.
• At least the same number of throws.
• The same number of breaks.
• At least the same number of positions.
3-18
• The same configuration in regard to momentary or locked positions.
• A voltage rating equal to or higher than the original switch.
• A current rating equal to or higher than the original switch.
• A physical size compatible with the mounting.
In addition, the type of actuator (toggle, pushbutton, rocker, etc.) should be the same as the original switch. (This is desirable but not necessary. For example, a toggle switch could be used to replace a rocker switch if it were acceptable in all other ways.)
The number of poles and throws of a switch can be determined from markings on the switch itself. The switch case will be marked with a schematic diagram of the switch or letters such as SPST for single-pole, single-throw. The voltage and current ratings will also be marked on the switch. The number of breaks can be determined from the schematic marked on the switch or by counting the terminals after you have determined the number of poles and throws. The type of actuator, number of positions, the momentary and locked positions of the switch can all be determined by looking at the switch and switching it to all the positions.
As already mentioned, switches do not fail very often. However, there is a need for preventive maintenance of switches. Periodically switches should be checked for corrosion at the terminals, smooth and correct operation, and physical damage. Any problems found should be corrected immediately. Most switches can be inspected visually for corrosion or damage. The operation of the switch may be checked by moving the actuator. When the actuator is moved, you can feel whether the switch operation is smooth or seems to have a great deal of friction. To check the actual switching, you can observe the operation of the equipment or check the switch with a meter.
Q17. What two types of meters can be used to check a switch? Q18. If a switch must be checked with power applied, what type of meter is used? Q19. A double-pole, double-throw, single-break, three-position, toggle switch is faulty. This switch has
a momentary position 1 and is locked out opposition 3. The voltage and current ratings for the switch are 115 volt dc, 5 amperes. No direct replacement is available. From switches A through I, in table 3-1, indicate if the switch is acceptable or not acceptable as a substitute. Of the acceptable switches, rank them in order of choice. If the switch is unacceptable, give the reason.
Q20. What should you check when performing preventive maintenance on a switch?
3-19
5A
5A
10A
5A
10A
10A
10A
3A
5A
Table 3-1.—Replacement Switches and Their Characteristics
mechanical motion. The movement of the solenoid may be used to close a set of electrical contacts, cause the movement of a mechanical device, or both at the same time.
Figure 3-17 is a cutaway view of a solenoid showing the solenoid action. A solenoid is an electromagnet formed by a conductor wound in a series of loops in the shape of a spiral. Inserted within this coil is a soft-iron core and a movable plunger. The soft-iron core is pinned or held in an immovable position. The movable plunger (also soft iron) is held away from the core by a spring when the solenoid is deenergized.
When current flows through the conductor, it produces a magnetic field. The magnetic flux produced by the coil results in establishing north and south poles in both the core and the plunger. The plunger is attracted along the lines of force to a position at the center of the coil. As shown in figure 3-17, the deenergized position of the plunger is partially out of the coil due to the action of the spring. When voltage is applied, the current through the coil produces a magnetic field. This magnetic field draws the plunger within the coil, resulting in mechanical motion. When the coil is deenergized, the plunger returns to its normal position because of spring action. The effective strength of the magnetic field on the plunger varies according to the distance between the plunger and the core. For short distances, the strength of the field is strong; and as distances increase, the strength of the field drops off quite rapidly.
3-20
While a solenoid is a control device, the solenoid itself is energized by some other control device such as a switch or a relay. One of the distinct advantages in the use of solenoids is that a mechanical movement can be accomplished at a considerable distance from the control device. The only link necessary between the control device and the solenoid is the electrical wiring for the coil current. The solenoid can have large contacts for the control of high current. Therefore, the solenoid also provides a means of controlling high current with a low current switch. For example, the ignition switch on an automobile controls the large current of a starter motor by the use of a solenoid. Figure 3-18 shows a cutaway view of a starter motor-solenoid combination and a section of the wiring for the solenoid. Notice that the solenoid provides all electrical contact for current to the starter motor as well as a mechanical movement of the shift lever.
3-21
If you suspect that a solenoid is not working properly, the first step in troubleshooting it is a good visual inspection. Check the connections for poor soldering, loose connections, or broken wires. The plunger should be checked for cleanliness, binding, mechanical failure, and improper alignment. The mechanism that the solenoid is connected to (actuates) should also be checked for proper operation.
The second step is to check the energizing voltage with a voltmeter. If the voltage is too low, the result is less current flowing through the coil and a weak magnetic field. A weak magnetic field can result in slow or poor operation. Low voltage could also result in chatter or no operation at all. If the energizing voltage is too high, it could damage the solenoid by causing overheating or arcing. In either case, the voltage should be reset to the proper value so that further damage or failure of the solenoid will not result.
The solenoid coil should then be checked for opens, shorts, and proper resistance with an ohmmeter. If the solenoid coil is open, current cannot flow through it and the magnetic field is lost. A short results in fewer turns and higher current in the coil. The net result of a short is a weak magnetic field. A high- resistance coil will reduce coil current and also result in a weak magnetic field. A weak magnetic field
3-22
will cause less attraction between the plunger and the core of the coil. This will result in improper operation similar to that caused by low voltage. If the coil is open, shorted, or has changed in resistance, the solenoid should be replaced.
Finally, you should check the solenoid to determine if the coil is shorted to ground. If a short to ground is found, the short should be removed to restore the solenoid to proper operation.
Q21. What is the operating principle of a solenoid? Q22. What is one example of the use of a solenoid? Q23. If a solenoid is not operating properly, what items should be checked?
RELAYS The RELAY is a device that acts upon the same fundamental principle as the solenoid. The
difference between a relay and a solenoid is that a relay does not have a movable core (plunger) while the solenoid does. Where multipole relays are used, several circuits may be controlled at once.
Relays are electrically operated control switches, and are classified according to their use as POWER RELAYS or CONTROL RELAYS. Power relays are called CONTACTORS; control relays are usually known simply as relays.
The function of a contactor is to use a relatively small amount of electrical power to control the switching of a large amount of power. The contactor permits you to control power at other locations in the equipment, and the heavy power cables need be run only through the power relay contacts.
Only lightweight control wires are connected from the control switches to the relay coil. Safety is also an important reason for using power relays, since high power circuits can be switched remotely without danger to the operator.
Control relays, as their name implies, are frequently used in the control of low power circuits or other relays, although they also have many other uses. In automatic relay circuits, a small electric signal may set off a chain reaction of successively acting relays, which then perform various functions.
In general, a relay consists of a magnetic core and its associated coil, contacts, springs, armature, and the mounting. Figure 3-19 illustrates the construction of a relay. When the coil is energized, the flow of current through the coil creates a strong magnetic field which pulls the armature downward to contact C1, completing the circuit from the common terminal to C1. At the same time, the circuit to contact C2, is opened.
3-23
A relay can have many different types of contacts. The relay shown in figure 3-19 has contacts known as "break-make" contacts because they break one circuit and make another when the relay is energized. Figure 3-20 shows five different combinations of relay contacts and the names given to each.
A single relay can have several different types of contact combinations. Figure 3-21 is the contact arrangement on a single relay that has four different contact combinations. (The letters next to the contacts are the "forms" shown in figure 3-20.)
3-24
One type of relay with multiple sets of contacts is the clapper relay shown in figure 3-22. As the circuit is energized, the clapper is pulled to the magnetic coil. This physical movement of the armature of the clapper forces the pushrod and movable contacts upward. Any number of sets of contacts may be built onto the relay; thus, it is possible to control many different circuits at the same time. This type of relay can be a source of trouble because the motion of the clapper armature does not necessarily assure movement of all the movable contacts. Referring to figure 3-22, if the pushrod were broken, the clapper armature might push the lower movable contact upward but not move the upper movable contact.
3-25
Some equipment requires a "warm-up" period between the application of power and some other action. For example, vacuum tubes (covered later in this training series) require a delay between the application of filament power and high voltage. A time-delay relay will provide this required delay.
A thermal time-delay relay (fig. 3-23) is constructed to produce a delayed action when energized. Its operation depends on the thermal action of a bimetallic element similar to that used in a thermal circuit breaker. A heater is mounted around or near the element. The movable contact is mounted on the element itself. As the heat causes the element to bend (because of the different thermal expansion rates), the contacts close.
Relays can be described by the method of packaging; open, semisealed, and sealed. Figure 3-24 shows several different relays and illustrates these three types of packaging.
Figure 3-24 (E), (G) and (H) are open relays. The mechanical motion of the contacts can be observed and the relays are easily available for maintenance. Figure 3-24 (A), (B) and (C) are semisealed relays. The covers provide protection from dust, moisture, and other foreign material but can be removed for maintenance.
3-26
The clear plastic or glass covers provide a means of observing the operation of the relay without removal of the cover. Figure 3-24 (D) and (F) are examples of a hermetically sealed relay. These relays are protected from temperature or humidity changes as well as dust and other foreign material. Since the covers cannot be removed, the relays are also considered to be tamper-proof. With metal or other opaque covers, the operation of the relay can be "felt" by placing your finger on the cover and activating the relay.
Q24. What is the operating principle of a relay? Q25. How does a relay differ from a solenoid? Q26. What are the two classifications of relays?
3-27
The relay is one of the most dependable electromechanical devices in use, but like any other mechanical or electrical device, relays occasionally wear out or become inoperative. Should an inspection determine that a relay is defective, the relay should be removed immediately and replaced with another of the same type. You should be sure to obtain the same type relay as a replacement. Relays are rated in voltage, amperage, type of service, number of contacts, and similar characteristics.
Relay coils usually consist of a single coil. If a relay fails to operate, the coil should be tested for open circuit, short circuit, or short to ground. An open coil is a common cause of relay failure.
During preventive maintenance you should check for charred or burned insulation on the relay and for darkened or charred terminal leads. Both of these indicate overheating, and the likelihood of relay breakdown. One possible cause for overheating is that the power terminal connectors are not tight. This would allow arcing at the connection.
The build-up of film on the contact surfaces of a relay is another cause of relay trouble. Although film will form on the contacts by the action of atmospheric and other gases, grease film is responsible for a lot of contact trouble. Carbon build-up which is caused by the burning of a grease film or other substance (during arcing), also can be troublesome. Carbon forms rings on the contact surfaces and as the carbon rings build-up, the relay contacts are held open.
When current flows in one direction through a relay, a problem called "cone and crater" may be created at the contacts. The crater is formed by transfer of metal from one contact to the other contact, the deposit being in the shape of a cone. This condition is shown in figure 3-25(A).
Some relays are equipped with ball-shaped contacts and, in many applications, this type of contact is considered superior to a flat surface. Figure 3-25(B) shows a set of ball-shaped contacts. Dust or other substances are not as readily deposited on a ball-shaped surface. In addition, a ball-shaped contact penetrates film more easily than a flat contact. When you clean or service ball-shaped relay contacts, be careful to avoid flattening or otherwise altering the rounded surfaces of the contacts, YOU could damage
3-28
a relay if you used sandpaper or emery cloth to clean the contacts. Only a burnishing tool, shown in figure 3-26 should be used for this purpose.
You should not touch the surfaces of the burnishing tool that are used to clean the relay contacts. After the burnishing, tool is used, it should be cleaned with alcohol.
Contact clearances or gap settings must be maintained in accordance with the operational specifications of the relay. When a relay has bent contacts, you should use a point bender (shown in figure 3-27) to straighten the contacts. The use of any other tool could cause further damage and the entire relay would have to be replaced.
Cleanliness must be emphasized in the removal and replacement of covers on semi sealed relays. The entry of dust or other foreign material can cause poor contact connection. When the relay is installed in a position where there is a possibility of contact with explosive fumes, extra care should be taken with the cover gasket. Any damage to, or incorrect seating of the gasket increases the possibility of igniting the vapors.
3-29
Q27. How can you determine if a relay is operating (changing from one position to the other)? Q28. What items should be checked on a relay that is not operating properly? Q29. What is used to clean the contacts of a relay? Q30. What tool is used to set contact clearances on a relay?
SUMMARY This chapter has provided you with basic information on circuit control devices. The following is a
summary of the main points in this chapter.
CIRCUIT CONTROL DEVICES are used to apply or remove power and to select a function or circuit within a device.
A SWITCH is one type of circuit control device. Switches are classified in many different ways.
 An AUTOMATIC SWITCH will turn a circuit ON or OFF without the action of a person by using mechanical or electrical devices.
MULTICONTACT SWITCHES make possible the control of more than one circuit or the selection of one of several possible circuits with a single switch.
3-30
The POLES of a switch are the points at which current can enter the switch. The number of THROWS is the number of possible circuits that can be connected to each pole. The number of BREAKS is the number of points at which the switch breaks the circuit.
A ROTARY SWITCH is a multicontact switch with contacts arranged in a circular or semicircular manner.
A WAFER SWITCH is a rotary switch in which the contacts are on wafers. The wafers are mechanically connected by the shaft of the switch.
3-31
The ACTUATOR of a switch is the portion of the switch which is moved to cause the switch to change contact positions. The actuator could be a toggle, a pushbutton, a rocker, or, in the case of a rotary switch, a shaft and handle.
The NUMBER OF POSITIONS of a switch refers to the number of points at which the actuator can select a contact configuration.
3-32
A MOMENTARY POSITION of a switch is one in which the actuator will only stay as long as force is applied to the actuator. When the force is removed, the actuator (and switch) will return to a non- momentary position.
A LOCKED POSITION of a switch is used to prevent the accidental movement of the actuator to or from a specific position.
A SNAP-ACTING SWITCH is one in which the movement of the switch contacts is relatively independent of the actuator movement. This is accomplished by using a leaf spring for the common contact of the switch.
A MICROSWITCH is an accurate snap-acting switch and the operating point is preset and very accurately known.
3-33
The VOLTAGE RATING of a switch is the maximum voltage the switch is designed to control. A voltage higher than the voltage rating may be able to "jump" the open contacts of the switch.
The CURRENT RATING of a switch is the maximum current the switch is designed to carry; it is dependent on the voltage rating. Any current higher than the current rating may cause the contacts of the switch to melt and "weld" together.
The contacts of a switch can be checked with an ohmmeter if power is removed or with a voltmeter if power is applied to the switch. To check a switch, the actuator should be checked for smooth and correct operation, the terminals should be checked for evidence of corrosion, and the physical condition of the switch should be determined. If a substitute switch must be used to replace a faulty switch, the substitute must have all of the following:
At least the same number of poles, throws, and positions; the same number of breaks and an identical configuration in regard to momentary and locked positions; and a voltage and current rating equal to or higher than the original switch. In addition, the substitute must be of a physical size compatible with the mounting, and must have the same type actuator as the original switch.
3-34
A SOLENOID is a control device that uses electromagnetism to convert electrical energy into a mechanical motion. The magnetic field of the coil and core will attract the plunger of a solenoid when current flows through the coil. When current is removed, the spring attached to the plunger will cause the plunger to return to its original position.
If a solenoid fails to operate, check the terminal connections, the plunger and attached mechanism for smooth operation, the energizing voltage, and the coil of the solenoid.
A RELAY is an electromagnetic control device that differs from the solenoid in that the solenoid uses a movable core (plunger) while the relay has fixed core. Relays are classified as CONTROL RELAYS, which control low power COMMON circuits and POWER RELAYS or CONTACTORS which control high power circuits.
3-35
CLAPPER RELAYS use a clapper (armature) to move contact positions and accomplish the switching of circuits.
Relays are described by the type of enclosure. A relay may be OPEN, SEMISEALED, or SEALED.
3-36
If a relay fails to function, the movement of the contacts should be observed; the coil should be checked for opens or shorts; the terminal leads should be checked for burned or charred insulation; and the contact surfaces should be checked for carbon, arcing, and contact spacing.
A BURNISHING TOOL is used to clean the contacts of a relay. Files, sandpaper, and emery cloth should NOT be used.
3-37
A POINT BENDER is used to adjust contact spacing of a relay. No other tool should be used.
ANSWERS TO QUESTIONS Q1. THROUGH Q30. A1. To remove power from a malfunctioning device; to remove power from a device you wish to work
on and restore power when the work is completed; to turn devices on and off as the device is needed; to select the function or circuit desired within a device.
A2. Switches, solenoids, and relays.
A3.
a. Solenoid.
b. Switch.
c. Relay.
A4.  An automatic switch turns a circuit on or off without the action of a person (by using mechanical or electrical devices).
A5. A light switch, an ignition switch, television channel selector, etc.
A6. A thermostat, an automobile distributor, a limit switch, etc.
A7. Multicontact switches make possible the control of more than one circuit or the selection of one of several possible circuits with a single switch.
3-38
A8.
a. Three-pole, single-throw (triple-pole, single-throw) b. Double-pole, double-throw
c. Single-pole, double-throw
d. Single-pole, single-throw
e. Double-pole, triple-throw
f. Six-pole, double-throw A9.
a. Single-pole, single-throw, single-break
b. Single-pole, double-throw, single-break
c. Single-pole, single-throw, double-break
d. Single-pole, double-throw, double-break
e. Rotary
f. Wafer g. Double-pole, double-throw, double-break
A10. The type of actuator. A11. Two-position and three-position.
A12. A momentary switch.
A13. A locked-position switch.
A14. A microswitch.
A15. The maximum current a switch is designed to carry.
A16. The maximum voltage allowable in the circuit in which the switch is installed.
A17. An ohmmeter and a voltmeter.
A18. A voltmeter.
3-39
A19.
a. Not acceptable-single throw.
b. Not acceptable-double break.
c. Acceptable-choice #2 (different actuator). d. Not acceptable-single pole.
e. Not acceptable-no momentary position.
f. Acceptable-choice #1 (higher rating). g. Not acceptable-locked position incorrect.
h. Not acceptable-current rating too low.
i. Not acceptable-voltage rating too low.
A20. The switch operation for smooth and correct operation, the terminals for corrosion, and the physical condition of the switch.
A21. The magnetic field created in a coil of wire and core will attract a soft iron plunger when current flows through the coil.
A22. A starter motor and solenoid.
A23. The connections, the plunger, the mechanism that the solenoid actuates, the energizing voltage, and the coil of the solenoid.
A24. The magnetic field created in a coil of wire will attract aft armature causing a movement in sets of contacts.
A25. The solenoid provides a mechanical movement of a plunger (a moveable core) while the core of a relay is fixed.
A26. Control relays and power relays (contactors). A27. By observing the movement of the contacts if the relay is open or sealed with a transparent cover.
If the relay has an opaque cover, you can "feel" the operation of the relay by placing your finger on the cover.
A28. The coil should be checked for opens, shorts, or a short to ground; terminal leads should be checked for charred or burned insulation; the contact surfaces should be checked for film, carbon, arcing, and contact spacing.
A29. A burnishing tool.
A30. A point bender
AI-1
ACTUATOR—The part of a switch that is acted upon to cause the switch to change contact connections; e.g., toggle, pushbutton, and rocker.
AMMETER—A meter used to measure current.
ARC EXTINGUISHER—The part of a circuit breaker that confines and divides the arc that occurs when the contacts of the circuit breaker open.
ARMATURE—In a relay, the movable portion of the relay.
BREAK—In a switch, the number of breaks refers to the number of points at which the switch opens the circuit; e.g., single break and double break.
BURNISHING TOOL—A tool used to clean and polish contacts on a relay.
CONTINUITY—An uninterrupted, complete path for current.
DAMPING—The process of smoothing out oscillations. In a meter, damping is used to keep the pointer of the meter from overshooting the correct reading.
D’ARSONVAL METER MOVEMENT—A name used for the permanent-magnet moving-coil movement used in most meters.
DIRECT SHORT—A connection between two points in a circuit, such as between a component and ground.
ELECTRODYNAMIC METER MOVEMENT—A meter movement using fixed field coils and a moving coil; usually used in wattmeters.
ELECTROMAGNETISM—The relationship between magnetism and electricity.
ELECTROSTATIC METER MOVEMENT—A meter movement that uses the electrostatic repulsion of two sets of charged plates (one fixed and the other movable). This meter movement reacts to voltage rather than to current and is used to measure high voltage.
FERRULES—The cylindrical metallic ends of a cartridge fuse.
FREQUENCY METER—A meter used to measure the frequency of an ac signal. GALVANOMETER—A meter used to measure small values of current by electromagnetic or
electrodynamic means.
HOT WIRE METER MOVEMENT—A meter movement that uses the expansion of a heated wire to move the pointer of a meter; measures dc or ac.
IN-CIRCUIT METER—A meter permanently installed in a circuit; used to monitor circuit operation.
AI-2
LOADING EFFECT—The effect of a voltmeter upon the circuit being measured which results in an inaccurate measurement. Loading effect is minimized by using a voltmeter with an internal resistance many times higher than the resistance of the circuit being measured.
MAGNETIC TRIP ELEMENT—A circuit breaker trip element that uses the increasing magnetic attraction of a coil with increased current to open the circuit.
MEGGER—Common name for a megohmmeter.
MEGOHMMETER—A meter that measures very large values of resistance; usually used to check for insulation breakdown in wires.
METER—A device used to measure an electrical quantity; e.g., current, voltage, and frequency.
METER MOVEMENT—The part of a meter that moves.
MOVING-IRON METER MOVEMENT—Same as moving-vane meter movement.
MOVING-VANE METER MOVEMENT—A meter movement that uses the magnetic repulsion of the like poles created in iron vanes by current through a coil of wire; most commonly used movement for ac meters.
MULTIMETER—A single meter combining the functions of an ammeter, a voltmeter, and an ohmmeter.
NONTRIP-FREE CIRCUIT BREAKER—A circuit breaker that can be held ON during an overcurrent condition.
OHMMETER—A meter used to measure resistance.
OUT-OF-CIRCUIT METER—A meter which is not permanently installed in a circuit. Usually portable and self-contained, these meters are used to check the operation of a circuit or to isolate troubles within a circuit.
PARALLAX ERROR—The error in meter readings that results when you look at a meter from some position other than directly in line with the pointer and meter face. A mirror mounted on the meter face aids in eliminating parallax error.
POINT BENDER—A tool used to adjust the contact spacing on a relay. POLE—(1) One end of a magnet. (2) The number of points at which current can enter a switch; e.g.,
single pole, double pole, and three pole.
POLE PIECE—A piece of ferromagnetic material used to control the distribution of magnetic lines of force; i.e., concentrate the lines of force in a particular place or evenly distribute the lines of force over a wide area.
RANGES—The several upper limits a meter will measure as selectable by a switch or by jacks; e.g., a voltmeter may have ranges of 1 volt, 2.5 volts, 10 volts, 25 volts, and 100 volts.
RECTIFIER—A device used to convert ac to pulsating dc.
RELAY—An electromagnetic device with one or more sets of contacts which changes position by the magnetic attraction of a coil to an armature.
AI-3
RELUCTANCE—The resistance of a magnetic path to the flow of magnetic lines of force through it.
ROTARY SWITCH—A multicontact switch with contacts arranged in a circular or semi-circular manner.
SENSITIVITY—(1) For an ammeter: the amount of current that will cause full-scale deflection of the meter. (2) For a voltmeter: the ratio of the voltmeter resistance divided by the full-scale reading of the meter, expressed in ohms-per-volt.
SHORT CIRCUIT—An unintentional current path between two components in a circuit or between a component and ground which is usually caused by a malfunction in the circuit.
SHUNT RESISTOR—A resistor in parallel. In an ammeter, shunt resistors are used to provide range capability.
SNAP-ACTING—Changing position quickly with the aid of a spring.
SOLENOID—An electromagnetic device that changes electrical energy into mechanical motion; based upon the attraction of a movable iron plunger to the core of an electromagnet.
SWITCH—A device used to open or close a circuit.
TEST EQUIPMENT—A general term applied to devices used to test electrical and electronic circuits. THERMAL TRIP ELEMENT—A circuit breaker trip element that uses the increased bending of a
bimetallic strip caused by increased current to open a circuit.
THERMAL-MAGNETIC TRIP ELEMENT—A single circuit breaker trip element that combines the action of a thermal and a magnetic trip element.
THERMOCOUPLE METER MOVEMENT—A meter movement that uses the current induced in a thermocouple by the heating of a resistive element to measure the current in a circuit; used to measure ac or dc.
THROW—In a switch, the number of different circuits each pole can control; e.g., single throw and double throw.
TRIP-ELEMENT—The part of a circuit breaker that senses any overload condition and causes the circuit breaker to open the circuit.
TRIP-FREE CIRCUIT BREAKER—A circuit breaker that will open a circuit even if the operating 5 mechanism is held in the ON position.
TROUBLESHOOTING—The process of locating and repairing faults in electrical or electronic equipment.
VOLTMETER—A meter used to measure voltage.
WAFER SWITCH—A rotary switch in which the contacts are arranged on levels. Each level is electrically independent but mechanically connected by the shaft of the switch.
WATT-HOUR METER—A meter used to measure electrical energy.
WATTMETER—A meter used to measure electrical power.
AII-1
The International Symbols Committee has adopted prefixes for denoting decimal multiples of units. The National Bureau of Standards has followed the recommendations of this committee, and has adopted the following list of prefixes:
.1 10-1 deci d
.01 10-2 centi c
.001 10-3 milli m
.000001 10 -6 micro u
.000000001 10-9 nano n
.000000000001 10-12 Pico p
.000000000000001 10-15 femto f
.000000000000000001 10-18 Atto a
To multiply like (with same base) exponential quantities, add the exponents. In the language of algebra the rule is am · a n = am+n
To divide exponential quantities, subtract the exponents. In the language of algebra the rule is
AII-2
To raise an exponential quantity to a power, multiply the exponents. In the language of algebra (xm) n = xmn.
Any number (except zero) raised to the zero power is one. In the language of algebra x0 = 1
Any base with a negative exponent is equal to 1 divided by the base with an equal positive exponent. In the language of algebra x-a = 1/xa
To raise a product to a power, raise each factor of the product to that power.
To find the nth root of an exponential quantity, divide the exponent by the index of the root. Thus, the nth root of am = am.
AIV-1
INDEX-1
INDEX-2
INDEX-3
Information: The text pages that you are to study are provided at the beginning of the assignment questions.
1
Textbook assignment: Chapter 1, “Circuit Protection Devices,” pages 1-1 through 1-73. ___________________________________________________________________________________
1-1. Circuit measurement is used for which of the following purposes?
1. To find the weight of a circuit 2. To increase the power used in a circuit 3. To discover the length and width of a
circuit 4. To determine the reason a circuit is not
1-2. An in-circuit meter is used for which of the following purposes?
1. To reduce circuit losses 2. To monitor circuit operation 3. To control power to a circuit 4. To prevent circuit overload conditions
1-3. Out-of-circuit meters have which of the following advantages over in circuit meters?
1. They can be used on more than one
device 2. They are lighter weight 3. They are more rugged 4. All of the above
Figure 1-A.—Compass and wire (dc).
IN ANSWERING QUESTIONS 1-4 THROUGH 1-6, REFER TO FIGURE 1-A.
1-4. When S1 is closed, the compass needle will align itself in which of the following manners?
1. With magnetic north 2. With geographic north 3. Parallel to the conductor 4. With the magnetic field around the
1-5. If the current through the conductor is decreased, what happens to the magnetic field around the conductor?
1. It reverses 2. It decreases 3. It increases 4. It oscillates
1-6. When the current through the conductor
decreases, the compass needle will react in which of the following manners?
1. Point more to magnetic north 2. Move farther away from magnetic
north 3. Swing back and forth from east to
west 4. Vibrate rapidly back and forth around
1-7. The d'Arsonval meter movement is based on which of the following principles?
1. Moving vane 2. Electrostatic 3. Electrodynamic 4. Permanent-magnet moving-coil
2
1-8. Current through a meter results in the pointer. In d'Arsonval meter movement, what force produces this deflection?
1. Thermocouple action 2. Electrostatic repulsion 3. Mechanical spring tension 4. The interaction of magnetic fields
1-9. The hairsprings in a d'Arsonval meter movement perform which of the following functions?
1. They keep the pointer in the position
2. They aid the movement of the pointer when there is current through the meter
3. They make electrical connections to the meter movement
4. All of the above
Figure 1-B.Compass and wire (ac).
IN ANSWERING QUESTIONS 1-10 AND 1-11, REFER TO FIGURE 1-B.
1-10. If the frequency of the ac source is 5 Hz, how will the compass needle react when S1 is closed?
1. It will swing back and forth 2. It will point directly at the wire 3. It will point directly away from the
wire 4. It will vibrate rapidly around magnetic
1-11. If the frequency of the ac source is 200 Hz, how will the compass needle react when S1 is closed?
1. It will swing back and forth 2. It will point directly at the wire 3. It will point directly away from the
wire 4. It will vibrate rapidly around magnetic
1-12. What device allows a d'Arsonval meter movement to measure ac by converting ac to pulsating dc?
1. A pulsator 2. A modulator 3. A rectifier 4. A converter
1-13. What is meant by the term "meter damping"?
1. Moistening the felt pads 2. Smoothing the oscillations of the
pointer 3. Preventing excessive current through
the coil 4. Compensating for electromagnetic
1-14. Which of the following methods is used to dampen a meter?
1. Mount the meter in a mu-metal case 2. Install a fuse in one of the input leads 3. Incorporate an airtight chamber
containing a van 4. Provide a fluid reservoir and sponge
3
1-15. A d'Arsonval meter movement reacts to which of the following values of voltage?
1. Peak 2. Average 3. Effective 4. Peak-to-peak
1-16. What value of ac is indicated by a meter
scale?
1. Peak 2. Average 3. Effective 4. Peak-to-peak
1-17. Which of the following meter movements will measure either ac or dc without the use of a rectifier?
1. GMS 2. d'Arsonval 3. Electrostatic 4. Electrodynamic
1-18. What electrical property is reacted to by the electrodynamic, d'Arsonval, moving- vane, and thermocouple meter movements?
1. Power 2. Current 3. Voltage 4. Resistance
1-19. What electrical property is measured by an ammeter?
1. Power 2. Current 3. Voltage 4. Resistance
1-20. How are ammeters connected in an
electrical circuit?
1. In series with the load 2. In parallel with the load 3. In accordance with Lenz's Law 4. In series-parallel with the load
1-21. How does an ammeter affect the circuit being measured?
1. It acts as a resistances in series and
lowers the circuit current 2. It acts as a resistance in series and
raises the circuit current 3. It acts as a resistance in parallel and
lowers the circuit current 4. It acts as a resistance in parallel and
1-22. How is the effect that an ammeter produces in a circuit kept to a minimum?
1. By using a large resistor in series with
the ammeter 2. By using a large capacitor in parallel
with the ammeter 3. By ensuring that the meter resistance
is low compared to circuit resistance 4. By ensuring that the meter resistance
is high compared to circuit resistance 1-23. The ammeter with the greatest sensitivity
has which of the following characteristics?
1. The lowest amount of current for full- scale deflection indication
2. The highest amount of current for full- scale deflection indication
3. A low ratio of internal resistance to full-scale deflection indication
4. A high ratio of internal resistance to full-scale deflection indication
1-24. Ammeters measure various ranges through
the addition of which of the following components?
1. Shunt resistors in series with the meter
movement 2. Shunt resistors in parallel with the
meter movement 3. Capacitors in series with the meter
movement 4. Capacitors in parallel with the meter
4
1-25. What range of an ammeter should you use for an initial measurement?
1. The lowest range 2. The highest range 3. The mid-scale range
1-26. What portion of the ammeter scale should be used to take a final reading?
1. The upper half 2. The lower half 3. The mid-scale portion 4. Anywhere on the meter face
1-27. When, if ever, can you use a dc ammeter
to measure ac values?
1. When the ac is high frequency 2. For low values 3. Always 4. Never
1-28. Which of the following safety precautions should be observed prior to connecting an ammeter into a circuit?
1. Switch to the highest range 2. Observe proper dc polarity 3. Deenergize the circuit 4. All of the above
1-29. What electrical property is measured by a voltmeter?
1. Power 2. Current 3. Voltage 4. Resistance
1-30. A voltmeter should be connected in an electrical circuit in what manner?
1. In series with the load 2. In parallel with the load 3. In accordance with Lenz's Law 4. In series-parallel with the load
1-31. A voltmeter has an effect on the circuit being measured; what is this effect called?
1. Loading 2. Damping 3. Rectification 4. Eddy-current drag
1-32. To keep the effect of a voltmeter on a circuit to a minimum, the internal resistance of the voltmeter must have which of the following relationships to the circuit load?
1. Equal to 2. Lower than 3. Higher than 4. In proportion to
1-33. Which of the following types of meters can be made from a current sensitive meter movement?
1. Ammeter 2. Ohmmeter 3. Voltmeter 4. Each of the above
1-34. A voltmeter has a high sensitivity when it has which of the following characteristics?
1. Low deflection indication 2. High deflection indication 3. Low ratio of internal resistance to full-
scale deflection indication 4. High ratio of internal resistance to
1-35. Which of the following configurations extends the range of a voltmeter?
1. A resistor in series with the meter
movement 2. A resistor in parallel with the meter
movement 3. A capacitor in series with the meter
movement 4. A capacitor in parallel with the meter
5
1-36. What voltmeter range should be used for initial measurements?
1. The lowest 2. The highest 3. The mid-scale
1-37. The electrostatic meter movement reacts to which of the following electrical properties?
1. Power 2. Current 3. Voltage 4. Resistance
1-38. Electrostatic meter movements are used to measure which of the following current/voltage values?
1. Low voltage 2. Low current 3. High voltage 4. High current
1-39. Which of the following safety precautions should be observed when a voltmeter is used?
1. Deenergize the circuit before
connecting the meter 2. Start with the lowest range of the
meter 3. Connect the meter in series with the
circuit 4. All of the above
1-40. What electrical property is measured with
an ohmmeter?
1. Power 2. Current 3. Voltage 4. Resistance
1-41. An ohmmeter is used to check for which of the following conditions?
1. Continuity 2. Overheating 3. Overcurrent 4. Undercurrent
1-42. How should an ohmmeter be connected in an electrical circuit?
1. In series with the load 2. In parallel with the load 3. In parallel with the source 4. In series-parallel with the load
1-43. An ohmmeter can measure different ranges because of the use of which of the following components?
1. Range coils 2. Range resistors 3. Range capacitors 4. Range potentiometers
1-44. What area of an ohmmeter scale should be used when a measurement is taken?
1. Upper half 2. Lower half 3. Mid-scale portion 4. Anywhere on the meter face
1-45. Ohmmeter are classified by type. What are the two types of ohmmeters?
1. Series and shunt 2. Normal and reverse 3. Full- and half-scale
1-46. What is the most obvious differences in the two types of ohmmeters?
1. The ranges of the meters 2. The scales of the meters 3. The power sources of the meters 4. The size of the test leads of the meters
6
1-47. Which of the following safety precautions should be observed when an ohmmeter is used?
1. Always start with the highest scale of
the meter 2. Deenergize the circuit before
connecting the meter 3. Observe proper polarity 4. All of the above
1-48. Meggers (megohmmeters) are used to measure which of the following quantities?
1. Low voltage 2. High voltage 3. Low resistance 4. High resistance
1-49. When a megger is used to check the
insulation of a wire, which of the following indications should be considered normal?
1. ∞ 2. 0 3. 500 V 4. 1000 V
1-50. Which of the following safety precautions should be observed when a megger is used?
1. Do not use a dc megger to measure
circuits that are powered by ac 2. Always start with the highest scale
selection of the meter 3. Do not touch the meter leads when a
measurement is being taken 4. All of the above
1-51. A multimeter can be used to measure which of the following electrical properties?
1. Voltage 2. Current 3. Resistance 4. Each of the above
1-52. The function switch on a multimeter does NOT perform which of the following functions?
1. Selection of the meter range 2. Determination of the proper scale 3. Selection of ac or dc capability 4. Changing of the multimeter from an
1-53. One of the problems encountered in building a multimeter is that the meter movement gives different readings for the same values of ac and dc. Which of the following features of a multimeter will solve this problem?
1. A rectifier 2. An ac/dc switch 3. Separate scales for ac and dc 4. A mirror on the face of the meter
1-54. Why is there a mirror on the face of a multimeter?
1. To illuminate the meter face 2. To aid in reducing parallax error 3. To reduce the friction between the
pointer and the meter face 4. To compensate for the difference in ac
1-55. If the mirror on the face of a multimeter is used properly, where will the image of the pointer appear?
1. Hidden behind the pointer 2. Barely visible on either side of the
pointer 3. Clearly visible to the left of the pointer 4. Clearly visible to the right of the
7
1-56. Which of the following safety precautions does NOT apply to a multimeter?
1. Observe proper dc polarity when
measuring dc 2. Deenergize the circuit before
connecting the meter 3. Be sure the meter is switched to ac for
ac measurements 4. Never apply power to the circuit when
1-57. If a multimeter has no OFF position, and it is returned to storage, on which of the following positions should the meter be set?
1. +dc; highest voltage range 2. −dc; higher resistance range 3. Ac; highest voltage range 4. Ac; highest current range
1-58. When the current in a conductor is measured without the conductor being disconnected, which of the following meters could be used?
1. Multimeter 2. Hook-on voltameter 3. Induction wattmeter 4. Transformer voltmeter
1-59. Which of the following electrical quantities is measured by a wattmeter?
1. Power 2. Energy 3. Voltage 4. Current
1-60. Which of the following electrical
quantities is measured by a watt hour meter?
1. Power 2. Energy 3. Voltage 4. Current
8
Textbook assignment: Chapter 2, “Circuit Protection Devices,” pages 2-1 through 2-42. _________________________________________________________________________________
2-1. Circuit protection devices are used for which of the following purposes?
1. To protect people 2. To protect circuits 3. To guard against hazardous
conditions 4. All of the above
2-2. Which of the following conditions does NOT require the use of a circuit protection device?
1. Direct short 2. High resistance 3. Excessive current 4. Abnormal heating
2-3. When a point in a circuit, where full system voltage is present, comes in direct contact with the ground or return side of the circuit, which of the following terms applies?
1. Direct short 2. High resistance 3. Excessive current 4. Abnormal heating
2-4. When circuit current increases beyond the designed current carrying capability of the circuit, which of the following terms applies?
1. Direct short 2. High resistance 3. Excessive current 4. Abnormal heating
2-5. If the bearings of a generator were to fail, which of the following circuit conditions would probably occur?
1. Direct short 2. High resistance 3. Excessive current 4. Abnormal heating
2-6. How are circuit protection devices connected to the circuit they are protecting?
1. Alongside 2. In series 3. In parallel 4. In series-parallel
2-7. Which of the following two are circuit protection devices?
1. Electrical plugs and CO2 cartridges 2. CO2 cartridges and circuit breakers 3. Fuses and circuit breakers 4. Fuses and electrical plugs
THIS SPACE LEFT BLANK INTENTIONALLY.
9
Figure 2-A.—Recognition practice.
IN ANSWERING QUESTIONS 2-8 THROUGH 2-13, REFER TO FIGURE 2-A.
2-8. Which of the following is the schematic symbol for a fuse? 1. A 2. B 3. F 4. G
2-9. Which of the following is the schematic symbol for a circuit breaker?
1. A 2. B 3. F 4. G
2-10. Which of the following is an illustration
of an open cartridge fuse?
1. C 2. D 3. E 4. I
2-11. Which of the following is an illustration of a good cartridge fuse?
1. C 2. D 3. E 4. I
2-12. Which of the following is an illustration of a good plug-type fuse?
1. C 2. D 3. E 4. H
2-13. Which of the following is an illustration of an open plug-type fuse?
1. C 2. D 3. E 4. H
2-14. Which of the following factors is NOT used to rate fuses?
1. Size 2. Current 3. Voltage 4. Time delay
2-15. A fuse current rating has which of the following definitions?
1. The maximum current that can flow
2. The maximum current that will flow through a circuit if there is a direct short
3. The maximum current that will flow through a fuse without opening the fuse
4. The maximum current that will not "jump" an open fuse
10
2-16. A fuse voltage rating has which of the following definitions?
1. The maximum voltage that can exist
2. The maximum voltage that can exist in a circuit if there is a direct short
3. The maximum voltage across a fuse that will not cause the fuse to open
4. The maximum voltage across a fuse that will not jump the open fuse
IN ANSWERING QUESTIONS 2-17 THROUGH 2-19, MATCH THE TIME- DELAY RATING LISTED IN COLUMN B TO THE ELECTRICAL DEVICE LISTED IN COLUMN A. NOT ALL ITEMS IN COLUMN B WILL BE USED. A. ELECTRICAL
B. TIME-DELAY RATINGS
2-17. Electric motor
1. Fast
2-18. Lighting circuit 2. Delay
2-19. Meter Movement 3. Standard
4. Intermediate _______________________________________
2-20. What is the voltage rating for a fuse with
the designation F03D1R00B?
1. 32 V or less 2. 125 V or less 3. 250 V or less 4. 500 V or less
2-21. What is the current rating for a fuse with the designation F03B0R50B?
1. 1/2 amp 2. 1.5 amp 3. 3 amp 4. 50 amp
2-22. What is the time-delay rating for a fuse with the designation F03A20R0C?
1. Fast 2. Delay 3. Standard 4. Intermediate
2-23. What is the voltage rating for a fuse with
the designation F02B250V10AS?
1. 10 V or less 2. 32 V or less 3. 52 V or less 4. 250 V or less
2-24. What is the current rating for a fuse with the designation F03A125V5A?
1. 125 amp 2. 5 amp 3. 3 amp 4. 1/8 amp
2-25. What is the time-delay rating for a fuse with the designation F04C125V2AS?
1. Fast 2. Delay 3. Standard 4. Intermediate
2-26. What is the voltage rating for a fuse with the designation 3AG20125V?
1. 20 V or less 2. 90 V or less 3. 125 V or less 4. 250 V or less
2-27. What is the current rating for a fuse with the designation 3AG1032V?
1. 1 amp 2. 2 amp 3. 3 amp 4. 10 amp
11
2-28. What is the voltage rating for a fuse with the designation AGC5125V?
1. 12 V or less 2. 25 V or less 3. 51 V or less 4. 125 V or less
2-29. What is the current rating for a fuse with the designation AGC2125V?
1. 1 amp 2. 2 amp 3. 3 amp 4. 25 amp
2-30. What is the new military designation for a fuse with the old, military designation F03D1R50B?
1. F03A125V1.5A 2. F02B125V1.5A 3. F03A250V11/2A 4. F03B125V1.5A
Figure 2-B.—Fuseholder identification.
IN ANSWERING QUESTIONS 2-31 THROUGH 2-34, REFER TO FIGURE 2-B.
2-31. What type of fuseholder is shown in
figure 2-B(A)?
1. Clip 2. Post 3. Bayonet 4. Spring-loaded
2-32. What type of fuseholder is shown in figure 2-B(B)?
1. Clip 2. Post 3. Bayonet 4. Spring-loaded
2-33. When you use the fuseholder shown in figure 2-B(B), which connector should be connected to the power source?
1. Ground 2. Center 3. Inside 4. Outside
2-34. When you use the fuseholder shown in figure 2-B(B), which connector should be connected to the load?
1. Ground 2. Center 3. Inside 4. Outside
2-35. Which of the following methods will
provide an ABSOLUTE determination as to whether or not a fuse is open?
1. A visual inspection 2. A check of the fuse indicator 3. A voltmeter check of the fuse 4. A thermometer check of the
2-36. A fuse is removed from a circuit, checked with an ohmmeter, and found to be shorted. What action should be taken?
1. Discard the fuse 2. Check the fuse with a voltmeter 3. Put the fuse back in the circuit 4. Return the fuse to the supply
2-37. Which of the following methods should be used to check a .002 ampere fuse?
1. Use a megger and place a capacitor
in parallel with the fuse 2. Use a megger and place a capacitor
in series with the fuse 3. Use an ohmmeter and place a resistor
in parallel with the fuse 4. Use an ohmmeter and place a resistor
12
2-38. What should you use to remove a fuse from a clip-type fuseholder?
1. A scribe 2. A fusepuller 3. A screwdriver 4. A pair of pliers
2-39. Which of the following is a safety
precaution to be observed when a fuse is checked?
1. Turn the power off and discharge the
circuit before the fuse is removed 2. When you check a fuse with an
3. When you use a voltmeter to check a low current fuse, be careful to avoid opening the fuse by excessive current from the voltmeter
4. All of the above
YOU HAVE FOUND AN OPEN FUSE IN A PIECE OF EQUIPMENT AND HAVE REPAIRED THE CASUALTY. THE TECHNICAL MANUAL FOR THE EQUIPMENT SPECIFIES A FUSE CODED F02A125V3A. NO FUSES WITH THAT DESIGNATION ARE AVAILABLE. THE FOLLOWING FUSES ARE CARRIED BY THE SUPPLY SYSTEM. A. F03D3R00A B. 3AG3250V C. F02A3R00B D. AGC3125V E. F02D3R00C F. AGC5250V
IN ANSWERING QUESTIONS 2-40 THROUGH 2-46, USE THE INFORMATION IN FIGURE 2-C.
2-40. What fuse is a direct replacement?
1. A 2. C 3. D 4. E
2-41. What fuse is the best substitute?
1. A 2. B 3. C 4. F
2-42. What fuse is the second best in the fuseholder substitute?
1. A 2. C 3. E 4. F
2-43. What fuse is unacceptable because the physical size is incorrect?
1. A 2. C 3. E 4. F
2-44. What fuse is unacceptable because of the current rating?
1. B 2. D 3. E 4. F
2-45. What fuse is unacceptable because of the voltage rating?
1. A 2. B 3. C 4. D
13
2-46. What fuse is unacceptable because of the time-delay rating?
1. A 2. C 3. D 4. E
2-47. Before replacing a fuse, you should check for which of the following conditions?
1. Proper fit 2. Proper fuse 3. Both 1 and 2 above 4. Proper input voltage
2-48. Which of the following is NOT a safety precaution to be observed when a fuse is changed?
1. Be sure to "tag out" the fuseholder
when you remove the fuse 2. Remove the power from a circuit
before removing and replacing a fuse 3. Remove any corrosion from the
fuseholder before replacing a fuse 4. Be sure the fuse fits properly in the
2-49. When you perform preventive maintenance on fuses, which of the following is NOT a condition you should check?
1. Corrosion 2. Shorted fuse 3. Improper fit 4. Improper fuse
2-50. What is the total number of main
components in a circuit breaker?
1. Five 2. Two 3. Three 4. Four
2-51. Which of the following is NOT a type of trip element for a circuit breaker? 1. Thermal 2. Magnetic 3. Mechanical 4. Thermal-magnetic
THIS SPACE LEFT BLANK INTENTIONALLY.
14
IN ANSWERING QUESTIONS 2-52 THROUGH 2-54, SELECT FROM COLUMN B THE TRIP ELEMENT THAT IS DESCRIBED BY THE ACTION LISTED IN COLUMN A. NOT ALL ITEMS IN COLUMN B WILL BE USED.
A. ACTIONS
B. TRIP ELEMENTS
2-52. An electromagnet is connected in series with the load
1. Thermal
2-53. A bimetallic strip is heated by the load current
2. Magnetic
2-54. A bimetallic strip is heated by the load current and electromagnet is connected in series with the load
3. Mechanical
4. Thermal-magnetic _______________________________________
2-55. A circuit breaker that will trip even if the