Module 3—Introduction to Circuit Protection, Control, and Measurement
i - ix
, 1-1 to 1-10
1-11 to 1-20
, 1-21 to 1-30
1-31 to 1-40
, 1-41 to 1-50
1-51 to 1-60
, 1-61 to 1-70
1-71 to 1-73
2-1 to 2-10
, 2-11 to 2-20
1-21 to 2-30
, 2-31 to 2-40
2-41 to 2-42
, 3-1 to 3-10
3-11 to 3-20
, 3-21 to 3-30
33-31 to 3-39
AI-1 to AI-3
,, AII-1 to AII-2
AIII-1 to AIII-10
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.
NEW MILITARY DESIGNATION
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.
Figure 2-9.—New type military fuse designation.
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.
As you can see, the new military designation is much easier to understand than the old military
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.—Commercial designations for fuses
OLD COMMERCIAL DESIGNATION
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 x 1 1/4 inches
(6.35 millimeters x 31.8 millimeters) and has a standard time delay rating.
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
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?
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 fuse holderS. A fuse holder is a device that is
wired into the circuit and allows easy replacement of the fuse.
Fuse holders are made in many shapes and
sizes, but most fuse holders are basically either clip-type or post-type. Figure 2-11 shows a typical clip-type
and post-type fuse holder.
Figure 2-11.—Typical fuse holders.
CLIP-TYPE FUSE HOLDER
The clip-type fuse holder 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 fuse holders 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 fuse holder 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 FUSE HOLDER
Post-type fuse holders are made for cartridge fuses. The post-type fuse
holder is much safer because the fuse and fuse connections are covered with insulating material. The disadvantage
of the post-type fuse holder is that the fuse must be removed to visually check for an open. The post-type fuse
holder has a cap that screws onto the body of the fuse holder. 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 fuse holder. When the cap and
fuse are removed from the body of the fuse holder, the fuse is removed from the circuit and there is no danger of
shock or short circuit from touching the fuse.
Post-type fuse holders are usually mounted on the chassis of
the equipment in which they are used. After wires are connected to the fuse holder, insulating sleeves are placed
over the connections to reduce the possibility of a short circuit. Notice the two connections on the post-type
fuse holder 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 fuse holder 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.
Q19. Label the fuse holders in figure 2-12.
Q20. Which connector should you use to connect
the (a) power source and (b) load to the fuse holder shown in figure 2-12(A)?
Figure 2-12.—Fuse holder identification.
CHECKING AND REPLACEMENT OF FUSES
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
Before you look for the cause of an open fuse, you must be able to determine if the fuse is open.
CHECKING FOR AN OPEN FUSE
There are several ways of checking for an open fuse. Some
fuses and fuse holders 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
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.
Some fuses and fuse holders 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
Figure 2-13.—Open fuse indicators: Clip-type fuse holder with an indicating lamp.
Figure 2-13, view B, shows a plug-type fuse holder 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 fuse holder with an indicating
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.
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).
Remember there is no voltage drop across a straight piece of wire. Some plug-type fuse holders
have test points built in to allow you to check the voltage. To check for voltage on a clip-type fuse holder,
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.
PERSONNEL MAY BE EXPOSED TO HAZARDOUS VOLTAGE
Safety Precautions When Checking
Since a fuse has current through it, you must be very careful when checking for an open
fuse to 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 fuse holder.
• 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. REPLACEMENT OF
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
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.
• The best substitution fuse is a fuse with the same current and time delay ratings and a higher
• 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 fuse holder. If the fuse holder 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 fuse holder. Be certain the replacement fuse holder 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 fuse holder. When you insert the fuse in
the cap of a plug-type fuse holder, the fuse should fit tightly. A small amount of pressure should be needed to
insert the fuse and cap into the fuse holder body.
In clip-type fuse holders, 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 fuse holders used with knife blade 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 knife blade 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
Figure 2-14.—Contact between clips and fuses.
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 fuse holder or use clip clamps. Clip clamps are shown in
Figure 2-15.—Clip clamps.
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 fuse holder before replacing the fuse.
• Be certain the fuse properly fits the fuse holder.
PREVENTIVE MAINTENANCE OF FUSES
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 fuse holder terminals or the fuse itself. If corrosion is present, remove
it with fine sandpaper.
3. IMPROPER FIT. Check for contact between the fuse and fuse holder. If a piece
of paper will fit between the fuse and the clips on a clip-type fuse holder, there is improper contact. If the
fuse is not held in the cap of a plug-type fuse holder, 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.
Q26. You have removed an open fuse from a fuse holder and repaired the cause of the fuse opening.
The parts list specifies a fuse coded F02BI25VñA. There are no fuses available with that identification.
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. By manually opening and closing the contacts of a
circuit breaker, you can switch the power on and off. 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.
Introduction to Matter, Energy, and Direct Current, Introduction
to Alternating Current and Transformers, Introduction to Circuit Protection,
Control, and Measurement, Introduction to Electrical Conductors, Wiring Techniques,
and Schematic Reading, Introduction to Generators and Motors,
Introduction to Electronic Emission, Tubes, and Power Supplies,
Introduction to Solid-State Devices and Power Supplies,
Introduction to Amplifiers, Introduction to
Wave-Generation and Wave-Shaping Circuits, Introduction to Wave Propagation, Transmission
Lines, and Antennas, Microwave Principles,
Modulation Principles, Introduction to Number Systems and Logic Circuits, Introduction
to Microelectronics, Principles of Synchros, Servos, and Gyros,
Introduction to Test Equipment, Radio-Frequency
Communications Principles, Radar Principles, The Technician's Handbook,
Master Glossary, Test Methods and Practices, Introduction to Digital Computers,
Magnetic Recording, Introduction to Fiber Optics