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Module 3 - Introduction to Circuit Protection, Control, and Measurement
Navy Electricity and Electronics Training Series (NEETS)
Chapter 2:  Pages 2-11 through 2-20

Module 3 -- Introduction to Circuit Protection, Control, and Measurement

Pages i, 1-1, 1-11, 1-21, 1-31, 1-41, 1-51, 1-61, 1-71, 2-1, 2-11, 1-21, 2-31, 2-41, 3-1, 3-11, 3-21, 3-31, AI-1, AII-1, AIII-1, IV-1, Index

NEETS Modules
- Matter, Energy, and Direct Current
- Alternating Current and Transformers
- Circuit Protection, Control, and Measurement
- Electrical Conductors, Wiring Techniques, and Schematic Reading
- Generators and Motors
- Electronic Emission, Tubes, and Power Supplies
- Solid-State Devices and Power Supplies
- Amplifiers
- Wave-Generation and Wave-Shaping Circuits
- 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
Note: Navy Electricity and Electronics Training Series (NEETS) content is U.S. Navy property in the public domain.

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.

New type military fuse designation - RF Cafe

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 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.

Commercial designations for fuses - RF Cafe

Figure 2-10. - Commercial designations for fuses


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).


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

        Fuse equations

Q17. What are the voltage and current ratings for a fuse designated

       Fuse designations

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.


Typical fuse holders - RF Cafe

Figure 2-11. - Typical fuse holders.


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 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)?

Fuse holder identification - RF Cafe

Figure 2-12. - Fuse holder identification.


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 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 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 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

open-fuse position.


Open fuse indicators: Clip-type fuse holder with an indicating lamp - RF Cafe

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 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).


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.



Safety Precautions When Checking a Fuse

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 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.


• 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 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 point.

Contact between clips and fuses - RF Cafe

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 - RF Cafe

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.

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.

     (a) F03BI25V½A

     (b) F02BI25V⅜

     (c) F02GR500B

     (d) F02B32V½A

     (e) F02DR500B

     (f) F02A250V⅝

     (g) F02AI25V½A

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.


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