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Why are circuit protection devices necessary?
To protect people and circuits from possible hazardous conditions.
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What are the three conditions that require circuit protection?
A direct short, excessive current, and excessive heat.
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What is a direct short?
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.
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What is an excessive current condition?
A condition that is not a direct short but in which circuit current increases beyond the designed current carrying ability of the circuit.
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What is an excessive heat condition?
A condition in which the heat in or around the circuit increases to a higher than normal level
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How are circuit protection devices connected to the circuit they are intended to protect and why are they connected in this way?
In series, so total current will be stopped when the device opens.
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What are the two types of circuit protection devices?
Fuses and circuit breakers.
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In what three ways are fuses rated?
Current, voltage, and time delay.
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What does the current rating of a fuse indicate?
The amount of current the fuse will allow without opening.
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What does the voltage rating of a fuse indicate?
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.
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What are the three time delay ratings of fuses?
Delay, standard, and fast.
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Give an example of a device you could protect with each type of time delay fuse.
Delay-Motors, solenoids, or transformers. Standard-Automobiles, lighting or electrical power circuits. Fast-Delicate instruments or semiconductor devices.
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What are the voltage, current, and time delay ratings for a fuse with the designation (a.) F02DIR50B (b.) F02A250V1/8A?
- (a.) 125 volts or less, 1.5 amperes, delay
- (b.) 250 volts or less, 1/8 ampere, standard
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What are the voltage and current ratings for a fuse designated (a.) 3AG1/16125V (b.) FNA15/100250V?
- (a.) 125 volts or less, 1/16 ampere
- (b.) 250 volts or less, .15 ampere
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What is the new military designation for a fuse with the old military designation F05A20ROB?
F05B32V20A.
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What are three methods for determining if a fuse is open?
- Visual inspection
- Indicators
- Using a meter.
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You have just checked a fuse with an ohmmeter and find that the fuse is shorted. What should you do?
Put it back in the circuit. A good fuse will have zero ohms of resistance.
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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?
The ohmmeter causes more than 1/500 ampere through the fuse when you check the fuse, thus it opens the fuse.
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How could you check a 1/500-ampere fuse with an ohmmeter?
Use a resistor in series with the fuse when you check it with the ohmmeter.
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List the safety precautions to be observed when checking fuses.
- 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.
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A fuse is coded F02B124V1/2A, what would be the best replacement? (a) F03BI25V�A (b) F02BI25V3/8A (c) F02GR500B (d) F02B32V�A (e) F02DR500B (f) F02A250V5/8A (g) F02AI25V1/2A
- (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
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What two things should you check before replacing a fuse?
Check for the proper type of replacement fuse and proper fit.
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List the safety precautions to be observed when replacing a fuse.
- 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.
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What conditions should you check for when conducting preventive maintenance on fuses?
Improper fuse, corrosion, improper fit, and open fuse.
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What are the five main components of a circuit breaker?
- Frame
- Operating mechanism
- Arc extinguishers
- Terminal connectors
- Trip element.
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What are the three types of circuit breaker trip elements?
- Thermal
- Magnetic
- Thermal-magnetic.
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How does each type of trip element react to an overload?
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.
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What is a trip-free circuit breaker?
A circuit breaker that will trip even if the operating mechanism is held ON.
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What is a nontrip-free circuit breaker?
A circuit breaker that can be overridden if the operating mechanism is held ON.
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Where should you use a trip-free circuit breaker?
In current sensitive or nonemergency systems.
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Where should you use a nontrip-free circuit breaker?
In emergency or essential circuits.
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What are the three time delay ratings for circuit breakers?
- Instantaneous
- Short time delay
- Long time delay.
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What is selective tripping and why is it used?
- 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.
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What factors are used to select a circuit breaker?
The power requirements of the circuit and the physical space available.
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What type of circuit breaker is used on a multimeter?
A push button or push-pull circuit breaker (small size, low power).
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What steps are to be taken before beginning work on a 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.
- Tag the switch that supplies power to the circuit breaker.
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What items are you to check when working on a circuit breaker?
- 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
- Checkall components for wear.
- Check the entire circuit breaker for cleanliness.
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