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Protection: fuses, breakers, relays: board exam questions with solutions

On the REE exam under Electrical Engineering Professional Subjects and the RME exam under Technical Subjects. Try each question before you open its solution.

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Fuses and fault current · House panel: main breaker and four branch circuits · RCD protection: someone touches a live wire: the same circuits on a live bench, with meters you can move.

Questions

1. Why are time-delay (dual-element) fuses commonly used in motor branch circuits?

  1. They ride through motor starting inrush without blowing but still open on sustained overload or short circuit
  2. They open faster than ordinary fuses on every overcurrent
  3. They limit the motor's starting current
  4. They can be reset after operating
Show the answer and solution

Answer: A. They ride through motor starting inrush without blowing but still open on sustained overload or short circuit

The time-delay element tolerates the brief starting inrush (several times full-load current), while the short-circuit element still clears faults quickly.

2. What is the operating principle of a differential relay used to protect a transformer or generator?

  1. It measures the ratio of voltage to current at the relay point
  2. It compares the currents entering and leaving the protected zone and trips when they differ by more than a set amount
  3. It responds only to the magnitude of current in one line
  4. It trips on reverse power flow
Show the answer and solution

Answer: B. It compares the currents entering and leaving the protected zone and trips when they differ by more than a set amount

For an external fault or normal load the in and out currents balance. An internal fault produces a difference current that operates the relay.

3. An overcurrent relay rated 5 A is fed from a 300/5 A CT and set at a plug setting of 125%. What is the plug setting multiplier for a fault current of 3000 A?

  1. 10.0
  2. 6.4
  3. 8.0
  4. 12.5
Show the answer and solution

Answer: C. 8.0

Primary pickup = 300 × 1.25 = 375 A. PSM = fault current/pickup = 3000/375 = 8.0.

4. In a selectively coordinated system, a short circuit occurs on a branch circuit. Which device should open?

  1. The main breaker
  2. The feeder breaker and the branch breaker together
  3. All devices in series with the fault
  4. Only the branch-circuit device nearest the fault, on its line side
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Answer: D. Only the branch-circuit device nearest the fault, on its line side

Selective coordination confines the outage to the faulted circuit: the nearest upstream device clears the fault while the feeder and main stay closed.

5. During an insulation test of a motor winding, the insulation resistance is 1200 MΩ after 1 minute and 2400 MΩ after 10 minutes. What is the polarization index?

  1. 2.0
  2. 0.5
  3. 1200.0
  4. 3600.0
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Answer: A. 2.0

PI = R(10 min)/R(1 min) = 2400/1200 = 2.0. A low PI (near 1) suggests moisture or contamination.

6. A distance (impedance) relay operates on which measured quantity?

  1. The sum of the three line currents
  2. The ratio of voltage to current at the relay location
  3. The rate of change of frequency
  4. The difference between two CT currents
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Answer: B. The ratio of voltage to current at the relay location

An impedance relay measures V/I. Since line impedance is proportional to length, this tells the relay how far away the fault is.

7. An IDMT overcurrent relay has the standard-inverse characteristic t = 0.14 × TMS/(PSM^0.02 − 1). What is its operating time at a plug setting multiplier of 8 with a time multiplier setting of 0.2?

  1. 3.30 s
  2. 0.0040 s
  3. 0.66 s
  4. 0.027 s
Show the answer and solution

Answer: C. 0.66 s

8^0.02 = 1.04247, so t = 0.14 × 0.2/(1.04247 − 1) = 0.028/0.04247 = 0.66 s.

8. The available short-circuit current at a panelboard's line terminals is 18 kA. Which circuit-breaker interrupting rating is acceptable for breakers in that panel?

  1. 10 kA
  2. 14 kA
  3. 5 kA
  4. 22 kA
Show the answer and solution

Answer: D. 22 kA

A breaker's interrupting rating must be at least the available fault current at its line terminals. Only the 22 kA rating is ≥ 18 kA.

Other topics in the same subjects

DC circuits · AC circuits · Three-phase systems · Transients (RC, RL, RLC) · Semiconductors and diodes · Power electronics · Instrumentation and measurement · Control systems · Transformers · DC machines · Induction motors · Synchronous machines · Power systems and transmission · Illumination · Power plant engineering · PEC: branch circuits, ampacity and protection · Tools and test equipment · Motors, starters and control circuits · RA 7920 and ethics (electrical)

An independent, free reviewer: not affiliated with the PRC or the Boards. Subject weights and passing rules are from the PRC examination programs and the laws (RA 9292, RA 7920); check the current PRC program before you file. Code questions follow the Philippine Electrical Code 2017 Part 1; check them against your own copy of the Code.