IEC Overcurrent Protection 3 — Questions and Answers
Question 1: An inverse-time circuit breaker trips faster when the overcurrent is:
- Closer to its rated amperage
- Much larger than its rated amperage (Correct answer)
- Equal to its rated amperage
- Below its rated amperage
Correct answer: Much larger than its rated amperage
Inverse-time breakers have a trip time that decreases as the magnitude of overcurrent increases.
Question 2: Per NEC 240.85, a circuit breaker marked '120/240V, 2-pole' may be used on which system?
- 277/480V three-phase only
- 120/240V single-phase systems (Correct answer)
- 208Y/120V three-phase only
- Any voltage up to 480V
Correct answer: 120/240V single-phase systems
A breaker must be rated for the voltage of the system it protects; a 120/240V 2-pole breaker is rated for 120/240V single-phase systems.
Question 3: Which NEC section covers the location requirements for overcurrent devices in dwelling units?
- NEC 210.20
- NEC 240.24 (Correct answer)
- NEC 230.90
- NEC 250.30
Correct answer: NEC 240.24
NEC 240.24 specifies where overcurrent devices shall and shall not be located, including dwelling unit requirements.
Question 4: A 20A, 120V branch circuit supplies a single receptacle. The cord-and-plug-connected load must not exceed what amperage for continuous operation?
- 20A
- 16A (Correct answer)
- 15A
- 18A
Correct answer: 16A
Continuous loads on a branch circuit must not exceed 80% of the circuit rating (20A × 80% = 16A).
Question 5: What is the primary difference between a Type T and a Type W Class K fuse?
- Voltage rating
- Interrupting rating and let-through current (Correct answer)
- Physical size
- Number of elements
Correct answer: Interrupting rating and let-through current
Class K fuses are subdivided by let-through current and interrupting ratings; Type W has higher let-through than Type T.
Question 6: Per NEC 240.50, plug fuses of the Edison-base type shall only be used as a replacement in existing installations where there is no evidence of:
- Overfusing (Correct answer)
- Underfusing
- Correct sizing
- A time-delay type
Correct answer: Overfusing
Edison-base fuses may only replace existing fuses where there is no evidence of overfusing or tampering.
Question 7: A service disconnect rated 200A feeds a panel. Per NEC 230.90, the service overcurrent device must be rated at or below:
- 150A
- 200A (Correct answer)
- 250A
- 175A
Correct answer: 200A
NEC 230.90 requires the service overcurrent device to be rated not more than the ampacity of the service conductors, which here supports 200A.
IEC Overcurrent Protection 3
An inverse-time circuit breaker trips faster when the overcurrent is: