IEC Overcurrent Protection 5 — Questions and Answers
Question 1: Per NEC 240.60, cartridge fuses and fuseholders for 250V shall NOT be used on systems where the nominal voltage between any two conductors exceeds:
- 120V
- 250V (Correct answer)
- 300V
- 208V
Correct answer: 250V
Cartridge fuses rated 250V are limited to circuits where the voltage between any two conductors does not exceed 250V.
Question 2: Which overcurrent device characteristic describes the ability to interrupt a fault current without damage to itself?
- Ampere rating
- Interrupting rating (Correct answer)
- Voltage rating
- Time-delay rating
Correct answer: Interrupting rating
The interrupting rating specifies the maximum fault current an overcurrent device can safely clear without being destroyed.
Question 3: Per NEC 240.33, enclosures for overcurrent devices must be mounted in a:
- Horizontal position only
- Vertical position as close as practical (Correct answer)
- Diagonal position for drainage
- Any position approved by the AHJ
Correct answer: Vertical position as close as practical
NEC 240.33 requires overcurrent device enclosures to be mounted vertically unless this is shown to be impracticable.
Question 4: A Type S fuse adapter is inserted into an Edison-base fuseholder to accept which type of fuse?
- Class R fuse
- Type S plug fuse (Correct answer)
- Class T fuse
- Class H cartridge fuse
Correct answer: Type S plug fuse
Type S adapters accept only Type S fuses of the matching ampere rating, preventing installation of an oversized fuse.
Question 5: Per NEC 240.21(C), transformer secondary conductors are not required to have overcurrent protection at the secondary terminals if the primary overcurrent device rating does not exceed the value calculated using which multiplier for a 2-wire secondary?
- 125%
- 167% (Correct answer)
- 250%
- 300%
Correct answer: 167%
For a 2-wire transformer secondary, the primary OCPD may be up to 167% of the primary FLC as a substitute for secondary protection per NEC 240.21(C)(1).
Question 6: Which NEC article specifically addresses overcurrent protection for motors?
- Article 215
- Article 430 (Correct answer)
- Article 240
- Article 310
Correct answer: Article 430
NEC Article 430 contains the requirements for motor overcurrent protection, separate from the general rules in Article 240.
Question 7: A circuit breaker with a slash rating such as '120/240V' indicates it is suitable for use on:
- Any system up to 240V
- Both grounded and ungrounded 120V or 240V systems
- A solidly grounded system where the voltage to ground does not exceed the lower of the two values (Correct answer)
- Three-phase delta systems only
Correct answer: A solidly grounded system where the voltage to ground does not exceed the lower of the two values
A slash-rated breaker (e.g., 120/240V) is listed for use on solidly grounded systems where the line-to-ground voltage does not exceed the first number and line-to-line does not exceed the second.
IEC Overcurrent Protection 5
Per NEC 240.60, cartridge fuses and fuseholders for 250V shall NOT be used on systems where the nominal voltage between any two conductors exceeds: