USCG Exam PracticeElectrical troubleshooting and maintenance
A molded-case circuit breaker on a motor feeder incorporates both a bimetallic strip and an electromagnetic element. Which statement correctly describes the function of each element?
- A. The bimetallic strip trips instantaneously on short-circuit current; the electromagnetic element provides inverse time-delay tripping on sustained overloads
- ✓ The bimetallic strip provides inverse time-delay tripping on sustained overloads; the electromagnetic element trips almost instantaneously on short-circuit currentCorrect
- C. Both elements provide identical inverse time-delay tripping, with the electromagnetic element serving as a backup to the bimetallic strip
- D. The bimetallic strip protects against undervoltage; the electromagnetic element protects against overcurrent
Why B is correct
NEETS Mod. 3 §2-2 describes the thermal element (bimetallic strip) as providing inverse time-delay tripping on sustained overloads, and the magnetic element (electromagnet) as tripping almost instantly on the very high current of a short circuit. The roles are not interchangeable.
Cited:NEETS Mod. 3 §2-2