USCG Exam PracticeElectrical troubleshooting and maintenance
A molded-case circuit breaker protecting 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 provides instantaneous trip on short circuit; the electromagnet provides time-delayed trip on sustained overload
- ✓ The bimetallic strip provides time-delayed trip on sustained overload; the electromagnet provides near-instantaneous trip on short circuitCorrect
- C. Both elements provide time-delayed tripping, with the electromagnet responding to higher current levels than the bimetallic strip
- D. The bimetallic strip responds to voltage fluctuations; the electromagnet responds to sustained overcurrent
Why B is correct
NEETS Mod. 3 §2-2 states that the thermal element (bimetallic strip) trips on sustained overload with an inverse time delay, while the magnetic element (electromagnet) trips almost instantly on the very high current of a short circuit. These two mechanisms are often used together in molded-case breakers.
Cited:NEETS Mod. 3 §2-2