USCG Exam PracticeElectrical troubleshooting and maintenance
A molded-case circuit breaker protecting a motor feeder incorporates both thermal and magnetic tripping elements. What is the primary function of the thermal element?
- A. To trip almost instantaneously on the very high current of a short circuit
- ✓ To trip on sustained overload current with an inverse time delayCorrect
- C. To trip on reverse power flow from a regenerating motor
- D. To trip on undervoltage conditions at the switchboard
Why B is correct
NEETS Mod. 3 §2-2 describes the thermal element (bimetallic strip) as the element that trips on sustained overload with an inverse time delay, tolerating brief inrush but tripping faster as the overload increases. The magnetic element handles instantaneous short-circuit trips.
Cited:NEETS Mod. 3 §2-2