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Difficulty: MediumProtective Devices

Thermal-magnetic circuit breakers utilize two distinct internal mechanisms to provide comprehensive circuit protection. Which internal component provides an instantaneous trip to protect against severe short-circuit conditions?

  1. A
    A bimetallic strip that flexes as it gradually heats up
  2. An electromagnet that immediately pulls an armature to release the trip latchAnswer
  3. C
    A metal oxide varistor that clamps high voltage surges
  4. D
    A parallel shunt resistor that diverts short-circuit current to ground

Answer

An electromagnet that immediately pulls an armature to release the trip latch
Thermal-magnetic circuit breakers combine two tripping methods: thermal and magnetic. The magnetic component consists of an electromagnetic coil placed in series with the line. Under a severe fault such as a direct short circuit, the sudden surge in current generates an intense magnetic field that instantly draws an armature toward the coil, unlatching the switch mechanism within milliseconds to disconnect power safely.

Step-by-Step Solution

1
Identify the dual-protection mechanisms used inside thermal-magnetic circuit breakers.
The breaker includes a thermal element (bimetallic strip) for prolonged, moderate overloads and a magnetic element (electromagnetic coil) for large, instantaneous fault currents.
Different types of overcurrent faults require different response times to protect wiring without causing nuisance tripping during normal startup surges.
2
Determine which mechanism is designed to trigger instantly during a dead short circuit.
A short circuit causes a massive and immediate spike in current, which energizes the electromagnetic coil strongly enough to instantly attract a movable armature and mechanically trip the contact mechanism.
Electromagnetic response does not depend on slow thermal accumulation, allowing contact separation within milliseconds.

Key Concept

Dual-trip operational principles of thermal-magnetic circuit breakers
Estimated Time:1m 0s
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Topics

Protective Devices

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