Which mechanism in an MCB trips on prolonged overload?

A bimetallic strip that heats and bends

A solenoid coil pulling a plunger

How does an MCB respond to a short circuit?

The solenoid's strong field pulls a plunger and trips within milliseconds

The bimetal slowly bends over several minutes

It waits for the thermal delay to finish

How does the thermal trip time vary with overload current, and why is this useful?

Larger overload trips faster; small surges such as motor starting do not cause false trips

Larger overload trips slower; this protects motors

Trip time is the same for all currents

Larger overload never trips; only short circuits do

What does the arc chute do when MCB contacts separate?

It splits and cools the arc until it is quenched

It increases the arc to keep current flowing

It stores the arc energy in a capacitor

2081 Baishakh (new course) · 3 marks

Operating principle of MCB

Short answer

An MCB uses a bimetallic strip for overload: heating bends it, unlatching contacts with a time delay. For short circuits, a solenoid's magnetic field instantly trips the mechanism. An arc chute extinguishes the arc, and it can be manually reset.

Describe the operating principle of MCB.

A Miniature Circuit Breaker (MCB) automatically opens the circuit when the current exceeds its rated value, and can be closed again by hand after the fault is cleared. It uses two tripping mechanisms.

 In --o--[ bimetal strip ]--[ solenoid coil ]--o--- Out
         (overload trip)    (short-circuit trip)
                \              /
                  trip lever -> releases moving contact

1. Thermal (bimetallic) trip for overload.

A bimetallic strip (two metals with different expansion rates) carries the load current.
With a prolonged overload, the strip heats up and bends, and at a certain point it releases the latch.
The spring then opens the contacts. The time delay is inversely related to the current: a small overload takes some time to trip, while a larger one trips faster. This avoids false tripping on motor starting surges.

2. Magnetic (electromagnetic) trip for short circuit.

A solenoid coil is in series with the load.
During a short circuit, the very large current produces a strong magnetic field that pulls a plunger instantly, striking the trip lever and opening the contacts within a few milliseconds.

Arc extinction. When the contacts separate, the arc is drawn into an arc chute, where it is split, cooled and quenched.

Ratings are given in amperes (6 A, 10 A, 16 A, 32 A, etc.), with B, C or D curves for the magnetic trip level.

✓ Answer checked by Claude Opus 5.5 (high effort)

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