Electrical Installation Level 2 Practice Test

Overload protection in a circuit breaker relies on a heat‑sensitive mechanism known as a bi-metallic strip. When current flows above the breaker’s rating for a period, resistance heating warms the strip. Because the two metals have different coefficients of expansion, the heated strip bends or distorts. This bending releases the latch and opens the contacts, stopping the current. The thermal trip is designed to respond to sustained overcurrent, not just a sudden surge.

The magnetic trip coil, by contrast, reacts to very high instantaneous currents (short circuits) with a rapid magnetic force to trip. A fuse element is not a breaker at all—it melts open to interrupt the circuit and isn’t reset. An electronic sensor with a microcontroller can trip some modern breakers, but the classic, physically distorting component that handles overload is the bi-metallic strip.