Inside a thermal overcurrent trip

This is a demonstration of a classic thermal overcurrent device operating.

There is a fixed contact, and one on a bimetallic strip that is sized for the rated current.
When the rated current is exceeded significantly, the domed bimetallic strip heats up and suddenly clicks to the open position.

A plastic shim that was being held back by the closed contacts then pops into the gap between them, so that when the bimetallic strip cools and clicks closed again it can't make a connection.

Pushing the reset button pushes that shim out of the way and the contacts can close again and the unit resets ready for the next event.

This unit is very simple and not designed to break very high fault currents which may weld the contacts together. A full circuit breaker also has a magnetic trip mechanism to react quickly to a high current short circuit.