How does a thermal relay works?

The bimetallic thermal relay consists of a small bimetallic strip that is heated by an element connected in series with the supply. When the current rises above a preset value, the movement of the strip releases a catch which opens the trip contacts.

In recent years more modern electronic relays are used which simulate the thermal overload. Many of these relays also incorporate a memory, i.e. simulates the temperature rise / cooling curve of the winding.