When heated, copper expands more than iron. This makes the strip bend.
Thermal circuit breakers trip and turn off the circuit if the current is too great.
They work because the current heats up a bimetallic strip.
The bimetallic strip bends and this movement switches off the circuit.
These devices are quite slow to operate.
These circuit breakers can be found in shaving sockets. The small electric shaver current is too small to trip the breaker but most other appliances would cause the breaker to trip.
The current is made to flow through the heating element. Small currents do not heat the strip much so it does not turn off.
