A circuit breaker is a device that works this way: A spring holds two pieces of metal, called contacts, together. All the current in the circuit flows through these contacts. If the amount of current exceeds the safe limits , a bimetallic strip heats up, expands and bends, pushing against the spring holding the contacts together. This expansion overcomes the force of the spring and pushes the contacts apart. At that instant, no current can flow; we say that the circuit is open. The breaker can be reset by hand after the overload problem has been fixed.