What is an automatic circuit breaker?

Circuit breakers come in all shapes and sizes.

However, they all have one thing in common:
they are part of the electrical distribution box that interrupts the electrical circuit when a short circuit or overload occurs as a result of too high a current in the system.

Once a fuse, now an automatic circuit breaker

The purpose of a fuse is also to protect the wiring of electrical systems against damage caused by high electrical currents.
The difference is that a fuse can ‘blow’ and as a result will need to be replaced. A fuse consists of an electrically conductive strip or wire of copper, silver or occasionally a copper alloy that melts when a short circuit or overload occurs. This can primarily be found in old systems.

With an automatic circuit breaker, the overload protection consists of two elements connected in series:

• a magnetic circuit breaker that protects against short circuits;
• a thermal circuit breaker that responds automatically to overload.
   

The circuit breaker can simply be reset following an interruption. This is why fuses are increasingly being replaced by automatic circuit breakers.

What about residual current?

An electrical system is therefore protected by an automatic circuit breaker against short circuits and overloads, but not against residual current (earth leakage). Therefore, in order to protect people against electrocution and to prevent fire as a result of current leakage, there is yet another type of circuit breaker.

The residual-current circuit breaker with overload protection consists of a single compact housing in which the functions of a circuit breaker are combined with a residual-current device. The generic term for this is the residual-current device.

(See also: 'What purpose does earthing serve?')