Arc Fault Circuit Breaker
A circuit breaker with a push button to test it is more formally known as an arc fault circuit breaker or arc fault circuit interrupter (AFCI). When a power switch is turned off, the electrical potential between the two separating contact points does not die simultaneously, a temporary arc of electricity passes between the two contact points. This causes the characteristic snap noise heard when turning off a light, for example. These slight dips in current caused by arcs occur so often that normal circuit breakers are made insensitive enough to ignore them, else they would be thrown constantly. However, unintentional arcs as might be caused by faulty, damaged, or degraded wiring go ignored by traditional circuit breakers as well. These faulty arcs have contributed to thousands of house fires and hundreds of deaths, which is why the AFCI was invented. It is designed to differentiate between normal harmless arcs and faulty arcs, and protect against them.
Rather than being mechanical, the interior of an AFCI is largely electronic. It starts with two contacts on the exterior, one leading into the AFCI from the power supply, the other leading out of the AFCI into the home. On the inside, the supply contact is connected to one end of a circuit board on a spring-loaded catch. On the other end of the circuit board is the contact point leading into the home.
Normal, harmless arcing occurs in a period of milliseconds. Faulty arcing lasts longer because it does not originate from a loss of power, merely a redirection. If any minor surge or dip in current is detected for a period of more than ten milliseconds, the circuit board activates the spring-loaded catch, and severs the power connection. When you press the test button on the AFCI, it sends the same signal to the circuit board to make sure that it's still functional. Of course, the AFCI still operates like a normal circuit breaker in that if the dip or surge in current exceeds a five ampere threshold, the breaker is thrown as well.