A circuit breaker interrupts an electrical circuit whenever the current in the circuit exceeds a safe level. Unlike a fuse, which has to be replaced once it has "blown," a circuit breaker can be reset an unlimited number of times.
How Circuit Breakers Work
The essential elements of a circuit breaker are a switch and a current-sensitive mechanism that will move the switch and break the circuit when the current exceeds the breaker's rated level. In domestic breakers, the force that drives this mechanism is provided either by a bimetallic strip or an electromagnet. In the case of a bimetallic strip, the higher current makes the strip bend to move the switch. In the case of the electromagnet, the increased current increases the magnet's force, enabling it to move the switch via a linkage.
Lower voltage circuit breakers---the kinds that are used domestically---are generally divided into three groups. The most common is the molded case, or MCCB, type; the next step up is the insulated case, or ICCB type; and for medium voltage and amperage, the metal clad breaker.
Circuit breakers are rated for the wire capacity and load of the circuit of which they are a part. This is expressed in amps and volts. Circuit breakers are available in a vast range of sizes, suitable for protecting anything from a 110-volt domestic lighting circuit to a high voltage circuit carrying enough power for an entire town. Common voltage ratings for low voltage circuit breakers are 120 volts, 240 volt, 277 and 480 volts AC. Typical breakers for domestic 120-volt circuits are 20 and 15 amp. Large appliances such as water heaters, ovens, air conditioners and clothes dryers are protected by larger breakers on dedicated circuits.
Typical domestic setup
Main household breakers are typically 100 amp, although recently, with the increase in domestic electrical consumption, 150 amp and 200 amp main circuits have become more common. Circuit breakers for 120-volt domestic circuits take up a single slot in the electrical panel, while those for 240-volt circuits take up two slots.
Breakers for wet locations
Because of the risk of electrocution from a wet appliance or switch, you should always use a ground-fault circuit interrupter (GFCI) to protect circuits---and people---in potentially wet situations such as outdoors, the kitchen and the bathroom.