How Circuit Breakers Work


Electricity Basics

  • Electricity is measured in three ways: voltage, amperage and resistance. Voltage is the amount of electricity being "pushed" through a conductor (the wire), amperage is the number of moving electrons, and resistance is measured in Ohms and is literally the resistance the electricity is meeting, either by a piece of equipment, conductor thickness, or some other means.
    A circuit, at a minimum, needs a power source and a method of carrying the electricity from the power source to the equipment and then back (two wires). A circuit breaker is a device that measures and monitors the current flow across a circuit. If the current increases to a predetermined level, it will trip a switch, severing the flow of electricity past it.


  • A circuit breaker includes the following components: an actuator that is used to trip or reset the circuit breaker; an actuator which forces the contacts together or apart; contacts that allow current flow when together and disrupt current flow when separated (act as the "switch"); terminals; a bimetallic strip; a calibration screw; a solenoid; and an arc divider or extinguisher.

Types and Operation

  • The most common circuit breakers are the bi-metal thermal circuit breaker and the magnetic circuit breaker, or a combination of the two. The bi-metal thermal circuit breaker measures current flow by using two dissimilar metals, one of which expands when its temperature increases.
    The magnetic circuit breaker measures current flow through a magnetic field. As the magnetic field increases to a predetermined level, it becomes strong enough to move the mechanical linkage, which will trip a switch and disconnect the electricity.
    A magnetic circuit breaker can be immediately reset because as soon as it trips, the magnetic field is de-energized, essentially resetting the circuit breaker. The bi-metal thermal circuit breaker must be cooled before resetting it.

Ground Fault Interrupters (GFI)

  • The ground fault interrupter (GFI) is a modern circuit breaker that is incorporated into all new homes. The circuit breakers in your home are usually all centrally located at a circuit breaker panel. The ground fault interrupters are located throughout your home, typically incorporated into wall plugs. Using GFIs, the home is divided into "sections," protecting a series of outlets and/or equipment. Like a circuit breaker, the GFI will trip if there is an increase in current that is above a predetermined value. The reset switch on a GFI is on the outlet itself.

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