Aircraft primarily employ thermal circuit breakers which operate by detecting a sudden temperature increase for actuation. When a circuit reaches a specified temperature --- indicating a potential problem or excess of electrical current --- the breaker releases a bi-metal strip which deflects the current. This helps prevent a system-crippling short or fire which could render essential systems, such as landing gear deployment, navigation or communications, inoperable.
The Federal Aviation Administration allows pilots and crew to reset circuit breakers that have been tripped during the flight. However, the FAA cautions that resetting without tracking down the reason why the breaker tripped in the first place could be hazardous. Because of this, the FAA requires that the circuit breaker not be reset unless the flight captain judges it is imperative to do so for the safe conclusion of the flight or unless it is consistent with the procedures outlined in the flight crew operating manual. All resets must be recorded in the plane's maintenance log.
Some older aircraft may still use older model circuit breakers as the primary on/off switches for their many electrical systems. According to the Federal Aviation Administration, breakers used for this purpose have to be replaced with more modern designs such as thermal circuit breakers. Alternatively, these circuit breakers can be redesigned and reinstalled with appropriate switching devices.
