Circuit breakers are devices that interrupt the flow of electricity based upon a mechanical switch or the load current flowing through it. They are important safety elements in electric circuits, providing protection to both the circuit and users. Protection is provided by preventing over current or current leakage conditions that may cause electrocution or could damage components and lead to dangerous failure modes. They are used in applications such as main circuit isolation and automotive circuit protection.

There are large oil circuit breakers (OCBs) used in high voltage (HV) power distribution to electrically isolate HV power lines. They detect short circuit currents, and automatically switch the load circuit off and on several times to try to blow whatever is causing the fault (like animals or trees) off the line before they continually break the circuit. Automotive circuit breakers prevent dangerous overloading of the car battery and prevent wire harnesses being burnt out due to shorts. Such failures can cause fires or even explosions. They are very useful in faultfinding as they can indicate which circuit has the fault, and also help reduce the potential for catastrophic failure modes. For example –a failure on the heating circuit of a vehicle may trip the heating circuit breaker but will isolate the fault so the vehicle can still be safely driven until it is repaired.

Earth leakage circuit breakers (ELCB) and residual current circuit breakers (RCD or RCCB) are a very important component in the electrical wiring of every home. Building code regulations as an essential safety element usually requires them. ELCB detect small voltages or differential current flows in the earth circuit of a household. These have mostly been replaced today with RCDs that have less dangerous failure modes. RCDs detect a current imbalance between the phase and neutral wires. An imbalance over a certain trigger point set by regulation indicates the presence of a current flow in an earth – causing the breaker to switch off until manually reset.

Once tripped, a circuit breaker needs to be reset. This is usually by the manual operation of a pushbutton or switch after the fault has been eliminated. Some circuit breakers require maintenance and repair. Oil circuit breakers require regular oil cleaning as oil denatures over time and can have hydrophilic properties that reduce isolation. Filtering equipment may be part of a circuit breaker system or an accessory required. The oil itself may have to be replaced due to aging. In sulfur hexafluoride breakers, SF₆ is one of the accessories that have to be replaced. In vacuum CBs, the vacuum pump is one of the many accessories required. Electric arcs produced as a result of tripping damages the electrode terminals over time, and needs to be replaced. Electrodes are part of accessories of all types of breakers. The manufacturer of the breaker specifies an authoritative list of essential circuit breaker accessories. What quantities are to be stored are recommended by the manufacturer, but the can later be modified in the light of experience gained.