Circuit breaker

A circuit breaker is a safety device that guards electrical circuits against harm from excessive current, known as overcurrent. Its job is to stop the flow of electricity, shielding equipment and lowering the risk of fire. Unlike a fuse, which works only once and then needs swapping out, a circuit breaker can be reset—either by hand or automatically—to get back to normal operation. These devices are often found in distribution boards. Beyond safety, a circuit breaker can also serve as a main switch, letting someone manually cut power to or restore power for an entire electrical sub-network.

Circuit breakers come in many current ratings, from small units protecting low-current circuits or individual home appliances, up to large switchgear that guards high-voltage lines feeding a whole city. Any device that automatically cuts power when current gets too high—whether a circuit breaker or a fuse—is called an over-current protection device (OCPD).

**Origins** Thomas Edison described an early version of a circuit breaker in an 1879 patent application, though his commercial power system used fuses. Its purpose was to protect lighting wiring from accidental short circuits and overloads. A modern miniature circuit breaker, similar to those used today, was patented by Brown, Boveri & Cie in 1924. Hugo Stotz, an engineer who had sold his company to Brown, Boveri & Cie, was credited as the inventor on German patent 458392. Stotz’s invention became the forerunner of the thermal-magnetic breaker still common in household load centers.

Connecting multiple generators into an electrical grid required circuit breakers with higher voltage ratings and better ability to safely stop the growing short-circuit currents from these networks. Simple air-break manual switches created dangerous arcs when interrupting high-voltage circuits. These were replaced by oil-enclosed contacts, and later by designs using directed pressurized air or oil to cool and stop the arc. By 1935, the specially built circuit breakers at the Boulder Dam project used eight series breaks and pressurized oil flow to interrupt faults up to 2,500 MVA in three AC cycles.

**Operation** All circuit breaker systems share common features in how they work, but details vary widely based on voltage class, current rating, and type. First, the breaker must detect a fault. In small mains and low-voltage breakers, this is often done inside the device itself, typically using the heating or magnetic effects of electric current. Large-current or high-voltage breakers usually rely on protective relay pilot devices to sense a fault and trigger the opening mechanism. These relays often need a separate power source, like a battery, though some high-voltage breakers are self-contained with current transformers, relays, and internal power.

Once a fault is found, the breaker’s contacts must open to stop the circuit. This is usually done with mechanically stored energy inside the breaker, such as a spring or compressed air. A breaker might also use the higher current from the fault itself—via thermal expansion or a stronger magnetic field—to separate the contacts. Small breakers often have a manual lever to switch off or reset a tripped breaker. Larger units may use a solenoid to trip the mechanism and an electric motor to recharge springs, which then rapidly separate contacts when tripped.

The contacts must carry the load current without overheating and also withstand the heat of the arc created when opening the circuit. Contacts are made from copper, copper alloys, silver alloys, or other highly conductive materials. Their service life is limited by erosion from arcing. Miniature and molded-case breakers are usually thrown away when the contacts wear out, but power and high-voltage breakers have replaceable contacts.

When a high current or voltage is interrupted, an arc forms. The arc’s maximum length is roughly proportional to the voltage, while its intensity (or heat) is proportional to the current. This arc must be contained, cooled, and extinguished in a controlled way so the gap between contacts can again handle the circuit’s voltage. Different breakers use vacuum, air, insulating gas, or oil as the medium where the arc forms. Techniques to extinguish the arc include lengthening or deflecting it, intensive cooling in jet chambers, splitting it into partial arcs, zero-point quenching (opening contacts at the moment in the AC waveform when current and voltage are near zero, effectively breaking no load current at that instant—zero-crossing happens 100 times per second for 50 Hz and 120 times per second for 60 Hz AC), and connecting capacitors in parallel with contacts in DC circuits. Finally, once the fault is cleared, the contacts must close again to restore power.

**Arc Interruption** Low-voltage miniature circuit breakers (MCBs) use air alone to extinguish the arc. They contain arc chutes—stacks of mutually insulated parallel metal plates that split and cool the arc. Dividing the arc into smaller arcs makes it cooler, increases the arc voltage, and adds impedance that limits the current.