The hard part of a circuit breaker is killing the spark
Opening a switch sounds simple, yet pulling two contacts apart while a large current flows makes the electricity leap the gap as a glowing arc. Left alone, that arc can vaporise metal and blow the device apart. Every circuit breaker, from a household panel to grid switchgear, is really a machine for smothering this spark quickly.
A breaker has three jobs: notice a fault, open its contacts, and then kill the arc so the gap can hold back the voltage again. Small household units sense trouble themselves, using the heating or magnetic effect of excess current, while big ones rely on separate protective relays. Springs or compressed air then snap the contacts apart. Unlike a fuse, which is spent after one blow, a breaker can be reset and used again.
Designers have tried many ways to tame the arc. Miniature breakers push it into arc chutes, stacks of insulated metal plates that chop it into smaller, cooler pieces. Oil breakers boil some of their oil to blast a jet through the arc, while others use sulfur hexafluoride gas or compressed air. In a vacuum there is almost nothing to ionise, so the arc dies after the contacts part by less than 3 millimetres. Some designs time the opening for the instant the alternating current passes through zero, which happens 100 times a second on a 50-hertz supply. Most breakers quench the arc within 30 to 150 milliseconds of tripping.
The idea is old. Thomas Edison described an early breaker in an 1879 patent application, though his power system actually used fuses. The modern miniature breaker came from Hugo Stotz, credited on a 1924 patent for Brown, Boveri and Cie, and his thermal-magnetic design survives in home panels today. As power grids linked generators together, fault currents grew, and by 1935 the breakers at the Boulder Dam used eight breaks in series and pressurised oil to clear faults within three cycles.
Ratings matter. A typical home breaker can interrupt 10,000 amperes, and fitting one where potential fault currents run higher risks a breaker that stops a fault but explodes when reset.
Source: Circuit breaker
