Oil Circuit Breaker

Opening a switch on a high-voltage circuit does not stop the current. As the contacts part, the field across the tiny gap is enormous, the air ionises, and an arc forms that carries on conducting — hotter than the surface of the sun and perfectly happy to keep burning as the contacts separate further. The real problem in a circuit breaker is not opening the contacts but extinguishing what follows. The oil circuit breaker does it by opening the contacts under a bath of insulating oil: the arc's own heat cracks the oil into hydrogen, the sudden gas pressure blasts the arc apart, and hydrogen happens to conduct heat away better than almost any other gas. The arc creates the conditions for its own destruction.