EEI SOPD Practice Test 8

How a Circuit Breaker Interrupts Current

¹A circuit breaker is a switch built to open a circuit while current is flowing, even the very large current of a short circuit. Opening a circuit sounds simple, but it is not. As the breaker's contacts pull apart, current keeps flowing across the widening gap in the form of an arc, a column of extremely hot, glowing gas that conducts electricity.

²The breaker's real job is to put out that arc. In an alternating-current system, the current passes through zero many times each second as it reverses direction. At each of these zero points, the arc briefly loses its energy. If the gap between the contacts can be made to resist current at that instant, the arc cannot restart, and the circuit is open.

³Breakers do this in different ways. Some blow a blast of compressed gas across the gap to cool the arc and sweep the hot gas away. Others separate the contacts inside a sealed chamber that holds almost no air, so there is little material to carry the arc. Still others surround the contacts with special insulating gases that absorb the arc's energy quickly.

⁴Every method puts strain on the breaker. The arc wears away a little of the contact surfaces each time the breaker opens under load, and the wear is greatest when it interrupts a short circuit. For this reason, maintenance crews track how many times each breaker has operated and how much fault current it has interrupted, and they inspect or replace the contacts before the wear becomes a problem.