Definition – Air circuit breaker is a devised that is used for LT (Low Tension) electrical system i.e 415 Volt. It is used to switch On & Off of power and provides over current & short circuit protection. It is used in LT panels for switching & protection of load from various faults. Different relays/releases are used to trip circuit breakers in case of faults. These relays/releases are either installed inside the breaker or outside it. The sensing devices which are installed inside the breaker are called releases & these are internal features of ACB. Those installed outside of it are called relays. WORKING PRINCIPLE – The working principle of ACB is the same as we have discussed in HT circuit breakers. In HT circuit breakers additional arc quenching medium (like oil, air blast, SF6 & vacuum) is required due to high voltage but in LT system no additional quenching medium is required as arc is extinguished in normal ambient air but some special arrangements are done in the breaker to extinguish are arc.