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 variousfaults. Different relays/releases are used to trip circuit breaker in case of faults. These relays/releases are either installed inside the breaker or outside it. The sensing devices which are installed inside breaker are called releases& these are internal features of ACB. Those installed outside of it are called relays.

WORKING PRINCIPLEWorking principle of ACB is same as we have discussed in HT circuit breakers. In HT circuit breakers additional arc quenching medium (like oil, air blast, SF6 &vaccum) 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.

  • Arcing horns – Arcing horns are used with the fixed and moving contacts in order to increase the contact area and spreading of arc heat.
  • Arc Chutes – These are fitted just above the contacts & the function of arc chutes is to split the arc heat inside its chamber which causes arc in chamber becomes colder, lengthen & split therefore arc voltage becomes much larger than system voltage at the time of closing of ACB & finally arc is quenched automatically.

Important terms related with ACB –

  • Fixed type ACB – This is a breaker in which mechanism of breaker is fixed with the cradle. Breaker cradle/case is fixed with the panel. Breaker can’t be racked out from the panel.
  • MDO ACB –MDO stands forManually operated Draw Outtype of breaker. In this breaker, breaker is operated manually & breaker can be racked in and racked out with the help of a handle.
  • EDO ACB –EDO stands for Electrically operated Draw Outtype of breaker. In this breaker, breaker is operated electrically as well as manually & breaker can be racked in and racked out with the help of a handle.
  • Releases –These are relays which are fitted inside the breaker & these are used for various types of protection. Releases are of following types –
  1. Shunt release – This release is used to trip breaker remotely. When shunt release is energized it gives tripping command to trip the breaker & breaker mechanism disconnects the contacts.
  2. Under Voltage release – It operates when voltage goes below 80% of the supply voltage & trips. The purpose of UV release to protect the equipment from low voltage.
  3. Electromagnetic or Electronic release – ACBs are available in market with either electromagnetic or electronic release. In electromagnetic releases, trip setting are fixed in steps of percentage for fault protection.While in electronic releases, any setting can be done. Secondaly, the response time of electronic releases are very fast as compared to electromagnetic releases.
  • Poles – Circuit breakers are available in market in 4P, TP, TPN. Where 4P means 4 poles where all 3 phases (R,Y,B) & 1 neutral will operate when ACB operates. While in TP breaker, only R,Y,B-phases are part of breaker &operate when ACB operates & neutral is connected through a link & is not a part of ACB. TPN –  TPN stands for Triple Pole with Neutral where R,Y,B phase & neutral are part of ACB but when ACB operates then only R,Y,B operate & neutral doesn’t operate. In this case neutral remains connected (see diagram given below to understand it).

  • NO/NC Contacts – ACBs are available with certain minimum numbers of NO/NC contacts. These contacts are used for various types of metering & protection purposes. If additional NO/NC contacts can be provided on demand.


A typical range of ACBs  –  630A, 1000A, 1250A, 1600A, 2000A, 2500A, 3200A, 4000A, 5000A, 6000A.


Following points must be taken care before pacing the order of ACB

  • Current rating – ACB should be able to carry the full load current which is to be connected with it.
  • Poles – 4P or TP, TPN
  • Type of operation – MDO or EDO
  • Type of releases – Electromagnetic or electronic release,
  • Inbuilt releases – Like shunt, under voltage etc,
  • Fault current rating – 25KA, 35KA, 50 KA. It is available in kA (thousand amperes) which means that a ACB can withstand the fault current of short circuit,
  • NO/NC – Check no of NO/NC contacts in ACB.