MINIATURE CIRCUIT BREAKER (MCB)

Definition – MCB is a switching device which is used in LT electrical system. It provides protection against overload, short circuit. It is used for low current load with a range from 2A to 63A. It can be installed in panels & Distribution Boxes. It’s a very good replacement for fuse because after faults there is no need to replace fuse wire& only we have to switch it on again.

Working PrincipleWorking principle of MCB is same as MCCB only difference is that the current carrying capacity. MCB is used for low current load (2A to 63A) while MCCB is used for moderate current load (63A to 630A).

How MCB works on overload – Let us say a 10A MCB is connected with a load.  Under normal conditions MCB will carry 10A load. When overloading takes place say by 50% or 100% etc bimetallic strips of MCB starts heating up, on 50% overloading (15A) strips will heat slowly as compared with 100% overloading (20A).  When bimetallic strips bends it trips MCB. In other words we can say that MCB will trip faster when overloading value of current will be more.

How MCB works on short circuit – Short circuit occurs when phases or phase& neutral wires come in contact with each other due to insulation failure etc. On short circuit, a heavy current (i.e. 4-6 times more than normal current) flows in the circuit. This high current may damage the load or electrical circuits hence this dangerous high current needs to be stopped immediately (not like overloading protection which takes time). This is done with the help of a solenoid coil, when high current passes through the coil, a plunger which is inside the coil moves & trips MCB immediately.

Selection of MCB

  • Check type of load whether it is whether it is single phase or 3 phase load.
  • It should be capable to carry the rated current of load,
  • Check number of poles – SP, SPN, DP,TP, TPN &4P where

SP (for single phase load) = Single Pole which means only phase wire will be connected,

SPN (for single phase) = Single pole with neutral where phase will be On/OFF when MCB will operate but neutral will not isolate as it remains connected.

DP (for single phase load) = Double Pole which means phase & neutral wires will be connected& on fault both phase & neutral will be isolated,

TP(for 3 phase load) = Triple pole which means three phases will be connected with MCB & neutral will be separated from MCB,

TPN (for 3-phase load) = Triple pole with neutral where three phases & neutral are part of MCB & but 3-phase will operate with MCB operation & neutral will remain connected.

4P (for 3-phase load) = Fore pole which means 3 phases & a neutral will be connected& all 4 poles are linked with switching ON or OFF operations.

  • Fault current rating which should be in kA (thousand amperes) which means that a MCB can withstand the fault current of short circuit.

Ex – 40A DP MCB with 10kA fault current.

RATING OF MCBs

A typical range of single phase MCBs – 2A, 6A, 10A, 16A, 20A, 25A, 32A, 40A, 50A, 63A.

A typical range of Three phase MCB –  1A, 2A, 3A, 4A, 5A, 6A, 10A, 16A, 20A, 25A, 32A, 40A, 50A, 63A.

Note: MCBs are available with 10KA fault level.