Full form of MCB – The full form of MCB is Miniature Circuit Breaker.

What is MCB in electrical?

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.

How Does MCB work-

Working Principle –

Working 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).

Frequently Asked Questions (FAQs)– 

Q1) How much capacity MCBs are used in house wiring?

Ans) MCB’s range starts from 2A – 63A (complete range is given in above sections), but MCBs are used according to the load which starts from 10A till 63A. Some typical MCB capacities are given below –

i) 10A SP MCB – It is used for lighting circuits & 6A light plugs.

ii) 16A SP MCB – It is used for normal power circuits with 16A switch & sockets where 1000W maximum load is considered.

iii) 20A SP MCB – It is used for special power load with 20A switch & socket such as domestic heater (where load is more than 1000W).

iv) 25A SP MCB – it is used for domestic air conditioners.

v) 32A DP MCB / 40A DP MCB – It is used in each phase of three phase DB (depending on the load on each phase) for outgoing circuits.

vi) 40A TPN MCB / 63A TPN DB MCB – It is used as an incomer in three phase DB (depending on the load on all three phase).

(Note- MCB is a switching as well as protective device, therefore capacity/current rating should be selected very carefully (depending on the current rating of load) so that in case of fault, respective MCB should trip & isolate the faulty circuit from system to avoid further damage).

Q2) Which type of MCB is used for domestic wiring purpose?

Ans) MCBs are available in different curves also such as B, C, D.. . These curves represent the range of fault current suitable for different – different applications.

For domestic wiring purpose, B-curve type MCBs shall be used because in this case MCB will trip when fault current reach remains between 3-5 times of the full load current.

Q3) What is the use of MCB?

Ans) The purpose of usage of MCB in electrical system is to protect the load or equipment against overload & short circuit.

Q4) How a MCB is superior to the fuse wire?

Ans) MCB has following advantages over fuse wire –

1. No replacement is required in case of MCB when fault occurs (on over loading / short circuit), on the other hand fuse wires needs replacement on every fault in the circuit.
2. In case of fuse wire, there are chances of putting higher capacity fuse wire (than design) in the system to avoid repeated faults. This malpractice is safety hazard which may lead to a bigger fault in electrical system. While it is not possible in MCB.
3. MCB is a cheap & long durable switching devise as compared to fuse kit.

Q5) Why MCB trips again & again?

Ans) MCB provide protection to equipment against over-loading & short circuit. If MCB is tripping is again & gain, it means fault is there in the system. Therefore, we should first locate the fault & rectify it then we should switch it on again.

Q6) What is the difference between MCB & MCCB?

Ans) Full form of MCB is Miniature Circuit Breaker while MCCB stands for Molded Case Circuit Breaker. Function of both MCB & MCCB are same i.e a) work as switching device b) to protect equipment from overloading & short circuit. Only difference in both is current rating. MCB is used for small current rating loads while MCCBs are used for higher current rating loads.

MCBs are available from 2A – 63A while MCCBs are available from 63A – 800A.