What is an MCB?

MCB stands for miniature circuit breaker. It is a circuit protection device, capable of protecting a circuit or a piece of equipment from greater damages caused due to short circuits as well as overloads. It is a modular device and can be used as an alternative to fuses. It is capable of sensing overload and short circuit conditions and isolates the power before the whole circuit is damaged.

Single pole MCB

Working principle

It consists of a bimetallic strip and a plunger arrangement inside. Also, there is a latch arrangement made to hold the contacts closed, when it is turned ON. The bimetallic strip is designed to carry a specific amount of current through it. When the current flowing through it exceeds the limit, the bimetallic strip gets heated up and it bends. As it bends, it knocks down the latch holding the contacts. Higher the current faster shall be the operation.

Similarly, during short circuits, the current flowing through the miniature circuit breaker shall be several times the maximum limit. This activates the plunger arrangement instantaneously, which in turn knocks down the latch and opens the contacts. This is how a miniature circuit breaker works.

Learn Miniature circuit breaker in detail

Miniature Circuit Breaker – Principle of operation

How do MCB sense overload and short circuits?

During overloads and short circuits, the amount of current flowing through the MCB shall be higher than the limit. For example, overload is a condition at which a piece of equipment drains current more than the limit it is designed to operate. Also, in case of short circuits, the current flow shoots up several times the normal level. It is designed in such a way to sense these two situations and isolate the power. Higher the current flow quicker shall be its operation.

Electric Short Circuit

They are available in various ratings and sizes in the market. If the MCB is not optimally rated, there shall be a delay in operation and this may damage the equipment or circuit. Also, underrating of the device can result in nuisance tripping.

Frequently asked questions

What is an MCB used for?

They are used to protect a circuit against damages caused due to overload and short circuits. They are capable of interrupting the current flow in a circuit when it senses any abnormalities.

What causes an MCB to trip?

Overloads and short circuits cause an MCB to trip. A Trip is a condition at which the device automatically interrupt the power a circuit when it senses any abnormalities.

What are the types of MCB?

MCB can be classified based on the number of poles, trip curves, type of supply it can protect etc. It can have single-pole, double poles, triple poles or four poles. Based on trip curves it can be classified into Class B, Class C, Class D, Class K and Class Z. Based on the type of supply it can be classified into AC or DC.

Does MCB trip on earth fault?

An MCB never trips on earth faults. It trips only during short circuits and overloads. Earth faults can be sensed by RCD – Residual current devices and Earth fault relays

What is MCB trip C?

MCB with class C trip characteristics trips instantaneously when the current flowing through it reaches between 5 to 10 times the rated current. Suitable Domestic and residential applications and electromagnetic starting loads with medium starting currents.

What is kA rating of MCB?

kA rating of an MCB or an MCCB is the maximum current it can safely interrupt in case of a short circuit. If the current goes beyond this value, the circuit breaker could be damaged. kA rating is known as the short circuit withstand capacity or ultimate breaking capacity of a circuit breaker. It is denoted by Icu. Read more

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