Miniature circuit breaker (MCB) working principle

Rapidly growing demand for electrical installations composed of high-quality electrical components dictates new trends in designing low voltage circuit breakers. The miniature circuit breaker is the reflection of advanced circuit breaker technology. This article describes the operating principle of MCBs.

Table of Contents

What is a miniature circuit breaker?

A miniature circuit breaker provides overload and short circuit protection for conductors, motors, and starters. It prevents a possible fire and protects human life. It is the best protection device for the safety of the circuits and people.

The main function of a miniature circuit breaker is to protect circuits against the effects of thermal and magnetic overcurrents.

Protection against overcurrent is crucial for the electrical installation because it avoids damage on the wiring isolating characteristics, these damages can cause:

– Increase of the electrical energy consumption due to residual current caused by isolation issues in the wiring

– Personal accident by direct contact with damaged wires

When the circuit current exceeds the current written on the MCB, it protects the load by opening its contacts. When MCB opens (which is closed under normal conditions) it isolates the load from the main supply.

How does an MCB work?

The miniature circuit breaker opens automatically when an overcurrent passes through its contacts. When the factor causing excessive current is removed, it can be activated again thanks to the handle on it.

For a better understanding, we have to look at the thermal and magnetic operations.

Thermal operation

The thermal operation protects from moderate overloads. Under overload conditions, a thermo-metallic element (bimetallic strip) deflects until it operates a latching mechanism allowing the main contacts to open. It is also known as overload protection. Long-time over-currents can be dangerous because they reduce the life of the electrical installation, conductor, and components and if left unchecked could result in fire.

Magnetic operation

In magnetic operation, large overloads or short circuit current actuates a selenoid causing a plunger to strike the latching mechanism, rapidly opening the main contacts. It is also known as short circuit protection. The opening of the MCB’s contacts during a short circuit is complete in 0.5 milliseconds.

Miniature circuit breaker trip curves

Based on the tripping characteristics, MCBs are available in B, C, D, K, and Z curves to suit different types of applications.

B Curve: For the protection of the electrical circuits with equipment that does not cause surge current (lighting and distribution circuits) Short circuit release is set to 3-5 times the rated current (In)

C Curve: For the protection of the electrical circuits with equipment that causes surge current (inductive loads and motor controls) Short circuit release is set to 5-10 times the rated current (In)

D Curve: For the protection of the electrical circuits which cause high inrush current, typically 12-15 times the thermal rated current (transformers, X-ray machines, etc.) Short circuit release is set to 10-20 times the rated current (In)

K Curve: For protecting windings in motors and transformers and simultaneous overcurrent protection of cables. Short circuit release is set to 10-14 times the rated current (In)

Z Curve: For control circuits with high impedances, voltage converter circuits, and semiconductor protection, simultaneous overcurrent protection of cables. Short circuit release is set to 2-3 times the rated current (In)