Working Principle of Miniature Circuit Breaker (MCB) Explained!

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.

A miniature circuit breaker (MCB) 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 to 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.

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.

The figure shows a schematic drawing of the function of a bi-metal. The deflection of the bi-metal depends on the intensity and duration of the current. After a pre-determined deflection, which means after a certain time depending on the current intensity, it will activate the tripping mechanism.

In certain appliances, the deflecting bi-metal directly opens the contact. In the MCB however, the bi-metal only gives the signal to the switching mechanism which provides for the opening of the contacts. It is a special advantage that the characteristic of the thermal trip can be widely influenced by the design of the material and the shape of the bi-metal. Normally, the bi-metal is directly heated by the load current. With the low ratings of the MCBs, however, the bi-metal must be heated indirectly by a heater winding for low-rated currents in order to obtain enough energy.

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.

The electromagnetic trip consists essentially of a coil through which the load current flows. In this coil, there is a fixed and movable armature. If the current exceeds a pre-determined intensity, the armature will be attracted against the force of the spring. The switching mechanism is actuated by the lever on the right-hand side and provides for an opening of the contacts. This is the classical method.

Besides this, for breaking short circuit currents, it is enormously important to separate the contacts within the shortest possible time. The so-called switch-off delay must be short as possible. The switching mechanism however has springs and masses, and it takes a rather long time to move these elements.

In our schematic diagram, you can see on the left side, a hammerhead that provides direction for the opening contacts avoiding any delay by spring and additional masses. Of course, the tripping of the switching mechanism must be done in parallel to release the spring force, to bring the lever into the “off” position, and to prepare the switching mechanism for a new closing of the MCB. The hammer trip is one of two essential conditions for the limitation of short circuit currents.

Now it is time to have a look into tripping characteristics. Because MCBs have different tripping zones. 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)