The Operation of Miniature Circuit Breaker

- Aug 07, 2020-

The Operation of Miniature Circuit Breaker


There are three mechanisms provided in a single miniature circuit breaker to make it switched off. If we carefully observe the picture beside, we will find there is mainly one bimetallic strip, one trip coil and one-hand operated on-off lever.


The electric current-carrying path of a miniature circuit breaker shown in the picture is as follows. First left-hand side power terminal – then bimetallic strip – then-current coil or trip coil – then moving contact – then fixed contact and – lastly right had side power terminal. All are arranged in series.

If the circuit is overloaded for a long time, the bimetallic strip becomes overheated and deformed. This deformation of bimetallic strip causes, displacement of latch point. The moving contact of the MCB is so arranged by means of spring pressure, with this latch point, that a little displacement of latch causes, release of spring and makes the moving contact to move for opening the MCB.


The current coil or trip coil is placed in such a manner that during short circuit fault the MMF of that coil causes its plunger to hit the same latch point and make the latch to be displaced. Hence the MCB will open in the same manner.


Again when the operating lever of the miniature circuit breaker is operated by hand, that means when we make the MCB at off position manually, the same latch point is displaced as a result moving contact separated from fixed contact in the same manner.


Regardless of the operating mechanism – e.g. due to the deformation of the bimetallic strip, or due to the increased MMF of the trip coil, or due to manual operation – the same latch point is displaced and same deformed spring is released. This is ultimately responsible for the movement of the moving contact. When the moving contact separated from fixed contact, there may be a high chance of arc.


This arc then goes up through the arc runner and enters into arc splitters and is finally quenched. When we switch on an MCB, we actually reset the displaced operating latch to its previous on position and make the MCB ready for another switch off or trip operation.