We are going to discuss the working principle of miniature circuit breaker
In a miniature circuit breaker, there are mainly three mechanisms to make the switch off
They are, bimetallic strip, one trip coil and one hand operated on-off lever
In a MCB, current first enters through inlet power terminal to bimetallic strip, then to
current coil or trip coil, then to moving contact, then to fixed contact and lastly
current leaves through outlet power terminal. Here, you see all these are arranged in series
If over current flows for long time through this MCB, the bimetallic strip becomes overheated
and hence it is deformed. This deformation of bimetallic strip causes displacement of latch point
The moving contact of the MCB is arranged by means of spring pressure with this latch
point in such a way 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 during short circuit fault, the MCB will open in same manner
Again when the miniature circuit breaker or MCB is required to be switched off manually
the operating lever of the switch is operated by hand. When we do so, the same latch point is displaced and as a result moving contact is separated
from fixed contact and the MCB becomes in off position. So whatever may be the operating mechanism, that means due to overload, due to short circuit
fault or due to manual operation, actually the same latch point is displaced and same
deformed spring is released and this is ultimately responsible for movement of the moving contact
A miniature circuit breaker is always provided with arc splitters. During separation of moving and fixed contact, there may be a high chance of arc and this
arc 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. Thank you