A circuit breaker is ordinarily expected to provide a means to manually open and close its contacts. However, whenever it senses an overcurrent, it is also expected to open its contacts.
This function of the circuit breaker is determined by its trip unit.
For a circuit breaker of thermal-magnetic characteristic, the design of the elements in the trip unit are such that it senses high current which results from situation of short circuit and also, the resulting heat which occur in states of overloading. There’s also the some circuit breakers of thermal-magnetic properties are incorporated with buttons for “PUSH TO TRIP” purpose.
Some larger breakers could be tripped manually with the incorporated “PUSH TO TRIP” button which is on the surface of the circuit breaker. The tripper bar rotates upwards and to the right whenever the button is pressed.
The contacts are opened by the operating mechanism.
Overload states are sensed by circuit breakers of thermal-magnetic properties. These circuit breakers have bimetallic strips which sense the states of overload. When there’s ample overcurrent flowing through the path of the current, this leads to the buildup of heat and the subsequent bending of the bimetallic strip.
A bimetallic strip is composed of two distinct metals which are bonded together. Due to the difference in their thermal expansion properties, the bimetallic strip bends when heated. A rise I current gives a commensurate rise in heat.
The bending of the bimetallic strip depends on its degree of hotness. The removal of the heat source (opening of the contacts of the circuit breaker) leads to the cooling of the bimetallic strip and its return to its original state. This provides for manual resetting of the circuit breaker once the overload state is corrected.
Short Circuit Trip
As described earlier, the flow of current across blow-apart contacts of a circuit breaker produces opposite magnetic fields. Such opposing forces are not adequate for splitting the contacts in normal conditions of operation. However, when a short circuit occurs, those opposing forces significantly increase.
The conductor that passes close to the trip unit of the circuit breaker has the same current as the one flowing through the contacts. Sufficient force is provided to open the trip breaker and the trip unit by the magnetic field at fault current points.
A sudden interruption of the fault current happens as a result of the joint activities of the magnetic fields which forces the contacts to be apart while at the same time trip the circuit breaker.
The trip mechanism, which is in the trip unit has the tripper bar which holds the mechanism firmly in position. The mechanism will always be locked in position as long as it is held by the tripper bar.
The trip mechanism holds the operating mechanism in the “ON” position. The activation of the trip releases the trip mechanism and so the contacts are opened.