CIRCUIT BREAKERCircuit breaker is a mechanical switching device capable of protecting the circuit wiring, capable of making carrying and breaking currents under normal circuit conditions and also making, carrying for a specified time and breaking currents under specified abnormal circuit conditions such as those of short-circuit.A circuit breaker t consists of a thermal overload release, an electromagnetic short-circuit release, a tripping (operating) mechanism, the main contact system and the auxiliary contacts.The combination of fuse, contactor and thermal overload relay will be replaced by the starter combination of circuit breaker and contactor.Circuit breaker can be categorized as follows: Current-zero interrupting type of circuit breakers: In the case of an alternating current, the arc is extinguished automatically at each current zero. This property is employed in the current-zero interrupting type of circuit breakers and the re-striking of the arc is prevented. . The path of the arc is de-ionized by drawing away the heat-energy. The path of the arc is de-ionized by drawing away the heat-energy Current limiting type of circuit breakers: A device is said to be limiting when the current passing through it during a short circuit has an amplitude considerably lower than the prospective current.

In order to reduce the thermal and mechanical stress on the object to be protected, the current must be interrupted right during the initiation of the short circuit, before the full prospective value can be attained. This is achieved by: Quick opening of the main contacts. Rapid build-up of a high arc-voltage (moves the arc quickly away from the contact tips and guides it to the arc chamber)Current limiting circuit breakers can be either thermal-magnetic or electronic trip.They cannot contain fusible element.A current-limiting circuit breaker is one that has been certified by UL to limit the let-thru I^2t (I squared t) during a fault to not more than the I^2t available during a half-cycle of prospective symmetrical short-circuit current. Maximum let-through I^2t and peak current levels are published on curves available with the trip curves for the current-limiting breakers.

LOAD BREAK SWITCH:The load break switch is a mechanical switching device capable of making, car-rying and breaking currents under normal circuit conditions which may include specified operating overload conditions and also carrying for a specified time currents under specified abnormal circuit conditions such as those of short-circuit.

DISCONNECTOR:A mechanical switching device which, in the open position, complies with the requirements specified for isolating function.The operating mechanism itself may serve the purpose of the switch-ing position indicator provided in the "TRIP" position it indicates the position "OFF" only when all the moving contacts are in the "OFF" position.

The reason for a CB to be designed primarily for 400V may have a reduced breaking capacity at 690V is because of the following reason:A contact system is optimized for a particular rated supply voltage from the point of view of its switching capabilities. As for example a contact system designed primarily for 400V may have a reduced breaking capacity at voltages above 400V (at supply voltages lower than 400V, it is not critical). The reason of the reduction is the following: for quenching the arc due to the short-circuit current inside the arc chamber, an arc voltage which opposes the supply voltage is built-up between the contacts. The value of the arc voltage depends on the design of the contact system and the arc chamber (number of splitter plates and other factors). As long as this opposing voltage has a particular high value in relation to the supply voltage, the short-circuit currents can be efficiently limited and the arc quenched rapidly.