The MCB is an automatic, electrically operated switching device that was designed to automatically protect an electric circuit from overload currents and short circuit currents. It is a complicated construction made up of almost 100 individual parts. It can respond within milliseconds when a fault has been detected. Westinghouse Electric introduced the World’s first MCB and it initially had a porcelain base and cover mounted in a metal housing
Applications of MCBs
MCBs find wide application in residential, commercial and industrial operations. These applications include but
are not limited to:
Ø Power Supplies
Ø Programmable Logic Controller Input / Output (I/O) Points
Ø Lighting circuits
Ø Relay/Contactor coils
Ø Control circuits
Ø Motor circuits
The most essential feature of the MCB is the inverse-time tripping characteristic. This feature indicates the time required to trip the breaker to clear the circuit of any given level of overcurrent load. A typical inverse time tripping characteristic is represented in below figure.
Circuit Breaker Coordination
Time-current characteristic curves are essential for the proper coordination of circuit breakers. In the event of a fault, only the circuit breaker near to the fault zone only shall operate, leaving other circuits unaffected. If that MCB not operates because of some reason, the next upstream breaker shall operate. If that also not operates, then next upstream MCB and so on.
In the example below, three consecutive circuit breakers are shown which have been coordinated so that the tripping time of each breaker is greater than the tripping time for the downstream breaker(s) regardless of the fault magnitude.
Circuit breaker CB No-3 is set to trip if an overload of 2000A or greater occurs for 0.1 seconds. This have the minimum operation time. The circuit breaker CB No-2 will trip if the overload remains there for 1.0 seconds and the circuit breaker CB No-1 if the fault remains for 10 seconds. If the fault occurs downstream of breaker CB No-3 it will trip first and clear the fault. Circuit breakers CB No-2 and CB No-1will continue to provide power to the circuit. Similarly if fault happened between CB No-2 & 3, the CB No.2 will trip first. If it fails to operate due to some reason then CB No-1 will trip. As there is no current flow through CB No-3 it will not operate for this fault.
Each function of the tripping MCB unit also shall be mutually coordinated to prevent nuisance trips. If a circuit breaker is feeding a piece of equipment with large inrush currents for example, the instantaneous pickup value should be set higher than the short time pickup value. It is required to prevent tripping when the equipment is energized.