Emax Low voltage air circuit-breakers - ABB Download Center

Primary and secondary distributionSelective protectionSelectivity is normally actuated for tripping overcurrent protectiondevices in civil and industrial installations to isolate thepart affected by a fault from the system, causing only the circuit-breakerimmediately on the supply side of the fault to trip.The example in the figure highlights the need to coordinatetripping between the two circuit-breakers A and B so that onlycircuit-breaker B is tripped in the event of a fault in C, ensuringcontinuity of service for the rest of the system supplied bycircuit-breaker A.Whereas natural selectivity within the overload current range isnormally found due to the difference between the rated currentsof the load protection circuit-breaker and the main circuit-breakeron the supply side, selectivity can be obtained in the shortcircuitcurrent range by differentiating the current values and, ifnecessary, the trip times.Circuit diagram with selectivecoordination of protections1SDC200181F0001Selectivity can be total or partial:– total selectivity: only circuit-breaker B opens for all currentvalues lesser than or equal to the maximum short-circuit currentin C;– partial selectivity: only circuit-breaker B opens for fault currentsbelow a certain value; A and B are both tripped forgreater or equal values.In principle, the following types of selectivity are possible:Current selectivity, obtainedby setting the instantaneoustrip currents of the circuitbreakerchain to differentvalues (higher settings for thecircuit-breakers on the supplyside). This often results inpartial selectivity.6Example of current-type selectivity1SDC200182F0001Time selectivity, obtained byintentionally incorporating increasingtime-delays in the triptimes of the circuit-breakers furthestto the supply side in thechain.Example of time-type selectivity1SDC200183F00016/2ABB SACE