Emax Low voltage air circuit-breakers - ABB Download Center

By assuming some numerical values for the short-circuit currents,and considering some fault points, the following is theresult.For circuit breaker QF1, if a fault occurs at point B, the currentwill flow in direction A-B concordant to the reference directionor similarly, for a fault in A, the current direction will be B-A indiscordance with the reference direction.The different configurations can be resumed in the followingtable:Circuit-breaker Location of fault Measured current [kA] Direction Tripping timeQF1QF2A 15 Discordant t7BwB, C, D, E 10 Concordant t7FwB 15 Discordant t7BwA, C, D, E 10 Concordant t7FwThis installation aims at selectivity between QF1, QF2, QF3and QF4.On examining the table, we see that the only instance in whichthe fault current direction is discordant with that set for the circuit-breakerQF1 occurs in case of a fault in point A. The circuitbreakerQF1 must trip more quickly than the other circuit-breakers,since it is the one nearest to the fault. To this purpose, thetrip time t7Bw of QF1 must be set at:– a value below the time t7Fw of the circuit-breaker QF2, sincethe fault current is concordant with QF2 reference direction– a value lower than the time “t2” of protection “S”, if available,for the trip unit of the moulded-case circuit-breaker QF4.The instantaneous protection of QF4 shall be set in OFF orshall have a setting value I3 higher of the contribution givenby the motor to the short-circuit current.Moreover, the functions S and I of both QF1 and QF2 have beenset so as not to intervene before function D.Similarly to the process described for circuit-breaker QF1, toensure selectivity, circuit-breaker QF2 must trip first in the caseof a fault in B, and then with a delayed trip in the case of faultsanywhere else in the system.6The settings available for directional protection D, both for Fwand Bw, are the following:I 7=0.6...10xIn (tolerance + 10%) step 0.1xInt 7=0.20s...0.8s (tolerance + 20%) step 0.01sABB SACE 6/15