Emax Low voltage air circuit-breakers - ABB Download Center

MV-LV substation with multipletransformers in parallelThe rated current of the transformer is calculated following thesame procedure outlined in the previous section.The minimum breaking capacity of each protection circuitbreakeron the LV side must be higher than the highest of thefollowing values (the example is for machine 1 in the figure andapplies to three machines in parallel):– Ik1 (short-circuit current of transformer 1) in the event of afault immediately on the load side of circuit-breaker QF1;– Ik2 + Ik3 (Ik2 and Ik3 = short-circuit currents of transformers2 and 3) in the event of a short-circuit on the supply side ofcircuit-breaker QF1.Circuit-breakers QF4 and QF5 on the outgoing feeders must havea breaking capacity higher than Ik1 + Ik2 + Ik3; the contributionto the short-circuit current by each transformer obviously dependson the short-circuit power of the network to which it is connected,and on the line connecting the transformer and the circuit-breaker(to be determined on a case-by-case basis).1SDC200203F0001Ik2 + Ik3QF1 Ik1 QF2 QF3QF4QF5Ik1 + Ik2 + Ik3Circuit-breaker A Circuit-breaker B6ABB SACE 6/27