Emax Low voltage air circuit-breakers - ABB Download Center

Primary and secondary distributionBack-up protectionBack-up protection is required by the IEC 60364-4-43 Standardsand Annex A of the IEC 60947-2 Standard, which allow the useof a protection device with breaking capacity lower than theprospective short-circuit current at the points where it is installed,provided that there is another protection device on the supplyside with the necessary breaking capacity. In this case, thecharacteristics of the two devices must be coordinated in sucha way that the specific energy let through by the combination isnot higher than that which can be withstood without damage bythe device on the load side, and by the protected conductors.In the diagram in the figure, circuit-breaker B, located on theload side of circuit-breaker A, can have a lower breaking capacitythan the prospective short-circuit current in the event ofa fault in “C”, if circuit-breaker A is able to satisfy both of thefollowing conditions:– it has a suitable breaking capacity (higher than or equal tothe prospective short-circuit current at its point of installationand obviously higher than the short-circuit current in “C”)– in the event of a fault in “C” with short-circuit values higherthan the breaking capacity of circuit-breaker B, circuitbreakerA must provide a specific let-through energy limitingfunction, limiting it to a value that can be withstood bycircuit-breaker B and by the protected conductors.A fault in “C” can therefore cause a double interruption, howeverthe back-up protection must ensure that B always tripswithin the limits of its breaking capacity.It is necessary to choose switchgear combinations that havebeen verified by laboratory tests for this type of protection. Thepossible combinations are specified in ABB SACE documentsand PC programs (Slide rule kits, DOCWin, etc.) and shownhere for SACE Emax circuit-breakers.Back-up protection is used in electrical installations in whichthere is no essential need for continuous operation: when thesupply-side circuit-breaker opens, it also excludes loads thatare not affected by the fault. Furthermore, the use of this type ofcoordination limits the size of the installation and consequentlyreduces costs.NoteBack-up protection can also be implementedon more than two levels: the figureabove shows an example of coordination onthree levels. In this case, the choices arecorrect if at least one of the two situationsbelow is satisfied:– the circuit-breaker furthest on the supplyside A is coordinated with both circuitbreakersB and C (coordination betweencircuit-breakers B and C is notnecessary);– each circuit-breaker is coordinated withthe circuit-breaker immediately to the loadside of it, i.e. the circuit-breaker furthestto the supply side A is coordinated withthe next one B, which is in turncoordinated with circuit-breaker C.1SDC200187F00011SDC200188F00016Table showing coordination for back-up protectionSupply-side circuit-breakerBreaking capacityE2L - E3L 130 [kA] (at 380/415 V)Load-side circuit-breakerBack-up valueT4N 65 [kAT4S - T5N - S6N - E1B - E2B 85 [kA]T4H - T5S/H - S6S/H - S7S/H - E1N - E2N 100 [kA]T4L - T5L130 [kA]ABB SACE 6/13