Emax Low voltage air circuit-breakers - ABB Download Center

Switching and protection of generatorsEmax circuit-breakers are suitable for use with low-voltage generatorsemployed in the following applications:A - back-up generators for primary loadsB - generators disconnected from the supply networkC - generators for small power stations connected in parallelwith other generators and, possibly, with the power supplynetwork.In cases A and B, the generator does not operate in parallelwith the power supply network: the short-circuit current thereforedepends on the generator itself and, possibly, on the connectedloads.In case C, the breaking capacity must be determined by assessingthe short-circuit current imposed by the network at thepoint of circuit-breaker installation.The main points to check for generator protection are:– the short-circuit current delivered by the generator; this canonly be assessed if one is familiar with the machine’s typicalreactance and time constants. Here one can simply note thatlow short-circuit protection device settings are normally required(2-4 times In);– the thermal overload limit of the machine. According to theIEC 60034-1 Standard, this value is set at 1.5xIn for a periodof 30 seconds.For a detailed assessment, see the DOCWin program or specializedbooks on the topic.The wide range of settings offered by electronic trip units:PR121 Threshold I (1.5 to 15) x In Threshold S (1 to 10) x InPR122 Threshold I (1.5 to 15) x In Threshold S (0.6 to 10) x InPR123 Threshold I (1.5 to 15) x In Threshold S (0.6 to 10) x Inmakes SACE Emax circuit-breakers perfectly suitable for protectinglarge generators against short-circuit currents andagainst thermal overloads.66/32ABB SACE