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Earth Fault Loop Impedance with Safety Generator
Former Community Member
Do I take this into consideration? If so how do I size wire in a building with a safety generator? What is my "Ze" when the supply switches from DNO to the safety generator? How do diesel generators behave during faults?
In anything other than an induction set, it is more like something between a continuation of the total load current at the time and more like twice whatever that was, perhaps rising after a few seconds as the controller realises the volts are dropping, and opens the throttle. The voltage collapses catastrophically, so say if there was 100A of load current at 250v before fault came on, then you may get no volts at all and about 200A, and hopefully that is enough to prompt clear the fuse, and then the volts can rise back up again, and it settles back. Users see the lights flicker, and maybe the genset coughs a bit.
However that it is not enough to promptly clear the fuse then either the controller puts more fuel in, the genset slows and stalls, or a shear pin breaks depending on the design of the alternator and prime mover.
In an induction set the fault current rises until the volts drop to near zero, then it stops generating altogether, and the engine races away.
In anything other than an induction set, it is more like something between a continuation of the total load current at the time and more like twice whatever that was, perhaps rising after a few seconds as the controller realises the volts are dropping, and opens the throttle. The voltage collapses catastrophically, so say if there was 100A of load current at 250v before fault came on, then you may get no volts at all and about 200A, and hopefully that is enough to prompt clear the fuse, and then the volts can rise back up again, and it settles back. Users see the lights flicker, and maybe the genset coughs a bit.
However that it is not enough to promptly clear the fuse then either the controller puts more fuel in, the genset slows and stalls, or a shear pin breaks depending on the design of the alternator and prime mover.
In an induction set the fault current rises until the volts drop to near zero, then it stops generating altogether, and the engine races away.
