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Strange test results, generator supplied installation
Former Community Member
I did a bit of testing at an installation supplied by a generator and was confused by the test results. I had two testers with me: a megger loop tester and a MFT. The loop tester mainly refused to carry out the test due to the frequency being 51Hz but did give a result from a light switch of 0.9 ohms. The MFT gave a result of 20 ohms from the same light switch and similar (18 to 20 ohms) readings throughout. At the generator it gave a figure of 38.8 ohms. This doesn’t seem to make sense. I’ve done loop tests with both testers since arriving home and they seem to produce fairly similar results Could there be some feature of the way the generator operated that has led to this strange mix of results? Would the high supply frequency have an effect on the tester? HVe I done anything wrong when testing. I’d appreciate it if anyone can help.
The assumed value of 3-500A PSSC is OK for calculations Mike but the real SSC will be much less, as the excitation will probably collapse as powered from the output voltage. The effect of this is similar to assuming a high source reactance, probably capable of supplying only the rated output current or less. Protection of small generators from shorts is particularly difficult, but is the long slight overload which is most likely to cause damage as usual. Frequency control to with 1 Hz is also typical across the load range from a mechanical fuel injection system, larger machines may well have electronic speed control from a crystal so can give very good timekeeping. Generator supplies at this power level are best fitted with an RCD to deal with earth faults, short circuit faults are pretty unlikely in the kind of installation you describe and tend to be self limiting. A 32A MCB will probably provide output overload protection with a quick trip close to maximum output, look at the tripping time curves in the BBB.
