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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.
Personally I think it is easier to test R1 +R2 or R2 - depends how big the genset and what is being supplied, but I like to start at the genset with an earth fault relay or RCD,and then the fact the exact source impedance is all over the place does not affect the fact that there is always some level of ADS that will protect human life in all states, even if there is not enough oomph to take an MCB into the instant firing region at low loads. Then the MCB is only really there to catch overloads that creep up slowly.
How big is the genset in this case - if you know it is good for X kW you can estimate its equivalent source impedance by thinking about voltage drop, or maybe just pretending pssc was at least perhaps ten times max load current.. Though some electronic regulators can be set to have a negative eqivalent resistance where the voltage rises under load, to allow compensation for cable losses.
Personally I think it is easier to test R1 +R2 or R2 - depends how big the genset and what is being supplied, but I like to start at the genset with an earth fault relay or RCD,and then the fact the exact source impedance is all over the place does not affect the fact that there is always some level of ADS that will protect human life in all states, even if there is not enough oomph to take an MCB into the instant firing region at low loads. Then the MCB is only really there to catch overloads that creep up slowly.
How big is the genset in this case - if you know it is good for X kW you can estimate its equivalent source impedance by thinking about voltage drop, or maybe just pretending pssc was at least perhaps ten times max load current.. Though some electronic regulators can be set to have a negative eqivalent resistance where the voltage rises under load, to allow compensation for cable losses.
