Zs and RCD testing on 3 phase generator supply

davidjones89 · Oct 5, 2010

just a few pointers on this please, been to a property today where a large shed is supplied by a 3 phase genny. Inside the shed are numerous sockets, motors for opening large doors and lights. Started to do a Zs test on sockets, (ring main circuit) supplied by 32a mcb, within seconds RCD would trip off. Tried a few different sockets all with the same result.
Checked and double checked tester, no trip setting etc and still tripping RCD.
Did an RCD test on sockets, 1/2 times, 1 times etc. all times were over range

am i doing something wrong

no other tests were done as had to get away but if anyone has any pointers they would be much appreciated

telectrix · Oct 5, 2010

continuity and IR on dead circuit/s first. you could have a small leakage which is adding to your no=trip test current

Steve1412 · Oct 6, 2010

Normal installation testing on generators can throw up some strange things...came up against this many years ago..using far older test equipment....was unable to take any Zs readings on an installation powered by three phase genny...it turned out that as I initiated the test the generator sensed the increase in load (basically introducing small earth fault with tester) and upped the voltage to overcome the dip??????Mentioned it to a guru of electrical lecturing who explained it all to me but far too boring to comprehend....needless to say testing on generated supplies can be strange. The other thing I did remember was the question of can the generator supply the required fault current required to trip protection in required time....before genny blows up or something?????
Anyway...will get back to my Cad....good luck with the testing mate!!!