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Low current Zs test tripping RCBO

Sparkymania
Hi


Have a question about RCD testing. I'm not new to testing and have been doing it for quite a few years now but have come across something that has never happened before.


On Zs testing a circuits on a DB with all RCBOs every one of them trip on a 2 wire low test. I didn't find this out until I did the lighting as the sockets use 3 wire and I allow the default selection of 3 wire low. Using Megger MFT. All 3 lighting circuits did this. I used a 3 wire low (using 3 probes) and they didn't trip. I tried the socket circuits on 2 wire low and they did trip.

All load turned off.


I'm guessing the use of the neutral in the test allows some balancing current through the RCBO but then I don't know the technical differences between 2 and 3 wire low test and can't find any details on it anywhere. I suspect, even if I could find it it may go into so much detail it would go over my head.


The question is why are they tripping? No Load. IR tests all >999. 


Anyone got any Ideas?


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  • 0
    I think most (usually A-type) RCBOs these days are much more sensitive to residual currents that aren't nice pure sine waves than the traditional AC type RCCBs. That makes life much harder for loop testers - it's difficult to get an accurately measurable change in voltage from a maximum 15mA leakage current - so they usually resort to all sorts of tricks to use larger currents for very short durations to try to work out the loop impedance. In 3-wire mode where the meter has access to the L-N voltage and L-N loop it can deduce a certain amount from high current tests on that loop without risking tripping any RCD (even if it's only monitoring the changes in voltage caused by other loads) - in 2 wire mode everything has to be done on the L-PE loop so tests might have to be slightly more agressive for the L-PE loop or repeated more times to get a plausible reading - all of which gives an RCBO extra chances to trip.


    I've also had problems using 3-wire tests on some low rated (<=6A) RCBOs - seemingly (and annoyingly) they seem to be tripping on overcurrent rather than residual current - i.e. it's the :large currents on the L-N loop that's tripping them rather than the small L-PE test currents.


      - Andy.
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  • 0
    I think most (usually A-type) RCBOs these days are much more sensitive to residual currents that aren't nice pure sine waves than the traditional AC type RCCBs. That makes life much harder for loop testers - it's difficult to get an accurately measurable change in voltage from a maximum 15mA leakage current - so they usually resort to all sorts of tricks to use larger currents for very short durations to try to work out the loop impedance. In 3-wire mode where the meter has access to the L-N voltage and L-N loop it can deduce a certain amount from high current tests on that loop without risking tripping any RCD (even if it's only monitoring the changes in voltage caused by other loads) - in 2 wire mode everything has to be done on the L-PE loop so tests might have to be slightly more agressive for the L-PE loop or repeated more times to get a plausible reading - all of which gives an RCBO extra chances to trip.


    I've also had problems using 3-wire tests on some low rated (<=6A) RCBOs - seemingly (and annoyingly) they seem to be tripping on overcurrent rather than residual current - i.e. it's the :large currents on the L-N loop that's tripping them rather than the small L-PE test currents.


      - Andy.
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