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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
    Interestingly I've had a recent case whereby my MFT1741 trips a 30mA MEM RCCB on no trip loop testing but my old MFT1553 doesn't. I haven't come to any definitive conclusion yet as to why this is.

    Unfortunately due to a failure of the rotary selector on the 1553 I had to retire it coming up to Christmas, although if anywhere will repair it at a sensible price I probably will do that to have it as a spare. As it's no longer supported by Megger I'm not sure that they would carry out repairs on it.
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  • 0
    Interestingly I've had a recent case whereby my MFT1741 trips a 30mA MEM RCCB on no trip loop testing but my old MFT1553 doesn't. I haven't come to any definitive conclusion yet as to why this is.

    Unfortunately due to a failure of the rotary selector on the 1553 I had to retire it coming up to Christmas, although if anywhere will repair it at a sensible price I probably will do that to have it as a spare. As it's no longer supported by Megger I'm not sure that they would carry out repairs on it.
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