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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
    Thanks for the replies everyone.


    The RCBOs are all MK type AC (tested on AC setting). Fairly new as they are in a 3rd amd DB. I always test using the correct AC or A setting. Never had a problem like this until now. 


    Why would a low test trip even a < 6A RCBO? The L-E test current is only 15mA. How much neutral current is there on the 3 Lo test? I never thought if it as being high. I thought it was just using the neutral to drive the tester but, as AJJewsbury says, it is testing the L-N as part of the L-E test. It seems, then, that the L-N current is in amps rather than mAs.


    Still can't see how 15mA L-N is tripping all the RCBOs on the 2 wire though. They all passed the 1/2 test when RCD tested. If they are doing this on the test surely there would be a problem with nuisance tripping on loads with earth leakage up to that same amount which would render those RCBOs useless in many applications.
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  • 0
    Thanks for the replies everyone.


    The RCBOs are all MK type AC (tested on AC setting). Fairly new as they are in a 3rd amd DB. I always test using the correct AC or A setting. Never had a problem like this until now. 


    Why would a low test trip even a < 6A RCBO? The L-E test current is only 15mA. How much neutral current is there on the 3 Lo test? I never thought if it as being high. I thought it was just using the neutral to drive the tester but, as AJJewsbury says, it is testing the L-N as part of the L-E test. It seems, then, that the L-N current is in amps rather than mAs.


    Still can't see how 15mA L-N is tripping all the RCBOs on the 2 wire though. They all passed the 1/2 test when RCD tested. If they are doing this on the test surely there would be a problem with nuisance tripping on loads with earth leakage up to that same amount which would render those RCBOs useless in many applications.
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