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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?


Parents
  • 0
    Today I tried the L and E swap on an RCBO type A after installing some sockets in a container. It didn't work. 2 lo test worked fine, gave me the result. Then I thought I'd try Zs's idea or swapping the leads (or just changing the position of the probes; same thing). It tripped the RCBO. So i can't see how that method can be used to overcome the original issue.


    Also, got someone to look at the RCBOs in question while they were there doing something else. They are type A. Now I realise something that I had done wrong. I can't believe that I had done this after advising others about changing the tester setting from AC to A when they had problems. ("Change it from sine wave to sine wave with pulsating DC. What do you mean "what do you mean"? OK. Change it from squiggly line to squiggly line on tits").

    Mind you, it was the electrical laborer who was actually doing the Zs tests but that is no excuse as I should have realised. The tests were being done on AC setting. I can't believe I made such a stupid mistake. Unfortunately, I can't confirm that at the moment but there is still the server room and main switchboard with integral DBs to finish off and hopefully, it will be me that does them so I can test our whether this was the issue. I will post back on here if I can do that.

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  • 0
    Today I tried the L and E swap on an RCBO type A after installing some sockets in a container. It didn't work. 2 lo test worked fine, gave me the result. Then I thought I'd try Zs's idea or swapping the leads (or just changing the position of the probes; same thing). It tripped the RCBO. So i can't see how that method can be used to overcome the original issue.


    Also, got someone to look at the RCBOs in question while they were there doing something else. They are type A. Now I realise something that I had done wrong. I can't believe that I had done this after advising others about changing the tester setting from AC to A when they had problems. ("Change it from sine wave to sine wave with pulsating DC. What do you mean "what do you mean"? OK. Change it from squiggly line to squiggly line on tits").

    Mind you, it was the electrical laborer who was actually doing the Zs tests but that is no excuse as I should have realised. The tests were being done on AC setting. I can't believe I made such a stupid mistake. Unfortunately, I can't confirm that at the moment but there is still the server room and main switchboard with integral DBs to finish off and hopefully, it will be me that does them so I can test our whether this was the issue. I will post back on here if I can do that.

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