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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?


  • 0
    Just re-read my last post as I remembered I was going to try something out but couldn't remember what.

    That's the last time I go on any forum after drinking. Lo current Zs test with the type A setting. That's for RCD tests. Good job I drink after work and not before.

    Surprised no one picked up on that though.

    Don't drink and drive?

    More like don't drink and post.
  • 0
    At least one other poster often composes with an acoholic drink in hand * .


    I'm going to find the 15 min edit window hard to work with.


    Cheers


    (*) One whom your current interlocuter often refers to in the first person singular in fact. (aka perpendicular pronoun according to 'yes minister')

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