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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
    AJJewsbury

    I remember around 2003 using a Robin D-Lok on a new build hospital after using it to test Zs of circuits on basic RCDs. I then started to test Zs on circuits with RCBOs. These testers didn't have batteries and the display came to life when connected to the circuit under test. Every time I pressed the test button the display would disappear and come back after a delay of 1 to 10 seconds.

    It took an hour to work out what was happening.

    I suddenly occurred to me that the DC lock would not be working on a device that is using electronics to amplify the residual imbalance. I couldn't work out why, though, the power was going off then back on after a delay.

    So I decided to go down to the DB from the roof and there was one of the apprentices standing there. "The breakers keep on tripping so I reset them". I could have killed him.


    Zs

    I have sometimes tested with the probes the wrong way round accidentally but found it still tested out the same when I realised and swapped them. I wouldn't have thought of swapping them round in the case of this spurious tripping. Thanks for that. Unfortunately, I may not be going back there so I will be unable to try that out. As this is the first time this has happened it must be quite rare and I would need to try it on that specific DB were I know it's happening.
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  • 0
    AJJewsbury

    I remember around 2003 using a Robin D-Lok on a new build hospital after using it to test Zs of circuits on basic RCDs. I then started to test Zs on circuits with RCBOs. These testers didn't have batteries and the display came to life when connected to the circuit under test. Every time I pressed the test button the display would disappear and come back after a delay of 1 to 10 seconds.

    It took an hour to work out what was happening.

    I suddenly occurred to me that the DC lock would not be working on a device that is using electronics to amplify the residual imbalance. I couldn't work out why, though, the power was going off then back on after a delay.

    So I decided to go down to the DB from the roof and there was one of the apprentices standing there. "The breakers keep on tripping so I reset them". I could have killed him.


    Zs

    I have sometimes tested with the probes the wrong way round accidentally but found it still tested out the same when I realised and swapped them. I wouldn't have thought of swapping them round in the case of this spurious tripping. Thanks for that. Unfortunately, I may not be going back there so I will be unable to try that out. As this is the first time this has happened it must be quite rare and I would need to try it on that specific DB were I know it's happening.
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