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Discuss RCD's in series and how to test in the Electrical Forum area at ElectrciansForums.co.uk.

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  1. leep82
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    leep82 Regular EF Member

    Location:
    Stoke on Trent
    Business Name:
    L.M.P Electrical
    Im currently working on a site where im testing the supplies to the site cabins. Each cabin has its own 6 way single phase DB with a double pole RCD as a main switch. The DB's are then fed via commando plug and socket arrangement from a 32a RCBO within a TP&N DB. When the cabins were originally 1st fixed the dead tests were performed and recorded ( r1-r1, r2-r2, rn-rn, R1 + R2 and IR ). Now i am in the process of energising the cabins and performing my live tests and as you can see have two RCD's connected in series.

    In each cabin i have turned the main RCD switch off and in turn performed my RCD test on the supply side to prove the tripping times of the 32a RCBO back in the main DB. My question is will the readings ive obtained here be suitable to record as my RCD tripping times when i fill in the test results for my final circuits within the cabins? All the readings are within permitted values, only when i turn on the main RCD and try to perform the same test on the final circuits the RCBO in the main DB trips at x1 and the main RCD in the cabin DB trips at x5. Meaning that i havnt been able to prove that the main RCD in each cabin trips out at within the required time for x1.

    Im not sure why there are two RCD's connected in series like this as i havnt done the design. I would have thought the RCBO in the main DB would have been more than adequate
     
  2. Rpa07
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    Rpa07 2000 posts - only 46379 behind Telectrix! Electrician's Arms

    Location:
    Bristol
    Business Name:
    Ebenezer electrical
    Is the submain SWA? Does it need protecting? One way around it if it's ok as a special location requirement.
     
  3. westward10
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    westward10 In echoed steps I walk across an empty dream. Electrician's Arms

    Location:
    Northamptonshire
    It is standard for cabins to have rcd protection and obviously additional rcd protection is provided for the supply sockets. I know what I would do but it probably isn't advice I would give on the forum.
     
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  4. leep82
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    leep82 Regular EF Member

    Location:
    Stoke on Trent
    Business Name:
    L.M.P Electrical
    Yes the submain is a 4 core 35mm swa. TT system. A couple of the cabins were originally fed from a generator and so live readings were taken at the time. Now as more trades get onto site there is a need for the remaining cabins to be completed. I was going to use the readings taken for the the two cabins that were originally in use, would this be ok?
     
  5. westward10
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    westward10 In echoed steps I walk across an empty dream. Electrician's Arms

    Location:
    Northamptonshire
    Are the cabins which were originally in use the same cabins you are now needing to test.
     
  6. leep82
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    leep82 Regular EF Member

    Location:
    Stoke on Trent
    Business Name:
    L.M.P Electrical
    Yes only
    yes only two of them though, 9 cabins in total. The two that were already tested were fed from a generator which is no longer in use, and in both cases the supplies to each have been reterminated as the original cabling was not long enough
     
  7. westward10
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    westward10 In echoed steps I walk across an empty dream. Electrician's Arms

    Location:
    Northamptonshire
    How long since they were last connected then reconnected.
     
  8. leep82
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    leep82 Regular EF Member

    Location:
    Stoke on Trent
    Business Name:
    L.M.P Electrical
    Only a couple of months
     
  9. westward10
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    westward10 In echoed steps I walk across an empty dream. Electrician's Arms

    Location:
    Northamptonshire
    Well you have obtained the x5 test which is the requirement for additional protection so personally I would state the x1 as N/V as fault protection is provided at source.
     
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  10. leep82
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    leep82 Regular EF Member

    Location:
    Stoke on Trent
    Business Name:
    L.M.P Electrical
    Thanks for the advice
     
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  11. westward10
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    westward10 In echoed steps I walk across an empty dream. Electrician's Arms

    Location:
    Northamptonshire
    You never know if you try it enough times you may eventually obtain a x1 reading:D
     
  12. Leesparkykent
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    Leesparkykent You Rock Gmes Staff Member Trusted Advisor

    Location:
    Kent
    No RCD protection for the tails entering the 3phase DB that's TT? Would of been cheaper and possibly a better design to have an upfront time delayed RCD in a plastic enclosure with the tails run through it which would provide fault protection. Preferably bs1361 fuse carriers in the 3phase DB supplying the distribution circuits then 30mA RCD's as the main switches in the cabins.
     
  13. Wilko
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    Wilko Electrician's Arms

    Location:
    Berkshire
    Business Name:
    Wilko Electrics
    Hi - if you are in control of the site that may perhaps allow you a temporary wiring option to perform your testing?
    When I've struck this issue I've been fortunate enough that both have tripped during testing. Feeling optimistic I'd persist a while as Westward suggests :)
     
  14. westward10
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    westward10 In echoed steps I walk across an empty dream. Electrician's Arms

    Location:
    Northamptonshire
    This sounds a better set up however, it is now common practice for cabins to be connected by way of 32A couplers and assuming they are 32A in the OPs case they require additional rcd protection in compliance with Reg 415.1.1, that being 30ma.
     
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  15. Lucien Nunes
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    Lucien Nunes Mercury Arc Rectifier Trusted Advisor

    Location:
    London
    I've always overcome this problem by testing the second RCD using the test probes across its load-side line and supply-side neutral. The tester doesn't care whether the test current returns to earth or neutral, it should still perform a valid test on that RCD provided only the line goes through it. But because the test current returns via the the first RCD's neutral, that sees the tester as a legitimate load and shouldn't trip.
     
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  16. leep82
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    leep82 Regular EF Member

    Location:
    Stoke on Trent
    Business Name:
    L.M.P Electrical
  17. leep82
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    leep82 Regular EF Member

    Location:
    Stoke on Trent
    Business Name:
    L.M.P Electrical
    IMG_5577.JPG Just to be clear of the complete setup heres a little picture. The armoured cable that you can see is the 35mm 4 core that supplys the TP&N DB which in turn supplies the site cabins
     
  18. leep82
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    leep82 Regular EF Member

    Location:
    Stoke on Trent
    Business Name:
    L.M.P Electrical
    View attachment 37920 Just to be clear of the complete setup heres a little picture. The armoured cable that you can see is the 35mm 4 core that supplys the TP&N DB which in turn supplies the site cabins View attachment 37920
     
  19. leep82
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    leep82 Regular EF Member

    Location:
    Stoke on Trent
    Business Name:
    L.M.P Electrical
  20. leep82
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    leep82 Regular EF Member

    Location:
    Stoke on Trent
    Business Name:
    L.M.P Electrical
    sorry for the duplicate posts
     
  21. leep82
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    leep82 Regular EF Member

    Location:
    Stoke on Trent
    Business Name:
    L.M.P Electrical
    Just tried this method and it still trips the RCBO in the main DB
     
  22. Lucien Nunes
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    Lucien Nunes Mercury Arc Rectifier Trusted Advisor

    Location:
    London
    With no connection from tester to earth? Interesting, not sure why, anybody?
     
  23. Wilko
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    Wilko Electrician's Arms

    Location:
    Berkshire
    Business Name:
    Wilko Electrics
    If I understand correctly, it should by L and N to RCD load side as normal, but E probe to the incoming N for that RCD. So no test current in the supply earth.
     
  24. Karin
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    Karin Trainee Trainee Access

    Location:
    Kent
    that's how i do it on my kt64 with 3 probes :thumbsup:
     
  25. davesparks
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    davesparks Trusted Advisor

    Location:
    guildford
    It's hard to see from the picture but does that Earth leakage module not have adjustable settings?
     
  26. davesparks
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    davesparks Trusted Advisor

    Location:
    guildford
    I've just googled the part number and it is indeed adjustable, so you can wind its settings up to stop it tripping for the duration of the test.

    I'd suggest that, if it is compliant to do so, the trip should be set to something other than 30mA without time delay.
     
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