Special non rcd socket for pat testing

Discussion in 'Electricians' Talk' started by tina lucinda lane, Jan 13, 2021.

  1. tina lucinda lane

    tina lucinda lane Screwfix Select

    Yeah that had also been my thought
  2. Teki

    Teki Screwfix Select

    It usually passes through but the output is dereferenced from the neutral-earth link on the primary side. By joining two transformers back-back as @MGW did, you are essentially creating an isolation transformer.

    Here is a good website that explains:

  3. Lectrician

    Lectrician Screwfix Select

    Just spend to £60 and get the kit to do it correctly. Cheaper than a new circuit.
  4. tina lucinda lane

    tina lucinda lane Screwfix Select

    I can get the new circuit installed for £15 as well it at shortest will only run about a metre before becoming a socket, rest then legally can be installed by myself as its temporary in the interest of the law as its supplied by a plug if I'm not mistaken
  5. tina lucinda lane

    tina lucinda lane Screwfix Select

    As, the kit doesn't guarantee that it won't trip a main rcd
  6. The Happy Builder

    The Happy Builder Active Member

    So you’re then back installing a socket supplied from the transformer with the earth connected to the neutral, so is there an advantage in having the the transformer?
  7. The Happy Builder

    The Happy Builder Active Member

  8. The Happy Builder

    The Happy Builder Active Member


    There’s the diagram from the link that was posted, the second transformer being the isolation transformer.

    So just how would you connect a RCD to the isolation transformer to test it?

    Presumably by making a neutral to earth link down stream of the isolation transformer to connect a socket to plug the RCD into?
  9. Teki

    Teki Screwfix Select

    That's what it's designed for.

    Isolation transformers like that pass the earth from input plug to output socket. They were commonplace in repair workshops for TVs etc. No need for an additional socket, just plug the tester into the transformer secondary socket.

    These diagrams are more relevant from the webpage I linked to, in particular the second:


    The imbalance to test the plug in RCD is introduced by the tester and doesn't have any relevance on the deferenced neutral-earth link.
    Last edited: Jan 14, 2021
  10. Lectrician

    Lectrician Screwfix Select

    £15? Umm. Doubtful. I can't see a spark wanting to allow it either.
  11. Lectrician

    Lectrician Screwfix Select

    It does, they're used all the time. That, or construct your own test lead as described. Most sparks have that in their van.
  12. tina lucinda lane

    tina lucinda lane Screwfix Select

    He's family so family rate
  13. tina lucinda lane

    tina lucinda lane Screwfix Select

    But this has all been useful as it gives me great options that I hadn't considered thanks everyone
  14. The Happy Builder

    The Happy Builder Active Member

    Just to be clear, we are talking about RCD testing as part of PAT testing, in particular these Plug In RCDs


    The test button is wired back to the input terminals through a resistor, like it is in a fixed installation RCD, so a test button can test the RCD without a reference to earth to create an imbalance.

    When you plug a RCD tester in to one of these it doesn’t have access to the input terminals, so has to use the earth to create the imbalance and that trips both the plug in RCD being tested and the installation RCD that is upfront of it.

    To get the RCD tester to work downstream of the isolation transformer you would have to wire the test socket with an earth connection to the neutral as in figure 3 above, the one where is says the operator is unsafe :)

    But you don’t actually then need the connection to the real earth from the downstream side of the isolation transformer.

    However once you have made that connection to neutral you don’t actually need the isolation transformer as the upfront RCD won’t trip anyway whilst you are testing and if you don’t fit the isolation transformer you are actually still protected by the installation RCD.
  15. The Happy Builder

    The Happy Builder Active Member

    Just to add to that, it could be a lot easier depending on how the test lead is connected to the RCD tester.

    If you have test leads like this


    buy an additional set, then use one set to plug the live and neutral from the RCD tester into the Plug In RCD you are testing that is plugged into one side of a double socket, then use the second set to plug the earth from the RCD tester into the neutral of the other side of the double socket.
    tina lucinda lane likes this.
  16. The Happy Builder

    The Happy Builder Active Member

    I should have told you that yesterday :rolleyes:
  17. The Happy Builder

    The Happy Builder Active Member

    I was also taught to always do an earth loop test before doing a RCD test, because of the current you are introducing into the earthing system, which may be hazardous to people using the installation.

    You are using lethal currents to test RCDs that runs out through the earthing system, which is why when I’m testing other people’s equipment at home I’d rather run the test current out through the neutral, whilst maintaining the RCD protection at the socket.
    Last edited: Jan 14, 2021
    tina lucinda lane likes this.
  18. tina lucinda lane

    tina lucinda lane Screwfix Select

  19. tina lucinda lane

    tina lucinda lane Screwfix Select

    Yes that type and the plug version as well as extension leads with and in built rcd at there socket end (rare I know) as well as one's fitted with an inline rcd on them as well btw if it helps this is the rcd tester I own robin digital rcd tester kmp5404dl which has both an iec socket on it also has a lead to provide a reference to earth as well hence the need for a method to use it without tripping a main board rcd and knocking out part or all of an installation
  20. MGW

    MGW Screwfix Select

    Using a shaver output you have a IT supply so no problem tripping, the question is if the 200 VA is enough for your tests.

    But the question is if only using line and neutral you know you can make a rig to test the RCD so you just keep the rig in your tool kit, same as I have an old BA22d to 5 amp adaptor, very handy for testing lights, and it never leaves my test kit.

Share This Page