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Questions about RCDs and Earth Leakage Devices

tommatwalker

Good morning all,

I work in electrical and control systems safety and do a lot of annual inspections on machinery (specifically amusement rides) in the UK and around the world. A problem I come across sometimes is the testing of RCDs and earth leakage devices. There are a few scenarios that I'm hoping somebody would be able to help me out with!

1) I have recently come across a couple of installations with RCDs installed on systems that are on isolating transformers. Testing to earth does not trip them, testing between line and neutral doesn't trip them but their test button does trip them. Can anyone recommend a reliable method of testing them using the meter to get a trip time for them? In this scenario if the test button does not trip them is that a sign that the device itself has failed entirely or just a quirk of the installation?

2) I have recently attempted to test an RCD connected to a centre tap transformer (outside of the UK) which didn't function at all. The transformer is fed off a 200V Delta Delta supply and appears to be centre tapped and at the RCD I'm getting 60V to earth from the line and 48V to earth from the Neutral. Again I can't find a reliable way to test this but in this instance the test buttom doesn't work either. Is this a sign that this RCD has failed and should be replaced or am I not testing it right?

3) Intermittently when testing RCDs with my meter I can't get the RCD to trip when testing at the terminals but it will trip when I go further down the line. Can somebody explain to me why this would be as I've never found any rhyme or reason behind it!

For reference I am using a Megger MFT1741 meter if that makes any difference.

Thanks in advance! I am aware that supplies, transformers and earth leakage is a bit of a gap in my knowledge which I am looking for a training course or something to attempt to fill so any help that you guys can give would massively help me out!

Regards,

Tom

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  • 0

    1) Setting aside for a moment wondering what on earth what the fault condition is that the RCD on a non earthed transformer winding is going to protect against, the test method that is most likely to work is a variation of the one used to test an RCD downstream of another, without tripping the first one. That is to use the tester between the outbound live (like normal) and the incoming neutral (instead of earth) . This 'Z'  test (because of the directions of current flow) is very similar to how the test button works if you look at a device with the little sketch on the side.

    3)Some designs of compact RCD/RCBO use the outbound wires as the winding in the current transformer , and the core is hidden in the plastic terminal tunnel and the screw terminals are in effect putting the test current in on the wrong side.

    Testing from the load end or by vampire probes into the insulation of the outbound cables (naughty but useful) will give you a reliable trip.

    I'll have a think about 2

    Mike.

    Edit that was a cross post - but broadly in agreement with JP anyway.

  • 0 in reply to mapj1

    perhaps the easiest way to test the RCD itself is OK on the non-standard transformer is to take it out and wire it into a mains lead to test it.

    However, the self test may not be working if the pole to pole voltage is not enough to operate the internals - it sounds like it might be a 220V RCD on  a more or less 110V split phase circuit - in which case it is really the wrong sort of breaker for that application.

    Mike

  • 0 in reply to mapj1

    Hi Mike, thanks for this that's very helpful I can't see what protection you are getting from having and RCD in this circuit and I've asked the question before of manufacturers without getting much of a useful answer..... Thanks for that test method, it's not something that I was aware of but it is something that I will definitely use going forward. As for your answer for number 3 that is definitely news to me and explains an awful lot. I'll investigate the best method for testing them going forward!

    Tom

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  • 0 in reply to mapj1

    Hi Mike, thanks for this that's very helpful I can't see what protection you are getting from having and RCD in this circuit and I've asked the question before of manufacturers without getting much of a useful answer..... Thanks for that test method, it's not something that I was aware of but it is something that I will definitely use going forward. As for your answer for number 3 that is definitely news to me and explains an awful lot. I'll investigate the best method for testing them going forward!

    Tom

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