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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

    For point 3. I am aware that some makes of rcd stipulate not to test on the terminals of the rcd as it will not work. Sorry but i don't remember the make or makes. Maybe next time it happens make a note of the make and model and ask the manufacturer or google some instructions.

    Gary

  • +1 in reply to aligarjon
    aligarjon said:
    For point 3. I am aware that some makes of rcd stipulate not to test on the terminals of the rcd as it will not work. Sorry but i don't remember the make or makes. Maybe next time it happens make a note of the make and model and ask the manufacturer or google some instructions.

    That's very interesting, because 'at the terminals' is exactly where PD IEC/TR 62350 Guidance for the correct use of residual current-operated protective devices (RCDs) for household and similar use recommends the test is carried out (using, I would add, the upstream/downstream test method)! Clause 6.3 says:

    The testing current should be applied between the upstream and downstream terminal of the RCD.

    Perhaps they are pointing out that, following installation, a test to PE when carried out at the terminals might not operate as expected (or the test may not indicate as being completed properly with instruments to BS EN 61557-6 due to stored energy on the load side of the RCD)?

    If you do recall, , please let us know.

  • 0 in reply to gkenyon

    Thanks guys that's really helpful information. I'll start having a look at the manufacturers of RCD that cause this issue and the one's that don't

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