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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

    Tom

    If the RCD is connected to the supply side of the transformer it will not trip if you carry out the test on the secondary side of the transformer.

    Also if the transformer does not have a connection to earth on the secondary side (an isolating transformer) the RCD will not trip as there is no earth fault path.

    You can test the function of the RCD by doing an "up and down test".  This requires the RCD tester to do a 2 wire RCD test, your Megger  MFT will do this.Se the meter up to do an RCD 2 wire test. Place one probe on the incoming neutral to the RCD and the other probe on the outgoing line press the test button and take the reading.

    This test only tests the correction function of the RCD it does not simulate a fault to earth. Before carrying out an RCD test you should carry out a loop impedance test first to ensure there is an earth path. Modern test instruments like your Megger MFT will abort the RCD test if they detect a voltage rise on the earth and give you a warning.

    Hope this is helpful?

    Regards

    JP

Reply
  • 0

    Tom

    If the RCD is connected to the supply side of the transformer it will not trip if you carry out the test on the secondary side of the transformer.

    Also if the transformer does not have a connection to earth on the secondary side (an isolating transformer) the RCD will not trip as there is no earth fault path.

    You can test the function of the RCD by doing an "up and down test".  This requires the RCD tester to do a 2 wire RCD test, your Megger  MFT will do this.Se the meter up to do an RCD 2 wire test. Place one probe on the incoming neutral to the RCD and the other probe on the outgoing line press the test button and take the reading.

    This test only tests the correction function of the RCD it does not simulate a fault to earth. Before carrying out an RCD test you should carry out a loop impedance test first to ensure there is an earth path. Modern test instruments like your Megger MFT will abort the RCD test if they detect a voltage rise on the earth and give you a warning.

    Hope this is helpful?

    Regards

    JP

Children
  • 0 in reply to John Peckham

    Hi JP, The RCD is on the secondary side of the transformer and I'm glad to hear someone else say that the RCD won't trip without an earth path as that seemed like common sense to me! In an installation like this is the RCD actually going to give you any protection at all?

    Thanks for your help!

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