Questions about RCDs and Earth Leakage Devices

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

Parents
  • Sometimes RCDs are useful on IT systems with several circuits - on 1st fault the system remains safe and no disconnection is needed for safety (it in effect degrades to TN-S) but on 2nd fault (earth fault from the other pole) on a different circuit - the RCDs will then disconnect the two faulty circuits - leaving the remaining circuits energised and not relying on large fault currents to operate overload protective devices (which you might not have if you're behind a relatively small transformer, or if different circuits have their own independent earth electrodes, or the c.p.c.s between circuits might be relatively long).

     

    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

    Ah but it depends what you mean by "at the terminals" - at least two interpretations are possible - some might say the test conductor connected to the terminal in the same way as the normal outgoing conductor would be, other might be just meaning contact with the screw head. While I suspect many will assume the latter, the former may match the actual words better (and would be the only one to work with certain RCBOs).

       - Andy.

Reply
  • Sometimes RCDs are useful on IT systems with several circuits - on 1st fault the system remains safe and no disconnection is needed for safety (it in effect degrades to TN-S) but on 2nd fault (earth fault from the other pole) on a different circuit - the RCDs will then disconnect the two faulty circuits - leaving the remaining circuits energised and not relying on large fault currents to operate overload protective devices (which you might not have if you're behind a relatively small transformer, or if different circuits have their own independent earth electrodes, or the c.p.c.s between circuits might be relatively long).

     

    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

    Ah but it depends what you mean by "at the terminals" - at least two interpretations are possible - some might say the test conductor connected to the terminal in the same way as the normal outgoing conductor would be, other might be just meaning contact with the screw head. While I suspect many will assume the latter, the former may match the actual words better (and would be the only one to work with certain RCBOs).

       - Andy.

Children
  • Sometimes RCDs are useful on IT systems with several circuits

    And in this case, Test Method 2 (upstream/downstream) may be the only option, because in a healthy circuit in an IT system, there is no current return path through a sufficiently small resistance to permit the test to work.

    Ah but it depends what you mean by "at the terminals" - at least two interpretations are possible - some might say the test conductor connected to the terminal in the same way as the normal outgoing conductor would be, other might be just meaning contact with the screw head. While I suspect many will assume the latter, the former may match the actual words better (and would be the only one to work with certain RCBOs).

    That's a valid point, but does lead to the question of how to do the test safely if there is restricted access to the entry to the terminal for a test probe. 

    This may very well mean that, for applications where the upstream/'downstream method is necessary, such a product should not be selected, or other means provided to carry out such a test, since the designer of an electrical installation or machinery has a duty to ensure that the installation or machinery can be safely maintained.