RCD testing

Discussion in 'Electricians' Talk' started by acwizard, Nov 7, 2003.

  1. acwizard

    acwizard New Member

    What is the best way to test RCD's in consumer units? Is it possible to hire RCD test equipment?
  2. Bewlec

    Bewlec Member

    i dont think you can hire test equipment.
    press the test button on the rcd to check operation.
    if you are not happy with that you will have to get it tested to see if it is tripping in the correct times for safety reasons
  3. neil-sparks

    neil-sparks New Member

    I'm not 100% certain but believe you can actually now hire RCD testers from Edmundsons(electrical wholesalers) or Newey and Eyre
  4. acwizard

    acwizard New Member

    Thanks for this reply
  5. lastword

    lastword New Member

    You can hire test equipment from www.Instrotech.com they are based in Watford.

    The test button on the RCD is a functional / confidence test only.

    A full test involves ramping or stepping the test current and measuring at what point the RCD operates and how long it takes to operate. eg a 'standard' 30mA RCD must not open at 15mA, must open within 200mS at 30mA, and open within 40mS at 150mA. There's more, but I won't bore you.

    You can expect to pay around £300 for an RCD tester, more if it combines other functions.

    It will probably cost more to hire than getting a friendly local Sparky to come and do it for you.
  6. acwizard

    acwizard New Member

    Thanks lastword
  7. chtechie

    chtechie Member

    If you're near Milton Keynes I can do your RCD test for you - Drop an Email to ch-techie@ntlworld.com
  8. Divecomuk

    Divecomuk New Member

    You can get a combined RCD/Socket tester from Maplins.
    Probably not as good as ones you can hire. But for 9.99
    it's worth looking at!
  9. supersparky

    supersparky New Member

    i dont think that will do all the tests some how, either that or im missing something when i brought mine for £210?

  10. acwizard

    acwizard New Member

    Thanks chtechie
  11. lastword

    lastword New Member

    The RCD tester from Maplin:

    1. Only checks RCDs up to 30mA. If you have a TT supply (don't ask!) the main RCD in your CU should be a 100mA device.
    2. Only checks trip current levels, not timing.

    I warned you there was more and it could get boring, but here goes:-
    Testing for both operating current and time is important because RCD protection relies on limiting the time for which a fault current flows, not the current itself. A 30mA RCD doesn't limit the fault current to 30mA. It simply starts to open as the current ramps through 30mA on it's way to something like 0.25A for someone with wet hands, or 1A for someone in a bath. A quarter of an Amp for 40mS is unpleasant, but is unlikely to do you any harm. A quarter of an Amp for half a second will very likely kill you.

    It's like touching your hand on a boiling kettle. If you're quick there's no harm, leave it there and you get a blister. You get 100C in both cases, but time makes a big difference.
  12. unphased

    unphased Screwfix Select

    ......A quarter of an Amp for 40mS is
    unpleasant, but is unlikely to do you any harm. A
    quarter of an Amp for half a second will very likely
    kill you.....

    Just to clarify on amperage that can kill. 50mA is enough to kill..that is 50 thousandths of an amp. A quarter of an amp is very high. I think you are getting a little confused here with fault current tripping RCD's and time characteristics. In fault situations we need as big a current as we can get ie. hundreds of amps. The bigger the amperage the quicker devices will operate. This is why when we install a circuit we MUST check the earth fault loop impedance. If resistance is too high the current is reduced and in the worst cases will be insufficient to operate overcurrent devices. This is why in TT systems RCDs are so important because earth fault loop impedances are poor and overcurrent devices operate less efficiently.

    In the case of an RCD the 30mA/100mA/200mA etc is an indication of sensitivity to current, however the characteristics of the device are such that it will trip in split seconds irrespective of the rating unless it is an S type which is designed to have a delay. The difference in mA rating enables circuits to be protected with the most appropriate rating. Their use in split load distribution boards affords us with an ability to discriminate between the circuits so that loss of circuits under a fault condition due to an RCD tripping can be controlled rather tripping out an entire installation.
  13. lastword

    lastword New Member

    I don't think it's me that's getting confused here unphased. The original post asked about testing RCDs and subsequent posts were suggesting the possibility of some 'reduced function' testers. My point was about the importance of testing for both operating current and time. Thus my example of '40mA for 40mS....' to emphasis that point.

    Your last paragraph is confusing me. You appear to be saying that different sensitivity (30mA, 100mA, etc) is used to provide discrimination for cascaded RCDs. That's wrong. It's the time delay in S type RCDs that provides discrimination (and that's another reason for testing operating times).
  14. unphased

    unphased Screwfix Select

  15. sparky Si-Fi

    sparky Si-Fi Well-Known Member

    Youir best bet is to get a sparks in,as it involves live test equipment and if you are not competent to have 230/240VAC going up your arm,i would do that.

    when you are testing RCD's which are designed to give supplementry protection against shock ie. 30ma there are in fact 2 tests which need to be performed,the later being for the device above

    1 Is the normal tripping current of the device (namely 30Ma)which must dis-connect within 200ms
    2. Is what we call the fast trip or *5 the tripping current which should not exceed 40ms

    3The 'test' button which is on the RCD only tests the internal mechanics of the device,it does not test the 'healthyness' so to speak of the earth fault loop

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