RCD testing question

It is not without contention. Personally, I'm with you, but the official counter argument goes the self test button does not require the CPC, so before you test the CPC,  check the RCD functions on an imbalance so when you do use the CPC for the instrument test, it is RCD protected. The idea is that if the CPC is high resistance and the RCD is faulty you may liven up a load of earthed metal for the duration of the test, and the RCD will not disconnect you. Now any modern  tester will limit the time the voltage on a faulty CPC anyway, even without the RCD working as it should, (stopping the test early if the N-E difference exceeds much more than 50V) so the point is a bit moot.

Mike.

(and it is not unknown for a stuck RCD to detonate its internal test resistor, as they are only rated to take the 230V for a few tens of msec)

edited for clarity.

I agree Mike. I have a nasty feeling that this changed because manufacturers were getting "faulty" RCDs back, which were just stuck. Of course it could be the usual H&S brigade who don't understand, but then perhaps that is too kind. I do it the way I always have. If it doesn't work under all conditions it is useless so....

I think the reason these devices get stuck is because nobody reads the rcd instructions attached  to the front of the consumer units which makes clear that the RCD test button  should be pressed periodically. Used to be quarterly, now, I suspect, its whatever the manufacturers recommend.

Is it wise, therefore, to place the responsiblity for the major method of domestic earth fault protection in the hands of electrically non-competent householders?

Why the periodic test ?

The early current balance RCDs had nothing to provide the actuating force except the out of balance current itself, and this lead to a finely balanced 'hair trigger' mechanical arrangement that only needed the lighter components in the grease to evaporate freeze or a thin layer of oxide to form on a critical surface, and the sensitivity would at first greatly reduce, and perhaps not trip at all.

RCDs in barns and similar outbuildings are especially vulnerable and places where there are variations in humidity.

Periodic tripping 'polishes' the movement, and reduces this risk.

The modern kind have a far more bosky, and in many ways simpler movement with more in common with an MCB, This is 'fired' by a solid state switch, and do a require a good fraction of the L-N voltage to be present to trip it. The current balance transformer now only provides a signal for the electronics that then fires the actuator.

So we have eliminated a mechanical delicacy, but replaced it with potential vulnerabilities of an electronic kind and a constant but small dissipation.

Arguably we still need a regular test but there has been a subtle change of reasons.

M

Hi All, thanks for responses,

Agreeing with Mike, I thought that RCD testers limit the time and max voltage rise when testing specifically to avoid danger if doing the test on an unearthed circuit. Of course we would, on a new install carry out the tests in the correct order. I.E. make sure you have an earth path before doing ELI or RCD tests.

This was one of the answers to the question, "why do the tests in a specific order" on the old written 2391 papers before multi choice gave you the right answer to pick from .

I use the example that if you "unstick" a potentially sticky, (as it's never had a three or six monthly test) RCD and then do the one test now required and, Yipee it passes but 3 years later you cut the lawnmower lead and it's sticky again you die. Simples. On older split load arrangements it was not uncommon to see 63A or 40A 30mA  RCD taking the ring circuits, cooker with a socket, shower etc leaving only the lights on the 100A main switch. Leading I believe to someone, (NICEIC maybe?) doing a survey and finding that, a) no one tests them regularly and b) they did not operate in the required time when tested as a good proportion were overloaded. 

Interestingly GN3 still recommends the 1/2 test seemingly in contradiction to BS7671.

Kind regards

Marc

Marc Lambert said:

Interestingly GN3 still recommends the 1/2 test seemingly in contradiction to BS7671.

BS 7671 never specified a 1/2 times test, in reality.

I guess a better question, is whether it continues to be necessary to test RCDs at all, other than pushing the test button?

Hi

I think we all agree that pretty much no-one presses the test button. So yes, I will continue to test RCDs in accordance with BS7671 / GN3 or my engineering judgement.

Many thanks

Marc

Agreed from me ... another part to the 1/2 times test is that BS 7671 requires test method selected in accordance with relevant part of BS EN 61557 - which includes for a "non-tripping test".

BS 7671 doesn't say "don't do the non-tripping test" (the 1/2 times test) - it doesn't in fact say anything about the 1/2 times test, but it does say something about only doing one of the tripping tests.

Given the statement in Regulation 643.1 not being modified by any of the other Regulations, I would argue BS 7671 requires the non-tripping test.

Legh Richardson said:

I think the reason these devices get stuck is because nobody reads the rcd instructions attached  to the front of the consumer units which makes clear that the RCD test button  should be pressed periodically.

I have to confess that I wasn't aware of the need to press the test button until I did my training. In fact, I wasn't really aware that I had RCBOs in my outdoor (under cover, but otherwise exposed) CU. The test buttons had not been pressed for at least 21 years! As a result, when I came to press them, one did nothing and after I opened it manually, it refused to re-latch.

I suspect that the reason for pressing the test button at the end is a combination of doing what you were taught (monkey see, monkey do) and the fact that (reading left to right as you do) it is the last column in the Schedule of Test Results which is for the test button.

Having thought about it, I think press the button first. If it trips, then you have at least proved that the mechanism is sound.

I accept that the right thing is that the test button should be pressed, every 3 months, 6 months whatever.  But how practical is it to expect the householder to do it?

1. How many householders know that they are expected to test RCD’s regularly, and how often? On a mixed board would they know which needed to be tested?
2. It is unrealistic to expect householders to engage an electrician to push the button for them unless they are there anyway.
3. Many householders are uncomfortable “going into” the CU and push a button that  will turn the house power off with a thud. Bad enough resetting an RCD or MCB after a trip, even if they recognise which breaker has tripped.
4. Having tested the RCD, several appliances will need to have their clock resetting. In my house I have to reset the oven, microwave, washing machine and bedside clock/alarm.  Probably others that I will discover when I notice the time display is wrong.

David