Rcd fault

[wsoppitt]

Hello guys and girls , I have a question if I may.
Have to go and look at a intermittent fault on rcd , I'm told the circuit holds whilst using light appliances ( TV, etc ) but randomly trips the rcd on heavier loads ( hair dryer ) .
The manual operation of the test button also doesn't work when pressed.
1) could be a faulty rcd
2) could be N - E fault
I will test the button with loads disconnected 1st to check that but if that proves OK I will then start looking for the N - E fault.
My question is because the fault is intermittent what would be the best process for finding it , not had much experience of intermittent fault finding.
Thanks in advance.

 

[James]

once you have proved the rcd is not faulty,

odds are you are looking for a N to E fault.
that is unlikely to be intermittent, only the rcd tripping symptom is intermittent.

disconnect each circuit fed from the rcd (one at a time) test for continuity between N and E
when you have found the one that has continuity, you have probably found the fault.

I would start with the following in this order

oven/hob
immersion heater
outside lights
anything else outside
anything that looks a bit DIY

 

[littlespark]

If the button does not work, then it IS faulty.

 

[loz2754]

If the button does not work, then it IS faulty.

Not necessarily. A neutral to earth fault can prevent the test button from tripping the RCD.
With the outgoing line and neutral disconnected from the RCD, if the test button does not work then it is faulty.

 

[HappyHippyDad]

Not necessarily. A neutral to earth fault can prevent the test button from tripping the RCD.
With the outgoing line and neutral disconnected from the RCD, if the test button does not work then it is faulty.

This has happened to me twice now.
A really bad N-E fault had stopped the RCD test button from working (whilst loads were attached). It worked fine when loads disconnected.

I bet the RCD button works fine when the OP has disconnected the loads.

 

[pc1966]

I think it is safe to say if the test button does not work then there is a fault, but it might not be the RCD itself!

 

[HappyHippyDad]

It also wouldn't surprise me if this was a TT.

@davesparks explained to me a while back how a relatively high Ra (lets say 170ohms) may meet the regulations, but had an effect on whether the RCD would trip given a bad enough fault (or something similar to this). I wish I could find the thread as I remember it being one of those eureka moments when it all clicked in to place. Can't remember the maths at all now, but I think it was mainly based on the importance of a good Ra.

edit.. found the thread.. Why isnt the RCD tripping? | Page 3 | on Free Electrical Advice - Electricians Forums - https://www.electriciansforums.net/threads/why-isnt-the-rcd-tripping.176881/page-3#posts

 

[pc1966]

@davesparks explained to me a while back how a relatively high Ra (lets say 170ohms) may meet the regulations, but had an effect on whether the RCD would trip given a bad enough fault (or something similar to this).

That will not impact on the self-test button as they divert the test current L-N around the sense coil (e.g. L load to N supply).

What a high Ra does impact on is the ability of the RCD to trip on a N-E fault, as if Ra is much higher than Rn (which is practically always the case) such a small proportion of the load current along the N will bypass the RCD by going to E so it is not seeing that as faulty.

TT should still trip on a L-E fault as then Ra is low enough (or should be, by design!) that you get >In flowing at <50V to true Earth.

 

[pc1966]

I would add though that a typical good TT rod of 100 ohms only needs 3V to have 30mA flowing and so trip an RCD, you might find that sort of N-E voltage on a large system irrespective of the local volt-drop over neutral so in practice it will fire quite happily.