faulty RCD ?

maxj · Posted: Tue Oct 18, 2005 8:35 am Post Subject: faulty RCD ?

MY house was built in 75 with PVC cabling but with wire fuses and I understand the supply to be PME so I thought upgrading the consumer unit would be a good idea. Had the unit fitted over the week end inc a new shower fitted, originaly all as a split load. But as soon as some thing was switched on (RCD side) the RCD tripped. Even after disconecting all the RCD side circuts from the mcb's leaving only the new shower circut conected - resetting the RCD and then switching on the shower tripped the RCD.
Interestingly if the earth connection was removed from the consumer unit to the main fuse the RCD did not trip on a load being applied. The consumer unit is now wired without the RCD with just MCBs.
any ideas at what could be the problem ?
thanks

Adam_151 · Posted: Tue Oct 18, 2005 8:39 am Post Subject:

When you disconnected the circuits from the CU, did you disconnect the neutrals as well?, sounds to me like a neutral to earth fault, which would explain why when the main earth was removed, it didn't trip (This is not a recognised way of testing things, and is dengerous, btw, and I'd advise you not to conduct this test again)

maxj · Posted: Tue Oct 18, 2005 8:55 am Post Subject:

my sparky mate fitted the CU for me as a 'weekender' so i can't say for certain if the neutrals were also disconnected - but i'd guess not.
Would reconnecting back as a split load with all the nuetrals disconected and then connecting the nuetrals one by one identify which circut is the faulty one ?

thanks

Adam_151 · Posted: Tue Oct 18, 2005 9:02 am Post Subject:

maxj wrote:

Would reconnecting back as a split load with all the nuetrals disconected and then connecting the nuetrals one by one identify which circut is the faulty one ?

thanks

Yes, but make sure you do it on a circuit by circuit basis and not a cable by cable basis, obviously considering you'll most likely have ring finals in there...

Oh, and make sure you turn the breaker off for the circuit with disconnected neutrals.

This is not the ideal way of doing the test, but unless you own a insulation/continutity tester... (your sparky mate should have one though), he should have used it before fitting the cu tho...

maxj · Posted: Tue Oct 18, 2005 9:22 am Post Subject:

yea, you'd think a mate would do a 'puker' job and bring all his kit would'nt you....

one more question, if the fault is as you suspect, is my current instalation a cause for some imeadiate concern or something that will wait a few days

many, many thanks for your help

Adam_151 · Posted: Tue Oct 18, 2005 9:49 am Post Subject:

It has a 'potential' to be dangerous, there is a current flowing between neutral and earth and the connection is likely to be a bad one (bad connections can overheat), but because there the difference it potental between neutral and earth is likely to be small (especially on a PME system) the current is likely to be small, and its probabably been like that for years...

That said, it should be corrected as soon as possible, moving the damaged circuit to the non rcd side of the cu would fix the symtom, and if need be, can be used as a short term measure, but you need to find and fix the problem, I guess this is kinda obvious, but I'm saying it anyway just incase, as even some sparkys (admittedly cowboys) have sorted this kind of thing out by just fixing symtoms and not the problems

My advice would be to fix it ASAP, but IMHO, an extra day or two (considering the fault has probably been there for years) is unlikely to do much harm.

Once you have identified the circuit at fault, things to look for will be things like neutrals in backboxes that have been 'pinched' behind screws, cables that have been 'nipped' or have been nailed / screwed into , you'll probably have to split the circuit up to narrow down the fault

Spark123 · Posted: Tue Oct 18, 2005 11:27 am Post Subject: Re: faulty RCD ?

maxj wrote:

Interestingly if the earth connection was removed from the consumer unit to the main fuse the RCD did not trip on a load being applied.

Eeek,
Please do not try this ever again as it is extremely dangerous given the assumption you had the power turned on to apply a load.

ban-all-sheds · Posted: Tue Oct 18, 2005 10:56 pm Post Subject: Re: faulty RCD ?

Spark123 wrote:

maxj wrote:

Interestingly if the earth connection was removed from the consumer unit to the main fuse the RCD did not trip on a load being applied.

Eeek,
Please do not try this ever again as it is extremely dangerous given the assumption you had the power turned on to apply a load.

If you're the only person in the house, and you do it in a controlled way, i.e. just to see if the trip stops, and whilst disconnected you are b****y careful what you touch, how so "extremely" dangerous?

__________________
I mustn't warn people that the "experts" on the plumbing forum can't be trusted to tell the truth.

Spark123 · Posted: Tue Oct 18, 2005 11:26 pm Post Subject: Re: faulty RCD ?

ban-all-sheds wrote:

Spark123 wrote:

maxj wrote:

Interestingly if the earth connection was removed from the consumer unit to the main fuse the RCD did not trip on a load being applied.

Eeek,
Please do not try this ever again as it is extremely dangerous given the assumption you had the power turned on to apply a load.

If you're the only person in the house, and you do it in a controlled way, i.e. just to see if the trip stops, and whilst disconnected you are b****y careful what you touch, how so "extremely" dangerous?

Extremely dangerous in the context of there is already a fault on his install, this bloke is DIY, not an electrician and most likely does not know the control measures or how to implement them which need to be invoked to do this properly and safely. The main earth in an installation (eebads) is installed to ensure the operation of a protective device in the event of a fault and to limit the rise in potential above earth potential of exposed-conductive parts during a fault. When disconnected these fundamental principals can't be fulfilled. For the sake of his mate coming back and doing some insulation testing, imho it is not worth the risk.

maxj · Posted: Thu Oct 20, 2005 9:59 am Post Subject:

Hi all

just to clarify, I am a competant enough DIY'er to do most jobs and also importantly... when to get a 'pro'. yep I could of installed the CU myself but I got my sparky mate to do it for me as a 'weekender' ( paid him for it ) and he disconected the earth to test the RCD.

thanks for all the feedback

JM

Spark123 · Posted: Thu Oct 20, 2005 4:43 pm Post Subject:

I wouldn't recommend anyone wether DIY or electrician disconnect an earth on an energized circuit to test an RCD, personally if an electrician who worked for me did this his feet wouldn't touch the ground.

delmel · Posted: Tue Oct 25, 2005 5:22 am Post Subject:

Was he wearing wellies and rubber gloves

slippyr4 · Posted: Tue Oct 25, 2005 8:31 am Post Subject:

maxj wrote:

I got my sparky mate to do it for me as a 'weekender'

Doesn't sound like he did much testing of the wiring before hooking it up to the new CU!!

Which is bad, because although you have PVC cable, it's 30 yrs old now, and fully has the potential to have some signs of decay.

----

Maybe i've not read through the thread properly, but i've seen nothing to preclude a live/earth fault - only the suggestion of neutral/earth fault. Thats probably a bit more dangerous.

jenki · Posted: Tue Oct 25, 2005 9:10 am Post Subject:

New CU - Split Load Board - Sounds more like 1 of the RCD protected circuits has 1 or more the neutral in the non-RCD protected neutral bar.

May not be a Neutral to Earth Fault at all - so to speak just that the RCD Phase and neutral our out of balance due to the neutral returning via a different path? Or have i missed something

maxj · Posted: Thu Oct 27, 2005 1:51 pm Post Subject:

no wellies, no gloves, thick rubber boots though.

The suggestion of a 30 year life for PVC is interesting, my understanding that PVC cable longevity in residential property is currently an 'unknown' as theoretically (given a 10 year test) PVC will survive the house provided certain contaminants like polystyrene and non-waterbased paints are kept away.

confused over the seperate nuetral bar for RCD side

thanks