RCD trip with a difference!

[pushrod]

Was at a relative's house (big and old) and they said they sometimes flicked a particular light switch and it caused the CU to trip. Said i would take a quick look!

It turns out it was a split load board about 6 years old with one rcd. All the light circuits were on the non rcd side and when it was tripping it was taking out the rcd and leaving the lights on!. Taking out a 6 amp mcb on the non rcd side isolated the light and there was nothing loose at the light fitting or the switch. They said it used to happen very regularly but was less frequent recently. Needless to say we couldn't get it to happen when i was there. The switch in question was a smiths time guard, that comes on at night, but other than that it appeared perfectly normal. Didn't have time for any tests.

Must admit i was pretty much at a loss why a circuit on one side should be taking out the rcd on the other side! Am i missing something obvious? Anyone any suggestions? cheers

 

[nickblake]

it could be a faulty RCD had the same sort of thing happen at my sister in laws she would switch on lights round her house and it would trip the RCD and her lights were on a different board i dont know why it happend i just did the installation was the same age too 6 years tested every thing and was all ok put in a new RCD and problem solved hasnt tripped since

 

[telectrix]

could that one light have a borrowed neutral. bit of a long shot

 

[rocker]

Yep I'm seconding this - the light will have a borrowed neutral off a socket circuit.

Could also be an insulation problem between 2 seperate circuits, I spose.

EDIT: Actually on re reading your post, probably not a borrowed neutral if it's only intermittent.

 

[telectrix]

just re-read OP and seen intermittent, so agree with rocker. i would check to see if wiring was notched in joists (with or without protection) and look for intermittent short between circuits.

 

[nickblake]

you could get a leakage clamp tester and see what the leakage is across the earths , i got one worth its weight in gold

 

[rocker]

Nick how do they work? Like what do you do, just clamp all the earths in that clamp bit???

 

[nickblake]

its taken me a while to get used to it but what you do is clamp it round the earth of that circuit and it gives you a reading in MA just like the current clamp meters but using the earth not the phase , so what i do is check each circuits earth and add them up i mean the majority of the times the readings are 0.02ma so you know there is very little leakage one place i did find it very handy was a schools it class the leakage there was 75ma its a cracking piece of kit for fault finding and also if you are doing a CDU upgrade or converting to RCD protection then you can calculate the circuit splits the only thing i wont do is give you the leakage on the main earth to the board the modle and make is a TIS TIS550 be great for the fault here as you could check the leakageTIS 550 Mini Leakage Clamp Meter

 

[ringer]

Why won't it give the leakage on the main earth to the board, Nick?
I got the Di-log DL6507 for a smidge over a ton, but not had the opportunity to put it to work on a real problem yet.

 

[WayneL]

Hi mate,

IR test N - E on RCD side of board - my money is on a neutral/earth fault on RCD side

 

[telectrix]

Hi mate,

IR test N - E on RCD side of board - my money is on a neutral/earth fault on RCD side

so why does the RCD trip only when light on non-RCD side is switched on?

 

[ringer]

I always thought a N-E fault would involve current flowing from L (via load) to N to E, but is this possible?....
A N-E fault exists on a RCD circuit, on another (non RCD) circuit there is major earth leakage. This earth leakage current feeds from the earth into the (RCD protected) neutral via the N-E fault, then the RCD trips?

 

[WayneL]

I always thought a N-E fault would involve current flowing from L (via load) to N to E, but is this possible?....
A N-E fault exists on a RCD circuit, on another (non RCD) circuit there is major earth leakage. This earth leakage current feeds from the earth into the (RCD protected) neutral via the N-E fault, then the RCD trips?

This is correct - and the RCD only notices when circuit is under load.
Also, the bigger the load, the more the leakage increases until - trip - which makes it an intermittent fault.

Now, think about this:

Follow the current.

You have a neutral/earth fault on the RCD side - say RFC.

You switch lights on (non-RCD side)

Current flows Line - Load - Neutral - Neutral bar............now it should then go through the main switch and down the supply neutral, to the transformer.

BUT..... there's another path - via the neutral feeding the RCD, up the neutral of the faulty RFC, and down to earth.

It will still register as an imbalance on the RCD and trip.....and it only needs 30mA to travel this path.

Just an idea......I might be wrong

 

[ringer]

I think you might be wrong I think I might be wrong too
If there is a N-E fault on the RCD circuit, then the RCD will trip in normal use well before any extraneous (to this circuit) current is able to trip the RCD.

 

[kung]

long shot but when the light is on try walking round the house every where ! without seeing how its wired could be loose floorboards nail intermittently bridging l-e on a cable when floorboard is walked on or even nipping one cable say line on poss a lighting circuit and through another cable say cpc of rfc that would trip rcd and leave lighting on etc or damp etc. a long shot i know but doesnt sound like an obvious fault and if thats whats causing it might not always show on an ir or continuity test.

 

[WayneL]

I think you might be wrong I think I might be wrong too
If there is a N-E fault on the RCD circuit, then the RCD will trip in normal use well before any extraneous (to this circuit) current is able to trip the RCD.

A good point Ringer.

But with N/E faults being 'load dependant', it could be a case of the straw that broke the camels back.

In other words, the normal circuits on the RCD side are running just under the 'trip limit', and the additional imbalance, from the lighting (or another circuit), cause the trip.

It might look like the lighting circuit is causing the trip, but it could just be an accumulation of loads.

Can anyone think of another reason why a circuit on one side of the board will trip the RCD on the other??

 

[nickblake]

Still think its an RCD fault its been installed for 6 years so that rules out the shared borrowed neutrals may be wrong but thats what id put my money on

 

[telectrix]

why not ramp test the RCD?

 

[telectrix]

exactly what i said earlier, kung

 

[pushrod]

Thanks for all the replies, i had a chance to pop round there for literally 15mins this morning and just had time to test the rcd and it looks like the "laurels" go to Nick in this case for suggesting a faulty rcd as i couldn't get it to give a sensible reading in about 3 goes, just kept saying "trip". Didn't even have time for a quick IR tests on N-E so still unsure if there is a fault there as well so some of the explanations as to why it was happening could well be right as well. Apologies though as i reckon i held back a vital clue which was the very uneven distribution of load. There was a total max mcb rating of more than 140A on the 63A rcd!
Yesterday I asked this on the RCD Diversity thread.

"Anyone know how quickly an rcd cooks if you exceed it's rated capacity. Have you got much leeway and for how long?
Also what is the most likely damage? does it just take out the trip and leave you thinking it is still ok or does it stop it conducting altogether?"

There were no replies but i reckon i now know the answer

It is a crabtree board and a search for an 80A crabtree rcd hasn't shown anything locally. Will other makes fit crabtree? (Don't want to fit another 63A one) Reckon the load should be more evenly distributed but am guessing there are probs borrowed neutrals on the 3 or 4 lighting circuits and that is why someone has previously stuck them all on the same side!!!



​

 

[davelerave]

what type of light fitting?-out of interest

 

[nickblake]

Hi mate if its a crabtree load star board then the RCD is the same as the Wylex volex sector they are all made by electrium ...

 

[pushrod]

what type of light fitting?-out of interest

Just a triple pendant with BC incandesents. Thinking about it only one of the bulbs was now working (room no longer used much since daughter left home) so maybe the load was greater when the tripping was happening more regularly.

 

[pushrod]

Hi mate if its a crabtree load star board then the RCD is the same as the Wylex volex sector they are all made by electrium ...

Cheers

Just seen THESE any idea if they fit as well? Seem to be very cheap.

 

[nickblake]

oh my i hate them things with a passion WYLEX 80A 30mA DP RCD these are in screwfix may get them cheaper from senate , but ill be honest with you think the fault on the RCD would be to a failure in the electrionc side rather than the contacts over loading

 

[pushrod]

oh my i hate them things with a passion WYLEX 80A 30mA DP RCD these are in screwfix may get them cheaper from senate , but ill be honest with you think the fault on the RCD would be to a failure in the electrionc side rather than the contacts over loading

Not surprised, things are normally cheap for a reason

When i change it i'll probs try taking the old one apart and see if there are signs of overheating in there - be interesting to see if there is anything obvious.