Head scratcher and a half

[bigbaddave]

Right here goes, I have a large domestic house in Yorkshire which I've attended due to tripping RCD. Originally a dual RCD and I'm the third spark to look at this. Previous spark is a guy I work with and I know it's had a lot of time spent on it.

The side that was originally tripping had been swapped to individual RCBOS and the circuit that trips is upstairs lights however will only trip when a large current is pulled (kettle, toaster) on the kitchen sockets and this circuit is on the other side of the split loads board covered by the one RCD.

the tripping does not continue when the CPC is removed from the earth bar (upstairs lights) all class 2 fittings.

test results - IR global for DB (N-CPC ) 10.6 meg CPC - Live 15.7 meg L-N 21meg

IR between two n bars 999meg

IR between lives all 999 meg

RCD tests fine and been replaced along with RCBO

I'm due to go back but the contractor I work for is suggesting we just rewire the circuits we are having problems with as the cost of testing and time it's costing the company money not having the install working as it should. I Hate being beat.

 

[happysteve]

I'm only a trainee, and the last thing I'd want to do is offend by offering advice on egg-sucking, but if I were in this situation I would:

- Have a good read of chapter 9 of the BEAMA RCD Handbook.
- Do a ramp test (with and without loads on other circuit on)
- Ascertain whether it is the transient nature of a large kitchen circuit load being added, or whether it was a cumulative thing
- Try to work out if the two circuits in question are physically close to one another

Tests done are all at DC. The fact that the IR results come out nice and shiny, but there is still a problem, may indicate it is a coupling issue (inductive or capacitive).

Best of luck - let us know how you get on

 

[Matt85]

Have you tried IR between neutral bar and lives on opposite rcd

 

[uksparks]

Just one thing to think about, I'll tell you my story first.

i went to change a fuse box for split board, did all my testing including rings, disconnected a leg at a time etc, so both cpc legs on the ring, got what appeared to be a good end to end.

anyway when I changed it, I took ALL conductors out to change it to find that it wasn't continuous but linked via another circuit, which the cpc of that was connected to the bar at the time I pre tested it.

so maybe have you got a couple of circuits linked some how? Just a thought?

 

[Marvo]

I also hate being beaten by a fault.

Sometimes it's better to do live tests if it's a slippery one (obviously after a risk assessment ). Get hold of 2 x earth leakage clamp meters and clamp the L+N simultaneously which supply (or used to supply) the RCD's on both sides of the board. Monitor them both simultaneously to ascertain the leakage under different load conditions.

 

[bigbaddave]

I'm only a trainee, and the last thing I'd want to do is offend by offering advice on egg-sucking, but if I were in this situation I would:

- Have a good read of chapter 9 of the BEAMA RCD Handbook.
- Do a ramp test (with and without loads on other circuit on)
- Ascertain whether it is the transient nature of a large kitchen circuit load being added, or whether it was a cumulative thing
- Try to work out if the two circuits in question are physically close to one another

Tests done are all at DC. The fact that the IR results come out nice and shiny, but there is still a problem, may indicate it is a coupling issue (inductive or capacitive).

Best of luck - let us know how you get on

Hi mate, you mention about these circuits being close together. That was origannly what I suspected a interconnection of some for however I can't see where is would be possible as they are on separate floors and seperate sides of the building.

as for a ramp test this was performed for each circuit and even when on load 20 ma ish for both

 

[uksparks]

I don't even know of its possible, but if it's only happening under load, is there any chance that it's just the fact that two circuits could be squashed hard together and its inducing a voltage on the other?

 

[bigbaddave]

Have you tried IR between neutral bar and lives on opposite rcd

Hi, I did test between the two neutral bars and beetween the two lives (each side and no interconnections seemed apparent) the only thing I can think is the kitchen sockets feed boiler / heating system and there is a lot of zone valves etc, There all housed in a airing cupboard upstairs. I'm wondering if there is a possibility that there is an interconnection which due to the stats, and possibility not showeing up as its dead IR test.

 

[Marvo]

To get a coupled voltage convincing enough to trip an RCD the two circuits would need to run in very close proximity for a hundred meters or more. I doubt it's a capacitive coupling problem if it's a domestic premises.

 

[bigbaddave]

Just one thing to think about, I'll tell you my story first.

i went to change a fuse box for split board, did all my testing including rings, disconnected a leg at a time etc, so both cpc legs on the ring, got what appeared to be a good end to end.

anyway when I changed it, I took ALL conductors out to change it to find that it wasn't continuous but linked via another circuit, which the cpc of that was connected to the bar at the time I pre tested it.

so maybe have you got a couple of circuits linked some how? Just a thought?

That was my gut feeling to be fair, the only thing that puts me off it is that it's not constant tripping, even the smallest load usually trips it (ie shared natural)

 

[bigbaddave]

I also hate being beaten by a fault.

Sometimes it's better to do live tests if it's a slippery one (obviously after a risk assessment ). Get hold of 2 x earth leakage clamp meters and clamp the L+N simultaneously which supply (or used to supply) the RCD's on both sides of the board. Monitor them both simultaneously to ascertain the leakage under different load conditions.

Its actually a kids home so testing or any work is usually a mare as all covers etc have to be put back on before we can juice it up (HSE will frown on it but you know it's like in the real world)

now I havnt tried that one, that could prove helpful

 

[Marvo]

Sometimes live testing is the only way to localise a fault. Take an apprentice for crowd control duty and get the hazard cones, tape and signs out or maybe go after hours. I seriously think it's the best chance you have of finding the problem if several experienced guys have alread tried and failed.

 

[bigbaddave]

Sometimes live testing is the only way to localise a fault. Take an apprentice for crowd control duty and get the hazard cones, tape and signs out or maybe go after hours. I seriously think it's the best chance you have of finding the problem if several experienced guys have alread tried and failed.

totally agree mate, I'll be dammed if I don't find it tho. Cheers for the help guys

 

[UKMeterman]

My money would be on a N E fault on the upstairs lights. If this is a TNCS supply then there is not enough volts N E at the consumer unit to drive 30mA through the fault untill someone turns the urn on. When the urn is on there is a small voltage drop on N between the consumer unit and the cutout. This then drives current through the N-E fault and click... First look at all the lights and look for any heat damage.

 

[NDG Elecs]

My money would be on a N E fault on the upstairs lights. If this is a TNCS supply then there is not enough volts N E at the consumer unit to drive 30mA through the fault untill someone turns the urn on. When the urn is on there is a small voltage drop on N between the consumer unit and the cutout. This then drives current through the N-E fault and click... First look at all the lights and look for any heat damage.

I had similar not long ago. Electric shower causing upstairs lights to trip. Client said it all started happening after loft was boarded. Lifted a couple of boards in the loft and found one bit of T&E slightly crushed. Peeled back the sheath and all cores looked fine though. Found the fault on the same cable approx 5 ft away. Only discovered it as I knocked the cable and heard the RCD trip. I think it was possibly a manufacturing fault as the cable was fine from the outside but both L and N insulation was charred to hell inside. The outer sheath was not squashed, though there was signs it had been bent a bit over the joists.

IR tests showed cable was reasonable before I found the fault ~100Mohms...

Not worked out the science bit with the showers involvement though. This was a TT.