RCD Tripping

[Nickj]

Hey all,

Downstairs lights are off. When you put the mcb on the rcd instantly trips.

I've done an insulation resistance test and I have >20MOhm on L-N, L-E, N-E.

I've checked the wiring at all the roses and all seems secure and correct.

What could be causing this?

There are 4 ceiling roses, one porch light, one toilet light with fan, one kitchen light (5 GU10's) and one outside light.

All bulbs have been removed and all switches are in off position.

My voltage tester picks up a continuity between line and earth at the ceiling roses, however when tested with multi tester it indicates no continuity between the two.

The house does have a damp problem.

I did remove the line connections on what I think is the first light on the circuit however the RCD still tips.

I'm totally confused by it

Thanks,

Nick

 

[Risteard]

When did this start, and was any work done before it started?

 

[Nickj]

Hello and thank you,

No work has been done to the house. I've been away however according to my aunt who was at the house it happened sometime around the major storm we had. It being a damp house I suspect it's caused by water/moisture however would that not show up on insulation resistance test?

 

[Risteard]

It might, but there isn't any guarantee that testing will find a fault. This is why the term "inspection" comes first in the phrase "inspection & testing". It is done first, but is also more important than testing. Testing merely supplements inspection.

That said if there was a lot of damp you would probably expect to notice that when insulation testing. But there could be other reasons not related to dampness for your problem.

 

[tony mc]

With all the lamps out does the RCD still trip once the MCB is switched on or does it hold ?

I take it that the RCD is tested and is working?

 

[Nickj]

Hello and thank you tony mc,

Yes all lamps are out, all switches are off. Even in that state the rcd trips when the mcb is switched on. RCD is working.

I'm at a total loss at to what to do next

 

[Richard Burns]

Based on your test results there is a problem from switched line to earth (possibly via neutral).
Since you tested with the switches off and loads removed the switched line core from the switch to the light fitting was not included in the test.
When you are testing at the ceiling rose then this is indicating a fault (with a voltage indicator?).
Leave the switches off and test IR at each light fitting from switched line to earth and one (or more) of them will show a low IR result.
If it is associated with storm conditions then I would start with outside lights and lights in the loft.

 

[tony mc]

It seems strange that you are getting 20Meg on your test but on resetting the Rcd it trips instantly.

How many other MCB's are protected by the RCD and have you tried switching the other MCBs off and removing the neutrals to see if the RCD holds when the light MCB is reset?

 

[Richard Burns]

Have you disconnected the fan?

 

[Deleted member 9648]

The thing is OP....if the IR really is >20 megs, and the RCD is not faulty, and the circuit is correctly connected in the DB....it wont trip. The only reason I can think of where you may be missing a fault is if you are disconnecting the earth at the DB to test,and there is a L or N fault to earth via a damp wall or something which is may not show up to the cpc. If you are disconnecting the cpc at the DB try an IR test L/N to the earth bar in the DB, or leave the cpc connected.

 

[Nickj]

Based on your test results there is a problem from switched line to earth (possibly via neutral).
Since you tested with the switches off and loads removed the switched line core from the switch to the light fitting was not included in the test.
When you are testing at the ceiling rose then this is indicating a fault (with a voltage indicator?).
Leave the switches off and test IR at each light fitting from switched line to earth and one (or more) of them will show a low IR result.
If it is associated with storm conditions then I would start with outside lights and lights in the loft.

I will try this when I'm next home on Wednesday ad see what results it gives. Thanks for at least giving me another avenue to explore

 

[Nickj]

It seems strange that you are getting 20Meg on your test but on resetting the Rcd it trips instantly.

How many other MCB's are protected by the RCD and have you tried switching the other MCBs off and removing the neutrals to see if the RCD holds when the light MCB is reset?

2 other MCBs protected by same RCD. I did swap the downstairs lights with upstairs (and moved neutrals over). Upstairs lights still worked on the previously downstairs RCD but downstairs lights tripped on the previously upstairs RCD as well.

Thanks

 

[Nickj]

Have you disconnected the fan?

Ah, good point. I didn't disconnect the fan. The downstairs toilet where the fan is located is very rarely used.

 

[telectrix]

i've had a similar fault on a socket circuit. RCD trips as soon as you energise the MCB. IR test gave >100Meg. turned out to be a nicked L cable in back of a socket, arcing to the back box. with socket fronts pulled forward there was no tripping. soon as 1 socket in particular was screwed back, RCD tripped.

 

[Nickj]

The thing is OP....if the IR really is >20 megs, and the RCD is not faulty, and the circuit is correctly connected in the DB....it wont trip. The only reason I can think of where you may be missing a fault is if you are disconnecting the earth at the DB to test,and there is a L or N fault to earth via a damp wall or something which is may not show up to the cpc. If you are disconnecting the cpc at the DB try an IR test L/N to the earth bar in the DB, or leave the cpc connected.

I ran the IR test with all wires still connected so -
MCB -> Earth bar
MCB - > Neutral bar
Neutral bar -> earth bar

I didn't expect to get > 20 MOhm so I then disconnected the live neutral and earth wires and tested with just the wires.

Still > 20 Mohm -> Scratches head

 

[telectrix]

back to trial and error then. unscrew all switches. then if still tripping, IR test L-E and S/L - E at each switch. inspect cables at each switch for any sign of damage.

 

[Nickj]

back to trial and error then. unscrew all switches. then if still tripping, IR test L-E and S/L - E at each switch. inspect cables at each switch for any sign of damage.

I'll let you know how I get on Wednesday. Can't wait! haha

 

[Pete999]

Hey all,

Downstairs lights are off. When you put the mcb on the rcd instantly trips.

I've done an insulation resistance test and I have >20MOhm on L-N, L-E, N-E.

I've checked the wiring at all the roses and all seems secure and correct.

What could be causing this?

There are 4 ceiling roses, one porch light, one toilet light with fan, one kitchen light (5 GU10's) and one outside light.

All bulbs have been removed and all switches are in off position.

My voltage tester picks up a continuity between line and earth at the ceiling roses, however when tested with multi tester it indicates no continuity between the two.

The house does have a damp problem.

I did remove the line connections on what I think is the first light on the circuit however the RCD still tips.

I'm totally confused by it

Thanks,

Nick

https://www.google.co.uk/search?q=b...nt=firefox-b&gfe_rd=cr&ei=XMBmV7OCO8HBaMT3ozg

Don't use those stupid volt sticks for fault finding, waste of time and money, you have probably reconnected the ceiling rose incorrectly, I hope the link attached helps, isolate the supply, prove dead and lock off and check which cables go where. I sounds like you somehow connected the supply to the rose in the wrong terminals, take your time, good luck.

 

[Nickj]

https://www.google.co.uk/search?q=b...nt=firefox-b&gfe_rd=cr&ei=XMBmV7OCO8HBaMT3ozg

Don't use those stupid volt sticks for fault finding, waste of time and money, you have probably reconnected the ceiling rose incorrectly, I hope the link attached helps, isolate the supply, prove dead and lock off and check which cables go where. I sounds like you somehow connected the supply to the rose in the wrong terminals, take your time, good luck.

Hi thanks for the comment,

I think you've misunderstood. I've not used a voltage stick. I used a voltage and continuity tester, gs38 approved, to ensure a ceiling rose was dead and isolatd before checking the wiring to the roses. Previously I had of course safely isolated at the CU and locked off. I still check wires before touching them even after the locking off to ensure its not getting a live from anywhere else, although in this instance with the whole power being off it was incredibly unlikely.

The voltage and continuity tester picked up continuity between earth and line. However my mft did not pick up a continuity.

All dead tests are satisfactory it is only when the mcb is switched on does the rcb trip.

Previous answers have given me some indication of how to proceed and I thank everyone for that. If anyone can think of anything else I could do to fill my wednesday with various tests I'd be happy to hear.

Thanks

 

[Pete999]

How many cables are at the ceiling rose you have been working on? let me know and we can see how to proceed.

 

[Nickj]

How many cables are at the ceiling rose you have been working on? let me know and we can see how to proceed.

Hi Pete999,

Thanks for taking the time to respond again, I have checked the wiring at all roses, all except one are loop in, out, switch line. One just has line and neutral in with switch line. I'm guessing it's fed from a junction box as none of the other roses have been spurred from.

In the kitchen the line and neutral have been brought to the switch and I assume that was done to feed the power to the outside light.

All wiring seems in tact and correctly terminated. I cannot see how a line-earth fault has developed. Insulation tests are fine. It really has me scratching my head.

 

[Risteard]

If you are disconnecting the cpc at the DB try an IR test L/N to the earth bar in the DB, or leave the cpc connected.

Since the publication of the 17th Edition insulation resistance testing is required to be carried out with the cpc connected to the means of earthing. (That's not to say that some/many don't incorrectly test with disconnected cpcs though, of course.)

 

[Pete999]

Hi Pete999,

Thanks for taking the time to respond again, I have checked the wiring at all roses, all except one are loop in, out, switch line. One just has line and neutral in with switch line. I'm guessing it's fed from a junction box as none of the other roses have been spurred from.

In the kitchen the line and neutral have been brought to the switch and I assume that was done to feed the power to the outside light.

All wiring seems in tact and correctly terminated. I cannot see how a line-earth fault has developed. Insulation tests are fine. It really has me scratching my head.

Rose with just two cables is probably the last light on the circuit, have you dissed anything at the CU?

 

[Deleted member 9648]

If you are certain no sensitive loads are connected it is worth IR testing at 1000v. In the past I've used this to break down a fault which does not show up at 500v. Leaving it on the circuit for a minute or two is sometimes enough for the fault to suddenly show up.

 

[tony mc]

I dont think disconnecting the fan will make any difference if you previously got 20Meg on line to neutral it must of be switched off or putting 500volts has possibily fried it but i doubt you would of got a reading of 20meg.

As wirepuller has pointed out if all your tests show 20meg then if the RCD is in good working order it will hold.
The RCD should hold on anything above 7666 ohms so you are clearly missing something.


Let us know how you get on!

 

[Perryway-]

is it a split load rcd board?