RCD VR80 instantly trips with no MCB's on

[abstractniche]

Hi,

Yesterday the electrics in half the house went off - and basically the VR80 RCD which protects 5 circuits on the left side of my CU won't reset - even with all the associated MCB's switch off. I have another VR63 RCD on the right hand side protecting a different set of MCB's that is fine.

When I try to reset it I can see a quick spark from the area of the top right hand screw of the RCD. From reading through this forum I see that a neutral earth fault could still trip the RCD even with the MCB's in the off position - is that correct? And to see which one is likely causing the problem I should disconnect the neutral's in turn ?

Would that be the neutral's on the bar in the top left of this picture (from another post) //www.diynot.com/forums/viewtopic.php?p=1147347

My CU looks exactly the same.

Also I have seen people suggest to make sure all cables / screws are tight.

My main question is that if I put down (off) the master switch in the CU then as long as I don't go anywhere near the top two screws and cables on the master switch on the very left hand side of the same picture which is where I can see mine comes into the box as well will I be safe tightening up things and disconnecting neutrals one by one ?

Thanks

Andrew

 

[1john]

I dont know, you will be working in a live enclosure, do you think it is wise? Have you got VDE insulated tools, what if you slip while tightening down a screw? Have you got a volt meter or multi meter (not a neon screwdriver) that you can use to test for dead?

Personally I dont feel it's a good idea for you.

 

[riveralt]

My main question is that if I put down (off) the master switch in the CU then as long as I don't go anywhere near the top two screws and cables on the master switch on the very left hand side of the same picture which is where I can see mine comes into the box as well will I be safe tightening up things and disconnecting neutrals one by one ?

Thanks

Andrew

Do you not have an isolator switch between the meter and your CU this would be your best option.

Failing that you will have to get a test meter and double check that the RCD you are working on is dead.

 

[abstractniche]

I have a multi meter.

I'm pretty sure the RCD is not live when the main switch if off, because if I turn the whole main switch down (off) then the RCD switch will reset back to on. However as soon as current is applied by turning the master switch back on the RCD instantly trips.

There is no breaker between the CU and the meter or the big 100A black fuse and the meter - I understand I'm not allowed to touch the master fuse and don't want to anyway.

 

[Chri5]

Have you unplugged everything on the circuits ?

First test is to see if an appliance or fitting wired on the circuits may have cause the rcd trip. So turn off cooker points, washers, basically everything and see if the rcd will reset.

Have you done any diy work recently, used a drill or done something that could hint at why the problem has come about ?
Have you checked that anything outside is dry, a water filled security light, a soaked socket ?

If you pulled all the neutrals and keep the mcbs off, logic would be that if the rcd doesn't reset the unit may have failed.

 

[abstractniche]

I tested the RCD and it was not live when the master switch was off so I checked everything was tight and then proceeded to remove the neutrals one by one from the associated neutral bar - the one on the left.

When I got to the third one, without the neutrals connected from the circuit the RCD would reset and turn on. This was a pair of black wires and is marked as 'Old Power Down' which corresponds as the downstairs power sockets that were on the old power circuit still don't work (and I guess why the tails were black and not blue - being from an older circuit).

I tried the pair individually, but I guess they must be a ring main because with either one of them connected to the neutral bar the RCD trips.

I had, or at least I thought I had turned off all appliances, and turned of all the plugs or at least all the ones that had a switch - they don't all. However it still won't reset with this 'Old Power' sockets ring connected.

As it trips even with the associated MCB off am I correct in assuming a Neutral / Earth short in one of the sockets? As I have turned off the appliances and sockets as far as I can that are on this ring, am I now looking for a short inside the sockets ?

At least the rest of the power is now back on and I don't need to go and get a new RCD.

Thanks for all your help.

Andrew

 

[JohnD]

Yes, it is a neutral/earth short. it is less likely a short in one of the sockets. Sockets rarely go wrong spontaneously (although switches sometimes wear out)

More likely is rodent attack of an underfloor cable, or water in an outdoor or shed fitting, or a nail through a wall or floor. Look for damage first. You might have to go under the floor. Look especially for outdoor lights that are wet. Park those loose neutrals somewhere safe. Choc-block is better than nothing, put them where they cannot possibly touch anything in the CU, or be touched by anyone. If the MCB is turned on they may become live

Look for anything not working that you did not realise was on the ground floor socket circuit.

 

[Adam_151]

It sounds like you have correctly identified the fault as N-E fault on that ring, it should be noted that as well as having the neutral wires disconnected, you should disconnect the lives from the breaker as well (becuase if it got accidentally switched on and there was a load connected then the disconnected wires would be at live potentional, suitably terminate or tape up the conductors so they cannot touch anything else in the DB.


As to where to go now... have a good think about if there are any outdoor sockets/fittings off this cirucit... any new pictures hung on the walls, squeaky floorboards re-fixed etc. In terms of actually fault finding an insulation resistance tester would be usful here, but you'll probably find the fault is low enough resistance to register on a multimeter.

Be careful of borrowed neutrals, its not beyond posibility that the fault is caused by a neutral borrowed from the ring for something on another circuit, and whatever load it is (cupboard/loft light???) hasn't been switched on since the rcd board was installed) - so probably safest for you to work with the main isolator switched off.

Test between neutral and earth on the cables at the DB, check the fault registers on your meter, then try and guess the mid point of the ring... take socket off and separate out the cables, go and check which of the two halves the fault is on, then break that half down, and keep splitting it in half until you have found it.; Remember the circuit needs to remain as a ring, and you may not just remove a damaged section without changing the breaker and confirming the design of the circuit still complies. After repair the circuit should ideally be fully tested before being brought back into service, at the very least as a DIYer you should check the end to end is not broken from the board with your multimeter

 

[abstractniche]

Thanks for tips on taping up neutrals and disconnecting the associated lives which I have done just in case. Am going around double checking nothing else running of the downstairs sockets. The pond pump must come from somewhere down there so am trying to locate that and checking for rotten / rodent attacked cables at the same time.

Will let you all know how I get on !

Andrew

 

[JohnD]

pond pump

Aha!

outdoors as well as wet!

 

[RF Lighting]

I'll put 5p on it being the pond pump