RCD trips when switching lights off

[JonS]

I have a very recent RCD protected split load consumer unit. The RCD side protects our 3 ring mains only. I live in a rural area with overhead lines and I am on PME.

The problems I am about to describe are all intermittent and we can go several days without problems, then encounter 6 or more occurences in an hour. The RCD never trips without one of the actions below, i.e. we don't wake up in the morning or come home to find it tripped. The weather makes no difference.

* When switching off lights, which are on the unprotected side of the CU the RCD protecting the ring mains will trip. it could be any light but I can almost always make it happen by switching off the pull-cord bathroom light by pulling it down and then 'bouncing' the switch until the RCD goes.

* When using some power tools and my wifes blender the RCD trips. The washing machine/hoover have never caused any probs.

* The garage is on the non-protected side of the CU but with its own separate RCD. When mowing the lawn we come back in to the house to find the other RCD tripped, even though the RCD supplying the mower has not.

With specialist kit I've carried out insulation tests up to 500v, RCD tests and all the obvious connectivity ones. Everything is within spec and as it should be. I have traced all wiring and gone round and made sure all connections are sound and that all metal boxes are properly earthed.

There is reams of stuff on the subject of nuisance RCD tripping but I've not yet seen one that mentions the problems I've been having with the lights.

Has anyone experienced anything like this? Is the bit about bouncing the bathroom light switch significant? I have considered changing over to an earth spike, is this likely to make a difference?

Any comments or help gratefully received!

Thanks

 

[andy]

wouldnt happen to have a 30mA protecting everything? if yes, then this is probably the problem. accumalative earth leakage from many appliances could go over the 30mA. or if you have any old tools they can. cookers are also prone to earth leakage

 

[Steve]

When switching off lights, which are on the unprotected side of the CU the RCD protecting the ring mains will trip. it could be any light but I can almost always make it happen by switching off the pull-cord bathroom light by pulling it down and then 'bouncing' the switch until the RCD goes

Please tell us how you think bouncing the cord about is going to make any difference to your circuits. As this is on the non-RCD side, are you sure your wife isn't doing something like blending or using power tools downstairs while you did this? This could be coincidence.

When using some power tools and my wifes blender the RCD trips. The washing machine/hoover have never caused any probs.

Anything with a motor is liable, at a certain point in its life, to nuisance tripping. ie old items or cheap items.

I have considered changing over to an earth spike, is this likely to make a difference?

DONT unless you like the idea of a less safe electrical system. Put another way, your entire house would need RCD protecting if you went down this road. Not a good idea, given your current problems.

 

[securespark]

Just a thought....have you got all the "N"'s in the right blocks?

 

[mattux]

hi

it does sound like a possible neutral problem, this sometimes happens if an protected neutral is in the unprotected side, but if its intermittent id rule this out.
the appliances that trip the rcd, try them in the garage to see if they do it there.
does the garage have its own earth, ie a 'spike' in the ground somewhere.
if you know what your doing you could try putting the rcd from the garage in the house cu, (BUT if youre slightly unsure do not do this).
can you post a photo of the inside of cu?

hth

matt

 

[Spark123]

It could also be a mains filter on a portable appliance.

 

[JonS]

Thanks for so many quick responses. Let me address them...

it does sound like a possible neutral problem, this sometimes happens if an protected neutral is in the unprotected side, but if its intermittent id rule this out.
the appliances that trip the rcd, try them in the garage to see if they do it there.
does the garage have its own earth, ie a 'spike' in the ground somewhere.

Only the 3 ring mains are protected by the house rcd. Only the 3 neutrals from the ring mains go into the protected RCD nuetral block, same with the lives. I've checked this a dozen times!

I've also tried the appliances in the garage - the garage RCD does not trip but the ring mains one does, even though they are on different sides of the split load! I'm thinking that this is caused by current flowing somehow between the sides of the board in the same unorthodox way as the lights since the garage and lights are both on the unprotected side. The garage does not have its own spike.

When I bought the house it had an old CU and as a temp measure I protected it all by using the RCD that now protects the garage. I did not suffer from the light problems but the thing did trip with the blender and tools. A solution that I might have to consider is putting the lights and ring mains on the protected side, at least it would get rid of the light problem, which didn't happen when the house was all on the same RCD. I'm holding back as this approach would plunge us into darkness when we got a trip from an appliance.

Please tell us how you think bouncing the cord about is going to make any difference to your circuits. As this is on the non-RCD side, are you sure your wife isn't doing something like blending or using power tools downstairs while you did this? This could be coincidence.

Bouncing the cord, or rather the switch opens and closes the circuit rapidly. I can confirm that this is not coincidental with any other activity, such as my wife using power tools (I wish ) This switch is particulaly prone to causing the prob but any switch in the house has done it at one time or another.

wouldnt happen to have a 30mA protecting everything? if yes, then this is probably the problem. accumalative earth leakage from many appliances could go over the 30mA. or if you have any old tools they can. cookers are also prone to earth leakage

No, only the rings mains. I've done experiments with nothing plugged into any sockets and the lights still take out the RCD. All appliances less than 3 years old. The cooker and hob is brand new. I've tested the RCD with a proper device at various current levels and I need to get the full 30mA setting dialled in to get it to trip. Of course, some outside influence could cause the 'background' current to rise that wasn't prevalent when I did the test ??

From an electrical engineering theory perspective, the spike caused by the opening of a switch or the switching of an appliance could lead to inductive/capacitive coupling between the circuits and the loss of current from one into the other and hence the imbalance which takes out the RCD. The theory seems plausable but I've never heard of anyone having this sort of problem. Wiring is usually run fairly close together. What could I do to possibly prevent this, if indeed this might be the problem.

Thanks

 

[securespark]

What would happen if you were to temporarily wire the lights off the rcd side?

Would it still trip then?

 

[pdcelec]

When pulling the cables in, one can burn the other. Possibly in this situation you have a low insulation resistance and when a load is applied, the fault appears. I would suggest a PIR. From a theoritical electrical engineering perspective in "theory" very much differs from the "real" world. (Not being sarcastic as I am qualified in both)

 

[Beefcake daddy]

I'm no expert like the guys who have posted already, so always thinking of simple solutions I can understand so forgive me if this is I naive stupid newby thought.

Could it be possible that whoever put the existing rings in (Assuming you just changed the CU and not the wiring) took a spur for the bathroom light from the ring main instead of having it on the lighting circuit? Or used a borrowed neutral/live from it?

 

[Spark123]

Could it be possible that whoever put the existing rings in (Assuming you just changed the CU and not the wiring) took a spur for the bathroom light from the ring main instead of having it on the lighting circuit?

If the light is on a fused spur from the ring the light will stop working when the RCD trips.

Or used a borrowed neutral/live from it?

If this were the case, when the light is switched on and pulls more than 30mA, the RCD will trip every time it is turned on.

++++++++++++++++

What is plugged into the socket circuit? Have you tried unplugging everything and then see if the RCD trips when using power tools etc?

 

[JonS]

To reiterate, ALL ring mains and lighting circuits have been confirmed seperate and confrim to current IEE regs. All wiring that is not buried under plaster has been inspected.

When pulling the cables in, one can burn the other. Possibly in this situation you have a low insulation resistance and when a load is applied, the fault appears. I would suggest a PIR. From a theoritical electrical engineering perspective in "theory" very much differs from the "real" world. (Not being sarcastic as I am qualified in both)

As I said, we've meggar'd all the circuits and from the lights to the ring mains. At 500V the IR is 300-600MOhms.

I've got a masters degree in electronics and my father-in-law is an electrical engineer with 40 years experience and we still can't get to the bottom of this !!

Some work colleagues have suggested damp walls might be playing a part? Naturaly we went round and looked at every box when we earthed them all properly, Apart from a bit of corrosion on the oldest boxes (Which we replaced, one with plastic) there was no sign of the kind of dampness that could steal 30mA, or rather intermittently transfer 30mA between lighting and rings. Any thoughts on this idea?

 

[Spark123]

It could also be a mains filter on a portable appliance.

What is plugged into the socket circuit? Have you tried unplugging everything and then see if the RCD trips when using power tools etc?

 

[JonS]

It could also be a mains filter on a portable appliance.

With nothing plugged in at all, the lights can, intermittently, take out the fully unloaded RCD.

 

[plugwash]

i'd try replacing the rcd maybe its become noise sesitive or something (a switch turning on or off can create a lot of electrical noise especially if its an old one or deliberately held part on)