New Consumer Unit RCD tripping at random

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We've just had a new CU fitted and the electrician who fitted it is stumped.

It's a 15 way box with 4 lighting circuits and freezer running from the main 100mA RCD and the rest running from a 30mA RDC. It's been in a month and initially tripped several times a day or night with no pattern to things in use at the time.

He initially tested the RCD and found it to be faulty and replaced it - it now only trips every 3 or 4 days.

By leaving various circuits off we have fairly confidently identified the problem as being the downstairs power ring. He has identified that this circuit has a lower leakage resistance - still many megohms and legal, but less than the other circuits. I'm not sure that this was enough to cause enough leakage, but we are getting desperate for ideas! We have spent three hours disconnecting each downstairs socket and testing the wiring between each. One length in particular - a spur out to the garage has the lowest resistance so we confidently disconnected that. Today it tripped again, even with this disconnected.

At the time it tripped, nobody was in the house and the only things running were a fridge, satellite and Tivo box (like sky plus) and a couple of computers plus clock radios and such like.

We are stumped. Ideas please?
 
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What was the reason for putting everything on an RCD?
 
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What was the reason for putting everything on an RCD?

The original box was an old conventional fuse box which had various extensions added over the years so needed to be replaced. Each 'addition' now has its own MCB, which seems sensible.

To the far right of the board is a 100A 100ma Wylex RCD WEMT 100/2. To the left of that are 4 MCBs for the lighting and freezer circuits. This has never tripped. Next, working left is a 80A 30mA Wylex RCD WR80/2 which cover a variety of MCBs to the left (NSD16, 20, 32, 40s) This is the RCD that trips, and it happens when all bar the downstairs power MCB are turned off. I am aware from reading through many of the multitude (500+!) of other posts on this forum that come up when you search for "RCD tripping", that to prove things properly, neutrals and lives need to be individually disconnected. I believe the electrician did do this when he was trying to find the problem.
 
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Steth said:
The original box was an old conventional fuse box which had various extensions added over the years so needed to be replaced. Each 'addition' now has its own MCB, which seems sensible.
I don't foresee anyone disagreeing with that, although JohnD might 'shriek' at the missed opportunity for using RCBOs (which, incidentally, would have made your existing problem a no-brainer).

To the left of that are 4 MCBs for the lighting and freezer circuits.
This is the bit that seems unwise. Conventional wisdom puts lighting on the non-RCD side, that you have lights in the even of, for example, a ring fault. Also, if the RCD were to trip while the house were unoccupied for more than a couple of days, then in your case you'll lose the contents of your freezer.

Next, working left is a 80A 30mA Wylex RCD WR80/2 which cover a variety of MCBs to the left (NSD16, 20, 32, 40s) This is the RCD that trips, and it happens when all bar the downstairs power MCB are turned off.
The presence of the cooker, the TV, and "a couple" of computers on the RCD side will bring you close to the tripping level without any serious fault needed. Bear in mind that even if you shut down the computers, each power supply will still be leaking whatever it's prone to leaking.
 
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Curiouser and curiouser

We had a fairly uneventful day with only downstairs and upstairs power MCBs on. No problem until I turned off a downstairs fluorescent light (on the lighting circuit, not controlled by the 30mA RCD, but covered by the main (far right) whole house 100mA RCD. This triggered the 30mA RCD which covers the downstairs power but I wasn't able to duplicate the fault.

What I did do though was to put the kitchen power ring back on to recool the fridge as it had been off all day. The kitchen power circuit doesn't have the cooker or freezer on it - they are also off the far right whole house 100mA RCD, nothing was drawing power in the kitchen, but as I turned the kitchen power ring back on it tripped its RCD, and did so repeatedly 6 or 7 times as we went round unplugging everything on the kitchen power ring. It still tripped on turn on with nothing in any of the kitchen sockets. I left it an hour and tried again and it reset with no problem.

Softus - are you saying that we shouldn't have the lighting and freezer circuits protected by the 'whole house' 100A 100mA RCD, simply by their MCBs?

Is it remotely possible that when he had all the wires (and there are a lot) hanging in mid-air, he made a mistake and has half a ring connected to two different MCBs, or would that not do this?
 
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Steth said:
We had a fairly uneventful day with only downstairs and upstairs power MCBs on. No problem until I turned off a downstairs fluorescent light (on the lighting circuit, not controlled by the 30mA RCD, but covered by the main (far right) whole house 100mA RCD. This triggered the 30mA RCD which covers the downstairs power but I wasn't able to duplicate the fault.
It's within the spectrum of possibilities that the lighting neutral is in the wrong place.

What I did do though was to put the kitchen power ring back on to recool the fridge as it had been off all day. The kitchen power circuit doesn't have the cooker or freezer on it - they are also off the far right whole house 100mA RCD
You're beginning to lose me - I thought you said that the cooker was on the 30mA RCD. :confused:

nothing was drawing power in the kitchen, but as I turned the kitchen power ring back on it tripped its RCD, and did so repeatedly 6 or 7 times as we went round unplugging everything on the kitchen power ring. It still tripped on turn on with nothing in any of the kitchen sockets. I left it an hour and tried again and it reset with no problem.
Oh dear.

Softus - are you saying that we shouldn't have the lighting and freezer circuits protected by the 'whole house' 100A 100mA RCD, simply by their MCBs?
Yes. However, if the freezer is in the garage, and plugged into a socket outlet, then you have a problem. But it's easily overcome by cutting off the fridge plug and wiring it into an FCU that you put in the non-RCD side. More difficult if it's a on circuit for, say, the garage (but not impossible).

Is it remotely possible that when he had all the wires (and there are a lot) hanging in mid-air, he made a mistake and has half a ring connected to two different MCBs, or would that not do this?
If the two MCBs were on different sides of the 30mA RCD then I would expect it to trip all the time. However, if on the same side, but on different MCBs, then potentially you have an extremely dangerous installation, because the pair of MCBs together will not have been designed to protect that ring, so they could each fail to trip in the event of a cable overload.
 
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It's just tripped again and yet again kitchen power MCB can't be reset without tripping the RCD. Nothing on bar 1 computer and the TV, nothing in the kitchen plugged in bar the fridge (which of course wasn't when it last tripped earlier - at least it was plugged in then but the kitchen RCD was off).

The cooker isn't on the kitchen power main, it's with the freezer (the freezer is plugged into the socket on the cooker panel)

Thanks for the advice, I can see no logic to it at all.

I may suggest to the electrician that he may have wired it incorrectly. He was as desperate as I was for ideas when he left at 10pm last week.
 
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Has he ramp tested the RCD?

What other tests did he carry out?

In any case, it sounds like it would be a good idea if he reviewed all of the connections inside the CU.
 
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Good morning

A quiet night with just upstairs and downstairs power on and no events. Just turned the kitchen back on to re-cool the fridge again and all well so far.

He tested the original RCD and found it to be faulty, triggering at minimal mA at 0 volts and on the up half of the cycle so I understand. He tested the replacement and said it was OK. I assume the ramp test is where I stood at the CU turning it back on as he had something plugged into a socket and kept increasing the level until it triggered.

He also tested the insulation of each individual circuit and found the downstairs circuit to have a lower (but not dangerously low) resistance and this gave us hope. We progressively removed sockets to isolate which part of the circuit was likely to be the problem. That pointed towards a spur that has a cable out to the garage (which has it's own RCD in a mini CU) and the outside floodlights. That spur was disconnected then and remains so, so although it warrants further investigation, can't be responsible for the ongoing woes.
 
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Softus - many thanks for your ideas to date.

The problem continues with random tripping. The only consistent cause of tripping seemed to be turning a particular (fluorescent) light off. I've replaced both the switch and the whole fluorescent light fitting. Still it triggers a trip, though possibly less frequently. Most trips appear to be random, often when very little is on bar fridge and freezer and a computer, though it doesn't happen any more frequently when my wife is cooking with frequent opening and closing of fridge and freezer.

I'm taken by the RCBO idea. Could they be simply swapped in in place of the existing MCBs? They seem expensive, but may be preferable to spending the rest of our lives with the power going off every other day. Is this idea 'a no brainer' because it would stop the problem happening, or simply because it would confirm exactly which circuit was causing the problem?

Is there not a test the electrician could do across each circuit to show if, and how much leakage there is on each?
 
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I'm taken by the RCBO idea. Could they be simply swapped in in place of the existing MCBs?

yes.

but you need to take the circuits off the existing 30mA RCD, otherwise they will both trip at the same time.
 
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It can often appear that a fault is on a particular circuit on the RCD side, as you have indicated by saying the kitchen. However, the fault can easily be on one of the other RCD protected circuits, and it just appears to be caused by the kitchen.

I suggest you test all circuits for insulation resistance, making sure to remove the neutrals from the consumer unit busbar when you test each circuit.

Then find the fault on that circuit by breaking the circuit down into bits.

You cannot fault find by turning MCB's on/off and switching appliances on and off. You never isolate the neutral when doing this, and neutral to earth faults will cause the RCD to operate equally as well as a live to neutral.
 
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it's very difficult to diagnose random RCD trips with just words..

the "no brainer" RCBO idea would most likely only confirm which exact circuit was to blame..
it might however cure the problem as you then have 30mA "leak" available for each circuit..

you might ( for example ) have 24mA leaking from one circuit and turning the fluorescent lamp on or off would take it over the limit and trip everything..

you could always try the "ooops bought too many" trick..

buy some, fit them, narrow down the fault then once it's fixed put the mcb's back in the board and take the RCBO's back..
 
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Steth said:
Is there not a test the electrician could do across each circuit to show if, and how much leakage there is on each?
 

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