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Hi I hope this is the right forum to ask some advice for what I hope is a straight forward question.

Short version: Is an RCD required in domestic installation where there are 2 consumer units, each with its own RCD?

The layout of the system is
Meter >
RCD (BG 80A/30mA) > split >
CU1 with RCD (MEMAD100ME) 100A 100mA + 10 trip: B32A for cooker, B16A for outside sodium lights, 8xB6A Lighting circuits

CU2 with RCD (MEMAD100ME) 100A 30mA + 5 trips, all B32A for sockets & gates

Long version: I'm having a problem with a nuisance tripping RCD. It will trip (taking all circuits in the house with it) and if you immediately reset it then its fine. This is happening seemingly at random perhaps once or more times a month. Its not when anything is obviously coming on, heating, kettle, iron, etc. The frustrating thing is that it is ALWAYS the RCD closest to the meter that goes taking everything with it. It has tripped once when we were on holidays, defrosting the fridge freezer. We have electric gates and it did seem to happen more often when they were moving (not every time) but I have turned them off its still happening occasionally, slightly more often in bad weather. I don't know if it matters but we are rural and have the power coming from overhead lines and a pole in the garden.

I've spoken to 2 electricians and both say its very very difficult and expensive to troubleshoot a fault like this and to be honest while I'm not on the bread line I can't afford a bill of hundreds and hundreds with no definitive solution. I'm nervous that they both went down the lines of 'turn everything off then on one at a time to see what does it' and when I explained it happened occasionally and not on demand neither made any reference to systematically testing each circuit, although this was maybe implied by the 'difficult and expensive' bit.

I was pondering how to try narrow it down and what the purpose was of the MEM AD100ME switches because they have never triggered. When I looked them up and realised they were RCDS as well. Does that sound right, essentially 2 in series? And with a lower maximum Amp?

It would just be much more straight forward if it was a single circuit that tripped not everything every time. At least if the 'last' RCD wasn't needed then it would trip one CU or the other and you'd have a start. There is space on each CU so I don't know if it'd be sensible to move circuits one at a time to narrow down the culprit?

I should make clear that I've every intention of getting an electrician to carry out any work but I don't want to be taken for a ride if there is something obviously wrong in the setup or a straight forward test that can be done (by a _good_ electrician) to track down which circuit is the problem, or if its a combination thats hovering close to the limit and being pushed over the top. If wiring needs replaced I'll do it of course if that's the safest thing to do.

Thanks for any help in advance, I just don't know who else to ask.

-Steve
 
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telectrix

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Arms
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sounds like you have an upfront RCD that may have been retro fitted when you had old CU's with no RCD protection. if so, once newer CU's were fitted, the upfront RCD shoild have been removed. post a pic or 2.
 

Spoon

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Arms
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or the tails from the meter to the CU's needed RCD protection maybe...
 

Risteard

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It's feeding a 100mA RCCB on one DB as I read it so that probably explains why it is there although better solutions exist.
 

Paignton pete

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Photos would help, but I feel the electricians you’ve had out may be right.
I would advise a full test and inspect. £185-£250 in domestic dependant on size of property, number of circuits etc..

This would highlight any issues, then another quote to rectify.

Sorry but it’s the only way I can see to find and resolve the problem.
 
I'd get someone to test the RCDs in case they are tripping too easily. Been having fun myself with a treadmill that trips an RCBO, sometimes...
 

telectrix

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I'd get someone to test the RCDs in case they are tripping too easily. Been having fun myself with a treadmill that trips an RCBO, sometimes...
stop over feeding the hamsters then. they're too fat.
 

Strima

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Pictures of your suppliers fuse, meter and boards would be a massive help.
 
  • Thread Starter Thread Starter
  • #11
Pictures of your suppliers fuse, meter and boards would be a massive help.
Meter
50501
Consumer Unit 1
50502
Unit 2
50503

Its the RCD in the first picture that has only ever tripped in the 8 years or so since we moved in.

There's also a single panel & switch for the shed which is switched off unless I'm out doing something.
50504
 

Attachments

I wonder why that first RCD is not just an isolating switch - presumably because it is a TT supply with a local Earth spike. If all the CUs have RCDs in them then another series RCD is inappropriate - especially if it is also a 30 mA one. Ideally RCDs should not be in series with each other. If it is a TT system that requires an overall RCD, then that one can be 100 mA or even 300 mA (needs to be check calculated after measuring the Earth impedance) which should stop the problem. Cheap and easy fix as long as the Earth spike is good and working properly.

However, with all your outside connections, I would also recommend a full EICR is carried out which will determine any Earth leakage due to damp problems. Such problems could cause an electric shock hazard on touchable metal parts (like gates).
 

Risteard

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Arms
Esteemed
I wonder why that first RCD is not just an isolating switch - presumably because it is a TT supply with a local Earth spike. If all the CUs have RCDs in them then another series RCD is inappropriate - especially if it is also a 30 mA one. Ideally RCDs should not be in series with each other. If it is a TT system that requires an overall RCD, then that one can be 100 mA or even 300 mA (needs to be check calculated after measuring the Earth impedance) which should stop the problem. Cheap and easy fix as long as the Earth spike is good and working properly.

However, with all your outside connections, I would also recommend a full EICR is carried out which will determine any Earth leakage due to damp problems. Such problems could cause an electric shock hazard on touchable metal parts (like gates).
As I pointed out earlier it is feeding a 100mA RCCB in one of the DBs.
 

Strima

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Arms
Esteemed
The lone RCD should be a time delayed/selectable with the one in the DB being 30mA. As it sits the one in the top board is doing the square root of nothing.

An EICR would give you more information about the state of your installation.

Also missing covers on the boards need sorting out as a matter of urgency.
 
  • Thread Starter Thread Starter
  • #15
Would it be too much of a stretch to think that IF the issue were in unit2 its RCB ought to trip first therefore is more likely to be something in Unit1 because the RCD there is never going to go before the lone one?

The sodium lights are off but maybe that doesn't matter? They are old (one of 2 doesn't work anymore) and are outdoors subject to the elements.

Is an EICR.... (electrical inspection?) something I should specifically ask an electrician to quote for? Is it like a test with recorded results?


Thanks for the replies so far, I appreciate everyone's time.

-Steve
 
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