RCD keeps tripping..

Thanks Rebuke - Will move the RCD over to protect the lighting cricuits in that case.

Building Control has been notified of work on kitchen and bathroom circuits already as part of wider work (I have installed 3 RSJ's to facilitate kitchen knock through to a Victorian out-house) - they should be coming to inspect first fix electrics and sign-off on the structural work next week (once I resolve the RCD issue and finish first fix in bathroom - which I can't without a stable power supply).

On that note - and moving away from the RCD issue a bit - I have been getting advice from 2 different electricians on the reconnection/wiring of the kitchen ring.

The first said better to add FCU's as spurs from the socket outlets (1 max per socket) which I did - but the other is saying that it could put too high a load on the circuit and is a potential fire hazard...which doesn't make sense to me - but obviosuly I am no expert! I did query it with LABC and they said its fine to run 13a FCU's as spurs as long as there is only 1 spur per socket outlet and the appliance connected to the FCUs is rated below 13a.

So given my solution works and is acceptable to BCO I would like to stick with it - EXCEPT if it is a fire hazard or in any way more dangerous than having a continuous ring without spurs. (obviosuly this is all RCD protected by 63amp RCD at the CU side) and the electric oven will run from a dedicated circuit on 6mm t/e. - any views on why having FCU's as spurs is a bad idea?

thanks.

Also to follow up on the RCD protection of the lighting circuit - I don't intend to do this bit myself - but since I have a spare RCD now, and I have spare terminals in the CU is it possible to run 2 RCD's in a single CU? e.g could I use 1 for the lighting circuits and bathroom circuit (for shower) and 1 for the kitchen ring, upstairs ring and downstairs ring?

Would this double the fault tolerance and keep one circuit working if a problem developed in another? It would be good to be able to keep the lights on if the dishwasher developed a fault for example. Or is this not the way to do it?

Probably time to get a spark in - once the bank balance allows it!

Providing the socket you're taking the spur from isn't itself a spur you'll be fine.

It would be better to have the FCUs on the ring, but it's not inherently dangerous not to, since the FCU will limit what can be drawn to 13A, which is less than the capacity of 2.5mm cable (assuming that's what you used).

As for why it would be 'better', things that come to mind are that it makes it easier if you want to add extra sockets/FCUs later on, as you can just spur from the FCU, rather than having to either find a socket that hasn't been spurred from, or extend the ring, The safety argument is that if e.g. the CPC (earth) conductor in one cable got damaged (but live and neutral were left intact so you didn't notice), you'd still have an earth from the other side of the ring, but then this argument would apply to lighting circuits etc as well, which aren't generally wired as rings, so is kind of mute.

The only other thing to look at is how many sockets and FCUs you have in total, as you need to make sure you don't try and draw too much load in normal use - i.e. if it's a 32A ring, then you should only be drawing a max of 32A (after diversity) at maximum usage. This applies regardless of if you spur or put on the ring though...

Depending on the design of your CU (number of neutral bars, and number of spaces etc) you might be able to fit two RCDs, but getting a spark in would probably be best, since changing the protection method (adding RCD) is notifiable, and if your existing notification was specific about bathroom and kitchen, then it probably wouldn't cover it...

Ok, thanks - its getting quite technical now so I think getting spark is going to be best option on the CU side of things at the least. I just tried moving the unprotected circuit back and its still tripping the RCD. Final questions now - at least on this matter.

Once I bgan rewiring the kitchen ring, could any of the connections I have made in there be causing the trip even though the MCB for the kitchen ring has been off for months? If so then this is more likely the source of the problem as that work is recent.

Also - I will make this place sound like a death trap now (luckily I don't live here yet) - but I have not yet completed all the supplementary bonding to new pipework - could this be the source of my earth leakage?

I will post a new topic on supp bonding - but need to check other posts about it first as the answers are probably out there already - suffice to say I decided to go with copper pipe and plastic push-fit in my infinite lack of wisdom - which is going to make supp bonding a pain in the a**e!

If the kitchen MCB (or more importantly the neutrals) are on the RCD side of the CU, then yes, it is possible that is the source of the fault. Do you have any unterminated wires anywhere (e.g. something you haven't finished wiring yet), that might be making a high resistance contact between earth and neutral (a low resistance contact such as a dead short would *probably* be enough to cause it to trip straight away, so wouldn't explain what you're seeing...)?

Not having bonding in place wouldn't cause earth leakage as such, so I don't think that could be the cause of the problem (and is it just supplementary bonding that you're missing (i.e. cross bonding pipework etc in your bathroom), or are you also missing the main equipotential bonding (bonding to incoming gas+water pipes etc) - if the latter, you need to fix this ASAP as it can leave you in a very dangerous situation!).

rebuke said:

bongos: I assumed that to be a typo, and he meant the RCD still tripped - if an MCB tripped but the equipment kept working, that's much more worrying, as it indicates a serious wiring fault!

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Apologies! If I had read it again (properly this time) I might have realised it didn't make sense.

Spark123 said:

Like I said above, smells of a N-E fault with TN-CS.

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Is this because the PEN conductor on a TN-C-S system could allow current fluctuations from the supply side back to the consumer RCD if you had a N/E fault on the consumer side? The resulting L/N imbalance causing the RCD to trip? (stop me if I'm talking bo//ocks - just trying to get this idea straight in my head!)

If it was TN-S then almost all the time there will be enough potential between N & E for a short between them to cause the RCD to trip hence it would be almost impossible to have it stay in.
With TN-CS the neutral and earth are common at the service head hence no potential exists between them at this point and the RCD can be reset. As soon as a heavy appliance is used a small potential can exist at the CU (caused by the resistance/impedance of the meter tails etc), this potential difference can cause current to flow back through the fault and the earth wire, imballancing the RCD and causing it to trip.

Ok, thanks for all your help everyone. I bit the bullet and followed Spark123's advice for identifying the problem circuit from the CU - disconnected neutral cables and MCB's for each circuit in turn - isolated the fault to the downstairs circuit. Upstairs circuit and kitchen circuit both work fine now and RCD does not trip. Now, I just need to double check 5 double sockets and 2 spurs to find out what is wrong.

Rebuke - On the bonding side - I meant supplementary - although extraneous is not in place at the moment either. There isn't really anything to extraneously bond to at the moment - I ripped out all the old water and disused gas pipes from the subfloor and replaced the incoming water main with MDPE - except the first 400mm of it which is located beneath the living room floor. This piece of pipe is more than 1.6m from the nearest electrical cable, 4 metres from the consumer unit and almost 2 metres from the nearest socket outlet. Does anyone know if its still classed as an extraneous conductive part and therefore need bonding to the main earth terminal block at the CU?

Gas pipes have also all been ripped out apart from the PEX or MDPE mains supply pipe. I presume when the corgi refits the boiler and runs gas pipes back to the meter (next to the CU) he will bond it with the same 10mm Earth Cable which runs to the Earth Block on the Fuseboard beneath the consumer unit.

Should I be doing anything else in the meantime or is it relatively safe given both incoming water and gas pipes are fed by plastic pipe?

The MDPE Water pipe does switch to copper in the kitchen where the stop cock is located - at this point I intended to re-attach the previous 10mm Earth cable with a bonding clamp marked Electrical Safety device, do not remove...this earth cable runs direct to the block on the fuseboard beneath the CU.

From there I was intending to crossbond the copper pipes in the Kitchen and bathroom using 4mm Earth cable and the small copper bonding clamps they sell in B&Q (rather than the bigger ones used for extraneous bonding - is this acceptable?)

Do I need to cross bond around the plastic push-fit beyond the kitchen and bathroom including up to the rads in each room? Should I also continue this cross bonding under the dining room floor and in the sub-floor below the consumer unit (a hot and cold water feed run in the subfloor near the CU to feed a new understairs cloakroom sink and toilet)

Thanks again!

Supplementary EB does not go back to the main earth terminal.

It has not been a requirement to SEB in a domestic kitchen since the demise of the 15th Ed. in 1990/1.

Thanks securespark - I did read that it was not a requirement, but when the electrics were redone (new CU, new wiring and new socket outlets/light fittings throughout) 18 months ago they ran a 10mm Earth under the dining room floor and into the kitchen where it was bonded to the incoming metal water supply - the metal water pipe has since been removed but I figured since the earth cable was in place I should probably just connect it to the copper beneath the stop tap. Am guessing the removal of the requirement was based on the advent of RCD protection of kitchen circuits - surely reconnecting the EB link won't pose any hazard? Glad I don't need to do the supp bonding in kitchen though as there's too many plastic push-fits for my liking (should have gone with PEX pipe too I guess).

The supp bonding was not intended to run back to the terminal block beneath the CU - that only has space for 4 x 10mm Earths - 1 protected the gas supply (and will do again once its refitted), 1 protected incoming water main at the kitchen near stop tap (which I intend to reconnect to the copper as above), 1 went up to the bathroom - and will be reconnected to the rad and water pipes - with crossbonding across plastic push-fit, plus supp bonding to the legs of bath, towel rail etc. The final 10mm Earth is bonded to the main incoming electricity cable and I won't be touching that!

Although I am doing the work under Part P I do intend to pay a spark to come out and inspect everything and provide an electrical safety cert + move the RCD along the CU to protect the lighting circuits (following advice earlier in the thread) after the council has signed off on everything.

Thanks anyway - its always good to double check and triple check things like this.

Spark123 said:

With TN-CS the neutral and earth are common at the service head hence no potential exists between them at this point and the RCD can be reset. As soon as a heavy appliance is used a small potential can exist at the CU (caused by the resistance/impedance of the meter tails etc), this potential difference can cause current to flow back through the fault and the earth wire, imballancing the RCD and causing it to trip.

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Thanks. Was getting myself all confused.