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Topic Title: Is this an acceptable way of complying with the 17th edition?
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Created On: 05 January 2010 02:59 PM
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 05 January 2010 02:59 PM
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Smith249

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Joined: 09 September 2004

Got a small job to do, bathroom is being re-fitted and wants a new ceiling light, extract fan and an on demand pump for the shower which will fit inside a locked cupboard within the bathroom. I'm going to fit an isolator inside the airing cupboard for the pump.

This is a small job, however the problem there is just a 8way fuse box, no RCD protection.

I don't want to put a 30mA in the tails, so how is best to achieve 30mA protection for the bits of the existing installation I am modifying? they do not want to have a new c/u fitted despite me strongly advising it.

Is it aceptable to put 2x 30mA RCD/fused spur units inside the cupboard, one for the lighting in the bathroom and one for the pump? - does it matter that the wiring before this is not RCD protected?


Supply TN-C-S
Bonds to gas and water are already in place and are 10mm with a 16mm from MET to cut out.

Thanks.
 05 January 2010 03:31 PM
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daveparry1

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I don't see a problem with that, maybe put the one for the pump on the ring circuit? It won't matter that the wiring before the fcu's isnt rcd protected, it's only the bathroom circuit's that are being modified. Other than that I suppose you could put a garage type c/unit with rcd and 2 x 6a breakers next to the existing c/unit, a bit ott though I suppose!
 05 January 2010 03:40 PM
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Smith249

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Dave, Thank You

I did mean I'd take the shower pump from the ring as there is a ring supply in there already as the heating clock is taken from it. All I would do with the lighting then is just route the bathroom through the other RCD spur.

How would I word the MWC? in departures from BS7671 do I put that wiring up to RCD spurs does not comply with the 17th edition? but the bit I modified does, is this acceptable?

I did consider putting a 2way C/U next to the fuse box one for the ring circuit I'm modifying and one for the upstairs lights, but how would you supply it? a DP henly block will look messy as currently there is just a small fusebox showing in the hall way and then how would you provide one point of isolation? then also I would have to do a 3 page EIC.
 05 January 2010 03:41 PM
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Testit

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Depends on the setup.. but assuming your wiring before the spurs is pre-existing or that it is surface mount anyway then it won't matter.. just fit the 2 RCD spurs.. if you are running new cable to the spur however, then you would need to make a judgement call about safety and if need be state it as a non-compliance...

You can also get inline RCDs that you could place on the outgoing cables at the CU.

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 05 January 2010 03:43 PM
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spinlondon

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The Regulations require the circuits to be RCD protected.
As a circuit starts at the DB, then the RCD should be at the DB to protect the circuit.
It is possible that a fault on the circuit before the RCD could introduce a fault current into the bathroom. There would be a greater likelyhood of this occuring if the circuit is a ring.
 05 January 2010 03:56 PM
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Smith249

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Testit, Yes cables upto where I want to put the spurs are existing, all contained within the fabric of the house, all I would be doing is adding a cable from a fused spur in the cupboard, clipped down where it will disappear under the floor and re-appear in the bathroom cupboard into an IP65 adaptable box where the flex from the pump will connect. And then the lighting will be surface in the cupboard and then surface in the loft
 05 January 2010 04:01 PM
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Smith249

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What about putting a 2 module enclosure next to the fuse box containing 2x single module RCBOS and just breaking into the cables that come from the fuses of each of the circuits?, that way I don't have to worry about splitting the tails or providing one point of isolation? which would be a problem if I put a garage type c/u next to the fuse box.
 05 January 2010 04:36 PM
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spinlondon

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That's the way I'd go, if there was no chance of upgrading the old CU.
 05 January 2010 05:21 PM
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Smith249

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Many thanks,

What would be the right certification if I went the rcbo route? could I use a MWC as its not a new circuit.

What would be best to join between the fuse and the rcbo for the ring? bit of 4mm ?
 05 January 2010 05:26 PM
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daveparry1

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The Regulations require the circuits to be RCD protected.
As a circuit starts at the DB, then the RCD should be at the DB to protect the circuit
-----------------------------------------------------
Not quite correct spin, the regs. say that circuit's IN the location need to be rcd protected, there's nothing wrong with putting an rcd/fcu outside the bathroom to feed circuits whithin.
 05 January 2010 05:29 PM
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Smith249

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I don't know which option to go for now!!

 05 January 2010 05:40 PM
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daveparry1

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Go with your original plan, nothing wrong with that, do two mwc's as you're working on two separate circuit's, no need to mention any non-compliances as there won't be any if you're not adding any wiring that's concealed before the rcd.
 05 January 2010 05:42 PM
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AJJewsbury

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Personally, in the circumstances, I'd go for RCDs (possibly RCCBs rather than RCBOs) in a new enclosure next to the existing CU. Showing the customer that might persuade them than a new CU might be neater after all...

The regs are a bit oddly worded. Technically RCD FSUs contain an overcurrent device (a fuse), so everything after them is a distinct circuit in strict BS 7671 terms. Yet an RCD close to the origin is clearly better from an imported touch voltage point of view, but by the letter of the regs an RCCB a few inches after the original fuse doesn't protect the entire circuit

say that circuit's IN the location

From memory, I thought it said OF the location, but I'd have to check.

- Andy.
 05 January 2010 05:49 PM
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spinlondon

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Originally posted by: daveparry1
The Regulations require the circuits to be RCD protected.
As a circuit starts at the DB, then the RCD should be at the DB to protect the circuit
-----------------------------------------------------
Not quite correct spin, the regs. say that circuit's IN the location need to be rcd protected, there's nothing wrong with putting an rcd/fcu outside the bathroom to feed circuits whithin.


Sorry the wording is: "all final circuits of the location."
As a circuit starts at the DB (see Appendix 15), and the Regulations do not state part of the final circuit, or just a bit of the circuit.
You need to ask yourself why is RCD protection required.
Then consider what would happen If you were to take for instance a feed from a non RCD protected ring circuit. Any fault to earth that were to occur before the RCD, would allow the fault current to enter the bathroom via the earthing, by-passing the RCD.
Depending upon the installation, this earth fault could be in a fridge, cooker, washing machine, kettle etc.
 05 January 2010 05:55 PM
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daveparry1

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If spin's theory is correct it will never ever be worth using rcd/fcu's ever again as this situation could arise anywhere, bathroom or otherwise!
 06 January 2010 09:30 AM
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dg66

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Surely any fault in any part of the system will see a potential rise on everything connected to the MET regardless of where the RCD is situated.

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Dave(not Cockburn)
 06 January 2010 10:47 AM
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AJJewsbury

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Surely any fault in any part of the system will see a potential rise on everything connected to the MET regardless of where the RCD is situated.

There will be a voltage across all conductors carrying the fault current (Ohm's Law). On a TN system, the final circuit CPC is likely to see the larger voltage difference, the MET is likely to remain much closer to true earth (depending on the exact location of the fault and the relative impedances of L and the CPC between the origin and fault verses Ze). On a TT installation (provided no significant parallel paths due to bonding) the bulk of the voltage difference would be external to the installation (usually in the ground around the electrode), together with a lower fault current, would mean a smaller voltage along the CPC.

- Andy.
 06 January 2010 10:50 AM
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potential

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Originally posted by: dg66

Surely any fault in any part of the system will see a potential rise on everything connected to the MET regardless of where the RCD is situated.


Precisely.

Potential rise on anything connected to the MET is hopefully made safe by bonding.
 06 January 2010 01:04 PM
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AJJewsbury

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The pre-RCD section of the circuit would often have much longer disconnection time that that provided by the RCD - having the RCD just outside the bathroom might be similar to importing a CPC from a non-bathroom circuit with up to a 5s disconnection time.

- Andy.
 06 January 2010 02:09 PM
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dg66

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Even if the RCD was at the CU and the fault was on a non RCD protected circuit then the cpc potential rise would remain until the fault was cleared,and if all bonding was correctly installed there would be no potential difference between exposed,extraneous conductive parts (well no more than 50Volts)

-------------------------
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Dave(not Cockburn)
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