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Topic Title: Whats the correct way?
Topic Summary: Spur from Ring
Created On: 24 April 2014 05:45 PM
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 24 April 2014 05:45 PM
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bak1950

Posts: 85
Joined: 24 June 2009

Say you were wiring a new shower pump. Decide to spur from a near ring. Ring has no RCD protection 30amp rewireable fuse and cables are buried in wall less than 50mm. So decide to install an RCD spur.
Ring is in 2.5mm. Pump only needs 1mm with a 3amp fuse.

So now for testing. You have already tested the ring.

So now you need to do a certificate. But hey do you test and certify only the new bit ie the RCD outwards onto a minor works, because that is all you are responsible for or do you go the whole hog with an installation certificate, to cover the ring as well, but of course this is not up to current standards due to no RCD protection. What fuse protection do you certify 30amp or 3 amp?
 24 April 2014 05:54 PM
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OMS

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The introduction of a fuse within the FCU, by definition, creates a new circuit so that's what needs to be covered on your EIC.

You could however take the view that it's an alteration to an existing circuit and use a minor works certificate

In either case, you should identify the FCU fuse as your protective device (plus the RCD) and note the lack of RCD protection onthe RFC as an abservation on the existing installation.

Personally, I would go the MWC route and fudge it

Regards

OMS

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Failure is always an option
 24 April 2014 06:01 PM
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mawry

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MWC for me!
 24 April 2014 07:35 PM
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daveparry1

Posts: 6374
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MWC for me too although I would put 30 amp for "protective device for the modified circuit" and also state that the addition is fed via an rcd fcu.
That question re the protective device, they do ask for the protective device for the modified circuit (which in this case is the ring) not protective device for the modified part!
Just my way of interpreting it,

Dave.
 24 April 2014 07:51 PM
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OMS

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Does a CPD not define a circuit then Dave ?

A spur off a ring doesn't actually need a FCU - we have one in this case because the OP wanted to RCD his new work - and introduced a fuse as a result - ergo we have a new circuit by definition.

A MWC suitably fudged should cover it

Regards

OMS

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Failure is always an option
 24 April 2014 08:13 PM
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daveparry1

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I think it would make more sense if they asked "protective device for the modified/extended part of the circuit".
 24 April 2014 08:27 PM
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spinlondon

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Unfortunately, the use of an RCD FCU would not comply with BS7671.
The requirement is for the circuit to be provided with RCD protection.
If you were to spur off of an existing circuit, the existing circuit would require RCD protection.
An FCU would not be considered as the origin of the circuit as far as BS7671 is concerned.
 24 April 2014 08:30 PM
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daveparry1

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Oh no, don't start all that again Spin!
 24 April 2014 08:44 PM
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spinlondon

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Not me starting anything.
Just pointing out what the requirements of BS7671 are.
Not my fault if people are unable to comprehend what is to my mind a rather basic requirement.
 24 April 2014 08:59 PM
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daveparry1

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So your argument is that the regs talk about "circuits of the location" if I remember correctly Spin? Now it's often pointed out that a cpd defines a circuit, so if that's the case we could say that the "circuit of the location" starts at the fcu? I don't fully agree with this because it would mean that if for instance we spurred from a ring to feed something like under cupboard lights etc. we would have to issue an EIC because a MWC cannot be used for a new circuit.
I must add that I don't usually get involved in all this pedantic play on words stuff!
 24 April 2014 09:39 PM
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spinlondon

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I'm assuming that you mean OCPD, not CPD.
The problem here, is not really to do with pedantic word play, it to do with safety.
In this particular instance, any earth fault current caused by a fault either in the fixed wiring of the ring or in an appliance plugged into the ring will be able to bypass the RCD FCU and enter the location containing the bath or shower.
Now if you believe that such a situation is quite safe, then I ask why bother with RCD protection at all?
 24 April 2014 09:40 PM
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ebee

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OK.
I`ve said it before so I`ll say it again.
As OMS quite rightly says , the OPD (3amp fuse) in the RCD FCU is the origin of this circuit.
This circuit itself is fed by a distribution circuit.
The distribution circuit , in this instance, is the "Ring Final Circuit".
Yes although a Ring Final is a final circuit it is still a dist circuit feeding the spur.
As OMS quite eloquently pointed out some time back , RCD protection on the RCD spur would not provide RCD protection to a bathroom etc location , in the truest sense, if a earth fault occurs in the dist circuit feeding it.
Something that I (and I suspect many others) had not considered.
In that respect we should really treat our new circuit as an extension of an existing circuit (ie Part of it) and not a new circuit in itself.

You could further this arguement right down to the installation cut out.


-------------------------
Regards,
Ebee (M I S P N)

Knotted cables cause Lumpy Lektrik
 24 April 2014 10:25 PM
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spinlondon

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Unfortunately BS7671 informs us that the origin of a circuit is at a distribution board.
Now if you want to call the FCU a DB, then please do.
Remember however that equipment is also required to satisfy the requirements of the relevant product standard.
As such, if the FCU satisfies the product standard for a DB then fine, it's a DB, and yes it can be considered as the origin of a circuit in accordance with BS7671.
Then of course there's the problem that BEAMA (the British Electrotechnical and Allied manufacturer' Association) state in their RCD handbook, that RDC FCUs are not suitable for providing protection to fixed wiring.
Considering that BS7671 requires that we take account of manufacturers' instructions, using a device for a purpose that the manufacturers' state the device is not suitable for, would to my mind be a slight cause for concern.
 25 April 2014 10:57 AM
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joepostle

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Just as a quick question as I would be interested in people's views, if this same FCU supplied a 'portable' appliance such as a dishwasher or washing machine etc. as opposed to fixed wiring / appliance how would that change the perspective on the appliance itself and the lead supplying the appliance (permanent wiring?). Obviously I understand the requirements for 'supply-side' for the FCU doesn't change!

Sorry to ask would could be the bleeding obvious but after observing this thread just would like to hear thoughts. Thanks.
 25 April 2014 10:15 PM
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spinlondon

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RCD FCUs are intende to provide additional protection for appliances.
 26 April 2014 03:32 PM
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AJJewsbury

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In this particular instance, any earth fault current caused by a fault either in the fixed wiring of the ring or in an appliance plugged into the ring will be able to bypass the RCD FCU and enter the location containing the bath or shower.

It can do that anyway - in any number of ways all perfectly compliant with BS 7671. There's nothing in BS 7671 to demand that there's a single CU at the origin next the the MET - it could be on the end of a submain (not uncommon with extensions) so faults on other circuits there would raise the potential on that CU's earth bar and hence on all other circuits supplied by the same CU (which could include your bathroom). Or earth fault currents could enter the bathroom via metallic pipework (even if bonded).
- Andy.
 26 April 2014 03:46 PM
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OMS

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Which is why the application of a bit of supplementary bonding to re establish the zone of protection is such a useful thing in terms of safety - in conjunction with RCD's.

The ideal would be to have the RCD close to the boundary of the special location along with the supplementary bonding in place.

Regards

OMS

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Failure is always an option
 26 April 2014 03:58 PM
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AJJewsbury

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Now if you want to call the FCU a DB, then please do.

Well, it is convenient to do so. If we didn't then recognising that a BS 1362 fuse is an overcurrent protective device and BS 7671's definition of a circuit and demand that circuits be connected to separate way of a distribution board, would mean the use of a fused connection unit at all would be contrary to BS 7671.
- Andy.
 26 April 2014 04:06 PM
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OMS

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Digging in for a 100+ posts Andy ?

OMS

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 26 April 2014 04:10 PM
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ebee

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I`m with you 200% on this OMS


(The RCD & Supp bond bit)

-------------------------
Regards,
Ebee (M I S P N)

Knotted cables cause Lumpy Lektrik
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