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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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 06 January 2010 02:37 PM
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betnwah

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Originally posted by: Smith249I 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?



http://www.theiet.org/Forums/F...tid=205&threadid=33863
 06 January 2010 02:54 PM
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AJJewsbury

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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

yes, but not on a CPC that entered the bathroom...

and if all bonding was correctly installed there would be no potential difference between exposed,extraneous conductive parts (well no more than 50Volts)

I don't think BS 7671 makes any such provision! Just try applying Ohm's Law to a circuit with Zs = 1 ohm and Ze = 0.35 ohms.

- Andy.
 07 January 2010 10:37 AM
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Testit

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Originally posted by: Smith249

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


Then essentially what you are doing is a minor works. If the integrity of the cable to where your FCUs are is OK then what you are putting in has to be compliant so an RCD spur at that point would be acceptable... there is no requirement to upgrade pre-existing. It would be preferable to RCD the entire circuit but fully acceptable to simply RCD spur the cabling you are putting in as a minor works... Wiska boxes are the best for your box too...

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 07 January 2010 10:53 AM
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spinlondon

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I disagree.
The Regulations require the circuit to be RCD protected. Not part of the circuit, or just a little bit of it.
When issuing the Certificate, you are signing to say that your work complies with BS7671. If the circuit is not RCD protected, then your work will not comply with BS7671, and you will be commiting fraud.
Might just as well not bother with any RCDs at all.
 07 January 2010 11:07 AM
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AJJewsbury

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I disagree.
The Regulations require the circuit to be RCD protected. Not part of the circuit, or just a little bit of it.
When issuing the Certificate, you are signing to say that your work complies with BS7671. If the circuit is not RCD protected, then your work will not comply with BS7671, and you will be commiting fraud.
Might just as well not bother with any RCDs at all.

But BS 7671 defines a circuit as starting at an overcurrent protective device - a 13A BS 1362 fuse is such a device and so a fused spur unit (with or without integral RCD) starts a new circuit; so using an RCD FSU the whole of that circuit is RCD protected. Hence the letter of BS 7671 is satisfied. Not ideal from a touch voltage point of view I agree and different solutions might well be superior, but I don't see a non-compliance.

- Andy.
 07 January 2010 12:21 PM
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spinlondon

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No, the definition in BS7671 does not state that a circuit starts at an overcurrent device. It states that a circuit is a collection of electrical equipment bladi blah, sharing the same overcurrent protective devices. Note sharing and devices (plural).
In order to clarify this, the 17th now has Appendix 15.
This has easy to read diagrams, and quite clearly shows/states that circuits start and end at DBs. The diagram also shows FCUs as being part of circuits in a similar manner to socket-outlets.
 07 January 2010 12:36 PM
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daveparry1

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I disagree.
----------
That's unusual for you spin! I get the impression that all your jobs end up with c/unit changes and complete re-wires.
Sorry to be so flipant but in the real world not every (or many even) jobs can be done to the ultimate level of technical excellence.
 07 January 2010 01:01 PM
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AJJewsbury

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Note sharing and devices (plural).

So does the wiring before the FSU share all the same protective devices a the wiring after it?

- Andy.
 07 January 2010 01:23 PM
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Testit

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Ewe

My understanding is that when extending a pre-existing circuit you are certifying that your extention has not impaired the safety of the existing installation and that it complies with the regulation.

When I fill out an EIC I tend to put FCU's on their own test results line as that part of the circuit is not protected by the OCPD at the consumet unit, but by the fuse within the FCU, unless of course you want to state that a 1mm cable on a 3amp FCU is protected by the 32amp fuse at the CU of course... then I'd be saying its a non compliance...

For the minor works in question you dont have to upgrade the entire circuit, but what works you do do have to be compliant, therefore fiting the RCD FCU will comply with the regulations for the cabling that is added to the pre-existing circuit and providing loading and readings are OK that it does not impair the safety of the existing installation.

Debates about what constitutes the circuit is a bit of a red herring in my view... for minor works it is what you install from the point of connection that needs to be compliant without efffecting the safety of the existing installation.. which requires you to take into account loadings, voltage drop, and the circuit utilised integrity. There has to my knowledge never been a requirement to upgrade an existing installation that has been installed to a previous version of BS7671, ond only a requirement to install new or added circuitry to current regulations.... so in this instance RCD spurs do the job... I agree with myself and nobody can tell me different...

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 07 January 2010 03:09 PM
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briandoherty

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Originally posted by: Testit
...never been a requirement to upgrade an existing installation that has been installed to a previous version of BS7671, ond only a requirement to install new or added circuitry to current regulations...


I don't think that's strictly true, particularly with regard to earthing and bonding, as seen from the following:

From 16th Edition (130-07-01) (underline emphasis is mine):
'No addition or alteration, temporary or permanent, shall be made to an existing installation, unless it has been ascertained that the rating and the condition of any existing equipment, including that of the supplier, which will have to carry any additional load is adequate for the altered circumstances and the earthing and bonding arrangements are also adequate'.

From 17th Edition (131.8) (underline emphasis is mine) (NOTE; text of regulation below taken verbatim from article in Wiring Matters Spring 2009, so I hope the words are accurate as my 17th Edition is in the office, and I'm not trudging through the snow to get it!):
"131.8 No addition or alteration, temporary or permanent, shall be made to an existing installation, unless it has been ascertained that the rating and the condition of any existing equipment, including that of the distributor, will be adequate for the altered circumstances. Furthermore, the earthing and bonding arrangements, if necessary for the protective measure applied for the safety of the addition or alteration, shall be adequate."

In both of the above clauses, there is a requirement not just to ensure that the addition / alteration is adequate, but also to ensure that existing earthing and / or bonding on which the addition / alteration relies are also adequate, the implication being that if these particular aspects of the existing installation are inadequate, then they must be improved / made adequate in order for the addition / alteration to comply.

Furthermore (and I've only just noticed this change), the clause referring to the 'altered circumstances' (i.e. the circumstances / installation following the addition / alteration) is different, perhaps significantly so; under the 16th Edition, the existing equipment had only to be adequate if it "will have to carry any additional load", but this qualification has been removed from the 17th Edition version, which means that the existing equipment has to be adequate even if the load is not being changed. This begs the question 'What does "adequate" mean?'; does it mean 'complies with 17th Edition', or just 'looks OK', or just 'well, a bit too warm but not too distressed', or whatever? For instance, if the existing CPD has not been selected to take account of derating of existing cables being run buried in thermal insulation, etc, then under the 16th Edition, provided the addition / alteration did not result in additional load on that part of the existing circuit, this issue could probably be ignored as there was no need for it to be "adequate". However, under the 17th Edition, even if the load on the circuit is not being increased by the addition / alteration, this existing installation must still be left "adequate" (i.e. even if the "altered circumstances" don't included additional load).

I'm thinking out loud here...maybe I'll think some more while others are tearing my words apart!

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Regards,

Brian
 07 January 2010 04:20 PM
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ebee

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To my mind the main thing to watch out for is RCDing the bathroom part of the lighting cct by a FCU feeding the bathroom from the existing Non RCD lighting cct. In this case supp bonding may not be omitted because the whole cct is not RCDd just the bathroom bit of it so an earth fault outside the bathroom would still cause the cpc to jump up in voltage until disconnection which could be 5 secs and the RCD would not effect this.

If you take the cct to have it`s origin at the DB and therefore the cct is not RCd just some of it then the relaxation of supp bonding clearly does not apply.

If you take the fuse in the RCD FCU to be the origing of that cct (ie just the bathroom light being by definition its own cct) then you might feel that the relaxation of supp bonding applies. But for the reason of the cpc rising I don`t think that would be wise.

Personally I still supp bond as if the relaxation doesn`t apply but that is because I am unhappy about the possible 7% failure rate of RCDs. I`ve only recently been made aware of the cpc scenario by one of our good members

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

Knotted cables cause Lumpy Lektrik

Edited: 07 January 2010 at 04:27 PM by ebee
 07 January 2010 04:25 PM
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AJJewsbury

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Furthermore (and I've only just noticed this change), the clause referring to the 'altered circumstances' (i.e. the circumstances / installation following the addition / alteration) is different, perhaps significantly so; under the 16th Edition, the existing equipment had only to be adequate if it "will have to carry any additional load", but this qualification has been removed from the 17th Edition version

Perhaps because some changes could adversely affect the rest of the circuit - e.g. extending a ring to move a socket could increase Zs on original parts of the ring, regardless of whether the load increased or not.
- Andy.
 07 January 2010 04:29 PM
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Testit

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Well I concede the bonding issue, but I was relating to the circuitry worked on as opposed to the equipotental bonding issue which is required to be in place.... though technically this isnt always the case as if the EB is adequate you wouldnt necessarily upgrade to 10mm if 6mm and satisfying fault ability if it meant ripping up a 7k kitchen for example....... otherwise I'm afraid I hold my guns....

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 07 January 2010 07:29 PM
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spinlondon

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There's a myriad of ways they could have worded this Regulation.
They didn't choose current using equipment as they did in the 16th, they didn't choose cables as they have in another Regulation.
They chose Circuit, why is this?
411.3.3, states RCD protection is required for socket- outlets, and mobile equipment. Why refer to these items sepsrately? Why not just state circuits supplying socket-outlets and mobile equipment?
This allows for additional RCD socket-outlets to be added to an installation where the circuits are not RCD protected.
522.6.7 refers to cables conceled in walls, not circuits that have cables conceled in walls.
This allows just the additional wiring of an addition to an existing circuit to be RCD protected.
If it was intended that just the parts of circuits that enter a special location be RCD protected, why not word it so that would be allowed?
They haven't done so, because they want the whole circuit to be protected. There's no allowance for additions and alterations.
In fact it is the opinion of some that if work is conducted on just one circuit of a special location, it is a requirement that all the circuits be RCD protected.
To help with any confusion over this, we have Appendix 15.
This indicates where a circuit starts, and also indicates that FCUs are part of a circuit, not the start of a circuit.
 07 January 2010 08:32 PM
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daveparry1

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This allows for additional RCD socket-outlets to be added to an installation where the circuits are not RCD protected
-----------------------------------------------------------
But earlier you were saying whole circuit's had to be rcd protected so what would be the point of fitting an rcd socket?
It would be to protect the user of equipment connected to that socket, just as the rcd spur feeding circuit's (maybe I should say parts of circuit's) feeding the location containing a bath or shower.
 07 January 2010 09:03 PM
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spinlondon

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Originally posted by: daveparry1
This allows for additional RCD socket-outlets to be added to an installation where the circuits are not RCD protected
-----------------------------------------------------------
But earlier you were saying whole circuit's had to be rcd protected so what would be the point of fitting an rcd socket?
It would be to protect the user of equipment connected to that socket, just as the rcd spur feeding circuit's (maybe I should say parts of circuit's) feeding the location containing a bath or shower.


Whole circuits of special locations need to be protected.
FCUs do not feed circuits. They are parts of circuits, just as socket-outlets are parts of circuits.
Circuits are fed from DBs or CUs.
 07 January 2010 09:04 PM
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perspicacious

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Where did spin's post of 09:00 pm just go to?

Is a Mod watching already?

Regards

BOD
 07 January 2010 09:11 PM
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daveparry1

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you can't read.
I said whole circuits of special locations need to be protected.
---------
You really are an obnoxious person aren't you,
How can you be talking about whole circuits of special locations when wer're talking about extending an existing circuit to feed a special location? I think you really mean the part of the extended circuit feeding the special location. Never mind, as you're of the opinion that I can't read, i'll leave it at that!
 07 January 2010 09:55 PM
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spinlondon

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Originally posted by: daveparry1
you can't read.
I said whole circuits of special locations need to be protected.
---------
You really are an obnoxious person aren't you,
How can you be talking about whole circuits of special locations when wer're talking about extending an existing circuit to feed a special location? I think you really mean the part of the extended circuit feeding the special location. Never mind, as you're of the opinion that I can't read, i'll leave it at that!


What does it matter if we are extending an existing circuit?
Once extended all of the circuit is of the special location.
If you were to run a new shower circuit, you would not say that only the part of the circuit is of the special location.

Not me stating that someone has posted something that they haven't.
I suggest that before you state that someone has posted something, you first read what it is they've posted.
If you are going to quote posts, why don't you quote them in their entirety?
For instance my post read,
"I never said anything of the sort.
No wonder you are having problems understanding the Regulations, you can't read."
I edited it, as I thought it was probably a bit over the top.
Why don't you look at Appendix 15, and then explain how an FCU can be considered as the start of a circuit?
 08 January 2010 06:44 PM
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AJJewsbury

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To help with any confusion over this, we have Appendix 15.
This indicates where a circuit starts, and also indicates that FCUs are part of a circuit, not the start of a circuit


I think you're reading too much into appendix 15.

Appendix 15 is Informative, not Normative; thus it is not the place to be looking for definitions.

Appendix 15 does not purport to describe all circuits in general, just options for ring and radial circuits for sockets and FSUs.

The diagrams are examples, illustrations, not definitions. There's nothing to state that everything on the each diagram is the same single circuit. If that was the correct reading, then the "fixed equipment" (including its supply lead) would have to be deemed to be part of a 32A ring.

I still says that a FSU meets the definition of a distribution board for the purpose of the regulations anyway.

- Andy.
IET » Wiring and the regulations » Is this an acceptable way of complying with the 17th edition?

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