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Topic Title: Zs measurement on MWC when no earth
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Created On: 06 January 2015 03:20 PM
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 06 January 2015 03:20 PM
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Nospark

Posts: 176
Joined: 05 October 2013

I've been asked to replace a domestic light fitting that is fed from twin and earth cable but the CPC isn't connected to earth. Other light fittings on the circuit are earthed. There is obviously a hidden JB or such like somewhere. I'd rather avoid trying to find where it has become disconnected because of buried cable, inaccessible ceiling voids etc. I would therefore like to replace it with a class 2 fitting. Is this acceptable? If so, how do I measure "Zs"?(R1+Rn?) And what is to be put on the Minor Works Certificate in the Zs field?
This of course raises doubts about the integrity of the line and neutral connections, but are we to start pulling houses apart because we suspect inadequate connections? That alone would keep me in work for several lifetimes!
 06 January 2015 03:26 PM
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Jaymack

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I suspect your handle says it all, but should you be carrying out electrical work?

Regards
 06 January 2015 05:15 PM
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AJJewsbury

Posts: 15812
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Clearly with no earth, the method of shock protection isn't going to be ADS. Hence ADS related parameters (R2, Zs and so on) have no relevance at all.

Missing earths on lighting isn't that unusual in older houses - I've found plenty where newish T&E is fed from an original twin with no c.p.c. at some point. Obviously a rewire is ideal, failing that adding an earth conductor is preferable (picking it off another circuit might be rough but doesn't appear to be prohibited by the regs). But in they end you're limited by what the customer is willing to pay for.

There is some guidance from some of the schemes about dealing with lighting circuits without c.p.c.s - (maybe someone can post a link?) - I'd use that as basis, but basically if you can use double-insulated fittings, copious notes on the certificate for both comments of the existing installation and departures of your work from BS 7671 (as the regs require an earth to be provided even at double insulated fittings in domestic situations), warning label at the CU about the lack of c.p.c.s and a strong preference for 30mA RCD supplementary protection and you should be able to sleep soundly.

- Andy.
 06 January 2015 05:18 PM
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AJJewsbury

Posts: 15812
Joined: 13 August 2003

BTW - a L-N loop test and insulation test should give you some idea if there's a significant problem with L / N.
- Andy.
 06 January 2015 05:21 PM
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Parsley

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 06 January 2015 05:35 PM
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daveparry1

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I think this is a completely different issue to where there's no earthing on the lighting circuit on for instance a c/unit replacement. If there is no earthing on the lighting circuit because it was installed that way then as said class 2 accessories and notes on the cert. (and a label on the c/unit with n/a in the Zs field on the cert.) is the way to go. In this case however, as the rest of the lighting circuit has earthing the problem is obviously being caused by a fault so this should be put right.
 06 January 2015 06:24 PM
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sparkingchip

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 06 January 2015 07:52 PM
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Grumpy

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I'm with His Parryship on this one. If the rest of the lighting has a cpc then there's at least one fault that needs fixing. (Sorry haven't the time/interest to read 7 pages of Spin digging a deeper and deeper hole)

-------------------------
Only dead fish go with the flow. Be a salmon!
 06 January 2015 10:07 PM
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leckie

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Originally posted by: sparkingchip

Remember this? We haven't been locked up lately!


I hadn't seen this thread before. It's fantasic.
 06 January 2015 10:27 PM
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Zs

Posts: 3814
Joined: 20 July 2006

Nospark,

Before your question disappears into us lot behaving badly.

I'd like to take a moment to commend you for finding out that the CPC is not connected. Particularly because this is not a normal case of an entire lighting circuit with no CPC but an individual fitting on an otherwise connected circuit.

I know plenty who only run their Z tests down L and N. This might not have shown on that test.

I could also name you twenty time-served electricians who would not have identified that.

An electrician who runs his tests is worth all of the other 20.

If you are still there...is this property looked after by any of the TN earthing systems?

Zs
 06 January 2015 10:38 PM
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leckie

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Nospark, read Andy Jewsburys reply, that gives you the info you need. Also read thought the BPG1 link that Parsley posted which gives some general principles to consider.
 06 January 2015 11:36 PM
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sparkingchip

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If you fit a class 1 light fitting to the CPC that is disconnected at some unknown point and then a live/ earth fault occurs in the fitting not only will that fitting become live, but also any other fittings including metal switches on the same side of the point of disconnection.

Also be aware of the dangers presented by someone using the CPC as a live for two way switching, which could be what the CPC has been used for.

Andy

Edited: 16 January 2015 at 08:09 PM by sparkingchip
 06 January 2015 11:41 PM
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mapj1

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That thread must have coincided with one of my away periods. Talk about herding cats..

Well, if you don't bring a valid CPC to the fitting it is not regs compliant. But if its double insulated its not actually going to make the fitting dangerous no to have an earth - if it had a flex and a plug it would be 2 core after all.

BUT, there is a big judgement call - do you think

a) there is a lurking fault, and once there was a CPC, but something has been tripped over in the loft or drilled though the middle, and is lurking waiting to bite,
or
b) do you think it was never actually connected, and is not any more dangerous than when it was put in, and there is no lurking fault.

depending on your answer depends what you can do next with clear conscience.

A cert with a note saying "departure from regs, no CPC provided to class 2 fitting",
is much less scary than
"suspect serious latent fault but cant be bothered to find it ".
That's no help really, as I m sure you knew that.

I presume a quick look at the nearest light switch and or ceiling rose don't reveal an obvious loose tail ?

-------------------------
regards Mike
 07 January 2015 07:29 AM
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Boolean

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Joined: 29 December 2014

A good quality tone/tracer might well trace the cable route

-------------------------


Charlie
 16 January 2015 08:00 PM
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Nospark

Posts: 176
Joined: 05 October 2013

Thank you everyone and than you Zs.
A fault finding I will go.
I have never found a reliable buried cable detector, could you recommend one?
 16 January 2015 08:13 PM
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sparkingchip

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A few weeks ago it took me several hours to change a bathroom light fitting as the CPC was not connect, however the several hours did include taking up and relay the landing carpet and floor

Andy
 16 January 2015 08:29 PM
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Nospark

Posts: 176
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and yes, TN system, (TNS if remember correctly)
 16 January 2015 09:47 PM
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leckie

Posts: 4319
Joined: 21 November 2008

Blimey, haven't you got this sorted yet!

It couldn't take that long
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