Yet more earthing questions

[rebuke]

So, having seen all the posts on here about bonding etc, I've investigated the current state of my house - From reading the wiki I believe I'm on a TN-S set up, as the incoming earth appears to come from the supply cable before it gets to the service head.

There are a few things that concern me though - firstly, the incoming earth conductor appears to be relatively small (~6mm), I was expecting it to be around 16mm based on the info in the wiki.

Secondly, I think that the supply to the adjoining property runs through my service head (there's two big cables going into it, one which comes from the ground, and one which goes down slightly and then straight through the wall (it's a party wall)) - both these cables have (unsheathed) earth wires coming off which are then joined in my earth block, does this mean that my neighbours earth runs through my earth block (which seems rather wrong to me - is it something I should get the DNO to sort out)?

Next - bonding sizes - at the moment I have (I think) 6mm going to the incoming water pipe (in a different room), and 6mm to a gas pipe - I gather from the wiki these should be not less than half the incoming earth and >= 6mm, so at the moment these are presumably OK, given the incoming earth is 6mm - but I guess I should probably upgrade them to 10mm so if I get the incoming earth sorted out they're still good?

Finally, I had a new gas pipe put in (during a service they discovered a small leak in the one buried in the concrete floor) - the old pipe is now capped off, but still exists (appearing by the consumer unit with the new one, behind a kitchen cupboard, and where there used to be a gas fire in the living room) - presumably it should still be bonded since it's still exposed metal work (the people who put the new pipe in just moved the bond from it to their new pipe)?

Sorry for all the questions - I have got access to a copy of the regs so am going to go and have a thorough read of the relevant bits, but I thought I'd ask on here as well...

 

[rebuke]

Doh - ignore most of my post, I was being an idiot and measuring the outer diameter, which obviously isn't the CSA of the conductor...

Looking at it again I think I do have 16mm incoming, and 10mm on all the bonding - so that just leaves the questions as to whether my neighbours earth should be running through my earth block, and whether I should bond the unused old gas pipe...

 

[Lectrician]

A photo would be good - We could then likely confirm cable sizes and the state of your incomming supply.

 

[ericmark]

Your question about shared supply head sorry don’t know the answer.
As to 6mm² this seems a common problem where originally there was a TT supply which did not require 10mm² earth connection reading the 17th Edition or BS7671:2008 it seems you should have 10mm² earth bonding however if you read Electrical Safety Council http://www.esc.org.uk/forum/forum_ed_thread.html EDQ12 then they seem to say you can retain the 6mm² earth. On a personal note where RCD protection is used I would say 6mm² is unlikely to cause any problems but where RCD’s are not used and the earth loop impedance is relied on to cause a MCB to trip with an earth fault than I would consider the BS7671:2008 recommendation of 10mm² is required.
As to the disused piece of gas pipe it is the same as any other metalwork within the house and you would not run earth wires to window frames etc.
The BS7671:2008 is not easy reading and even the Electrical Safety Council quote 701.415.2 as the regulation which only refers to bathrooms! Because both selection and calculation is permitted (543.1.3 or 543.1.4) which is further complicated by cross reference regulation 543.2.3 seems to say min of 10mm² but this refers also to 543.2.2 which refer to 543.2.6 which refers to 543.2.1 etc. Which means it is hard to read but regulation 544.1.1 says except where PME (Also called TN-C-S) conditions apply then min is 6mm² Table 54.8 is one where we have more than one supply yet 542.3.1 refers to 544.1.1 of which table 54.8 is a part for earthing conductor size with PME.
So S = √I²t/k with a TN-C-S supply I = 657 and with a TN-S supply I = 287.5 taking t as 5 and k from table 54.2 = 143 then S = 10.27mm² for TN-C-S supply and 4.5mm² for a TN-S supply since 6mm² is min then TN-S only requires a 6mm² earth.
Why have I made it so compacted well it is really to point out why there are so many conflicting reports as to the size of earth cable required.
Figures use 0.35ze for TN-C-S and 0.8 for TN-S which is what the supply authority is required to provide these may not be actual figures.
Or to put it simple don’t worry about it there is really no problem.
Eric
PS better not to add second post to first as count becomes 1 rather than 0 so less people look at it.

 

[ericmark]

Sorry &#8730 should have been square root sign.

 

[rebuke]

Pictures as requested (quality isn't ideal - this is all under the stairs where it's quite dark - I tried to use a flood to light it up, but camera still didn't focus too well).

In the terminals of the earth block, the cables as far as I can tell from left to right are:
16mm earth to consumer unit
10mm earth to water incomer (in a different room)
The two unsheathed earths from the two electricity board cables (one presumably is the incomer, the other presumably goes to next door)
10mm earth to gas pipe

I'm not sure what size the earths from the incoming cables are - they don't appear to be 16mm which is what I'd be expecting...

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• rebuke
• 16 Nov 2008
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• rebuke
• 16 Nov 2008
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• rebuke
• 16 Nov 2008
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• rebuke
• 16 Nov 2008
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[Lectrician]

It is exactly as you have stated in your post.

The earths to the lead sheahs are the property of the DNO, and are undersized really.

 

[rebuke]

Fair enough - I'll give them a ring during the week and see if there's anything they can do (I guess they might be able to switch it to TN-C-S rather than TN-S depending on the service head etc?)