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Topic Title: Shared earth rod
Topic Summary: is this OK?
Created On: 25 January 2013 09:17 PM
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 27 January 2013 06:38 PM
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OMS

Posts: 18919
Joined: 23 March 2004

Originally posted by: BrucieBonus

Thanks for replies folks. I'll try to describe in detail what is there as my sketching skills are non existent...



Victorian house converted into two flats with steps up to front door. Under these steps in a cupboard (accessed from outside so accessible to both flats) are - two gas meters (both unbonded), 1 water pipe (ditto), 1 earth rod with 16mm cable going to first floor flat (as far as I can tell as no sign of it in the ground floor (my) flat). Also elec supply cable runs through the cupboard - (it's the old lead sheathed and paper wrapped job). and into the ground floor flat.

OK - get a single bit of minimum 6mm2 and ideally 16mm2 to that cupboard from the intake - fit a henly block and pick up the two gas and one water supply in minimum of 6mm2 and ideally 10mm2 - stick a label on the block to advise the purpose.

You could connect the earth rod at this point as well



In a built in wardrobe - behind a load of boxing in, coats, shirts, shoes, ball gowns, etc I am pretty sure is the main head (for both flats). I could not get access to this at the time due to not having Bernard Cribbens and his removal men with me. However, I am 99% sure that this is there and will be some sort of TNS earthing system (or not as we now know).

The end of the earth you've just taken to the cupboard into a henly block at this point - stick a cable from the sheath to this block, add connections to the earthing for the fisrt floor flat and for the ground floor falt (probably existing and minimum 6mm2)

From there, the meter tails for the ground floor flat (16mm, plus an earth, but probably not 16mm) are run in trunking through a few more wardrobes, about 2-3m, to the board. No RCD. Can't tell where meter tails for the first floor flat are yet, but some boxing in in the entrance hall might be a likely route

There is NO landlords supply/board - just a single light in the shared hallway, which must run off one or other of the two flats.

OK - what protects the tails for the first floor flatagainst overcurrent - I'm assuming the RCD's if present are in the new works the flat end - if nothing then you need to shout


My experience in the past (with EDF) has been that they will NOT maintain a TNS connection as in 99% of cases it is via an old bonding clamp, which they swore blind was not fitted by them. In all cases in the past I have ended up having to fit a rod. They were NOT interested even to find out if the earth (and therefore cable) was deteriorating.

Sounds about right

However, Zs IF you are now saying that UKPN are going to be more interested, then this is the best route to take. I will advise my client to call them out to check the earthing arrangements.

sensible call - getting the TNS earth back is the best option to avoid sub main or tails based RCD's (but note if buried less than 50mm deep or not earthed screen or mechanical protection then you'll be stuck with 30mA - effectively a single front ender for each flat


It's just occurred to me that maybe the first floor flat rod was put in because the sparky couldn't get access to the main head (and therefore MET) so decided to ignore possible TNS, shove in a rod and then did the board change live!

may well have done - it wouldn't be that difficult with gloves and a bit of prep


OMS - back to your post - yes I concur, 2 installs, but common MET (or should be)

for sure - see above


My mission - should I chose to accept it, is to upgrade the ground floor DB. I would upgrade the tails (if required) and separate 16mm earth plus 10mm bonds. The water bond will be common (as the gas in practice). I will of course advise the first floor flat if necessary, but I'm not being contracted my them (at the moment).

swap out the DB and see if you can pick up where the gas and water enter the flat - bond those if you can, if not then don't stress over it


One question I have is, if it is to be TT, do I have to RCD protect the tails with 100 mA S type? I estimate no more than 3 m and run in trunking

Depends on where you TT from and what happens with that TNS earth

If it makes no demand on the TNS sheath then yes, you'll need RCD protection - 100mA TD will be fine plus say a 60A switchfuse


Sorry for the long post - thanks for your help and advice so far...

No worries - just be careful if those nice chaps at UKPN offer you TN-C-S - then you really will need to get a minimum of 10mm2 bonding in place (and ideally do it all in 16mm2 to avoid any issues about what's just bonding and what is both a CPC and a bonding conductor



regards

OMS

-------------------------
Failure is always an option
 27 January 2013 06:49 PM
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BrucieBonus

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OMS - thanks for the very speedy and comprehensive answer. That's enough info for me to price up at least and adjust once I've got more access/info and an answer from UKPN,

One further question... if it's going to be TT, then does making a link to the lead sheath make it some sort of lash up earthing?? The lead sheath is an exposed conductive part???( Not extraneous) I recall somewhere in the past, a suggestion to make sure that it can't become exposed - ie enclose it with insulating material.... Or am I barking up the wrong tree here

Thanks again for your time
 27 January 2013 07:51 PM
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slittle

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Don't link it to the lead sheath.

Consider the implications if the sheath loses it's connection upstream and manages to become connected to one of the phase conductors on your side of the break. Not likely but possible. Where's the fault current going to flow.......

If it's going to be TT, then it's TT with no connection to any cable that the supplier may or may not make available.


Stu
 27 January 2013 08:25 PM
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BrucieBonus

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that's what I was thinking Stu....
 28 January 2013 10:18 AM
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Avatar for OMS.
OMS

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One further question... if it's going to be TT, then does making a link to the lead sheath make it some sort of lash up earthing??


Not a lash up no - if you get the system earth re instated then it's not a problem (it's often done on PME as an added electrode).

If you are absolutely definite you are going to TT both flats (including those tails and sub mains), then don't make that connection from the henly block to the sheath.

Comments noted from Stu - a consideration though - if the sheath is lost totally back to the sub then you have oily paper only as an insulator - the cable will last about 30 seconds in the ground before it totally breaks down into short circuit conditions - it's barely credible and evenless likley than a lost PEN on TN-C-S

Regards

OMS

-------------------------
Failure is always an option
 28 January 2013 06:23 PM
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BrucieBonus

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Well a quick update - the customer phoned UKPN who are supposedly coming out within 24 hours to 'fix the earth for free'. Hopefully they'll prove my cynicism wrong. I'll report back!

(thanks for the heads up Zs!)

If they do get it back to a TN system, then I'll notify upstairs that they need to get their spark back to remove the rod and retest their Ze. I would have thought that UKPN should do this anyway, but will do it just in case

Fingers crossed!
 28 January 2013 07:10 PM
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slittle

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Originally posted by: OMS

One further question... if it's going to be TT, then does making a link to the lead sheath make it some sort of lash up earthing??




Not a lash up no - if you get the system earth re instated then it's not a problem (it's often done on PME as an added electrode).



If you are absolutely definite you are going to TT both flats (including those tails and sub mains), then don't make that connection from the henly block to the sheath.



Comments noted from Stu - a consideration though - if the sheath is lost totally back to the sub then you have oily paper only as an insulator - the cable will last about 30 seconds in the ground before it totally breaks down into short circuit conditions - it's barely credible and evenless likley than a lost PEN on TN-C-S



Regards



OMS



I was thinking more the mick and paddy effect on the cable OMS rather than corrosion, granted unlikely but...

Stu
 28 January 2013 08:01 PM
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OMS

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Fair point Stu -

OMS

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Failure is always an option
 28 January 2013 08:03 PM
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OMS

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If they do get it back to a TN system, then I'll notify upstairs that they need to get their spark back to remove the rod and retest their Ze. I would have thought that UKPN should do this anyway, but will do it just in case


Not really required - you could just leave it in and connect as I suggested

regards

OMS

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Failure is always an option
 28 January 2013 08:12 PM
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BrucieBonus

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Oh, OK. I suppose thinking about it, the rod will just be like any other metallic part in the earth that would be bonded...
 29 January 2013 09:05 AM
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OMS

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For sure Brucie - if you think of the PILC supply cable as just a big electrode laid horizontally in the ground and running near it will be gas and water pipes with multiple connections between them (ie the bonding in each house or flat) - then add a couple of illegal PME conversions along the street and a smattering of properties off that main with unrevealed N-E faults and you start to see a picture that suggests the "purity" of any earthing system is a complete fallacy - it's just a big buzzing mass of currents flowing in all sorts of pathways as well as through the ground.

That electrode of yours won't do any harm - and as the networks deteriorate over time and more plastic is installed for gas and water services it might just be a significant benefit when that distributor or service cable to the head starts to lose both earth and neutral.

I'm not suggesting that you would deliberatly rod down a property as a matter of course (although on PME that may be a different choice) - just that as it's there and you'll (probably) be getting your TN-S back then it's a pointless exercise to remove it - particularly as you can easily connect it in that gas meter cupboard anyway.

regards

OMS

-------------------------
Failure is always an option
IET » Wiring and the regulations » Shared earth rod

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