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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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 25 January 2013 09:17 PM
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BrucieBonus

Posts: 686
Joined: 20 February 2007

Is it OK for two flats in the same converted property to share the same rod, presuming it's well labelled as such and there is agreement by the two flat owners?

I ask as I looked at a job this evening in a ground floor falt where it looks like the top floor flat has a rod, but not the ground floor (couldn't get full access to check). Loop reading of 35 Ohms at a socket in the ground floor and 3036 board.

The main earthing conductor from the rod was 16mm

Thanks

BB
 25 January 2013 09:23 PM
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daveparry1

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So were loop readings similar upstairs? Could you see an earthing conductor between the two flats? Both flats rcd protected I presume?

Dave.
 26 January 2013 08:34 AM
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BrucieBonus

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Hi Dave

No, as mentioned I couldn't get access upstairs - this is to be arranged. And also as mentioned, the ground floor flat DB is rewireable fuses - no RCD. Obviously this would need to be changed!

At this stage I'm trying to establish if I have to fit another rod (which seems bonkers as they will be at most 1 m apart) or if I can use the existing one

Thanks

BB
 26 January 2013 09:15 AM
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daveparry1

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Yes Bruce I did note that you said downstairs has re-wirable fuses but I thought there would be a front-end rcd or elcb there. I don't see any real problem with the shared rod provided both flats are rcd protected, what i'm thinking is that a fault to earth in one flat could send volts up to the other one via the earthing conductor but I suppose the rcd would take care of that, albeit that they would probably both trip.
Bonding of both flats should be connected back to the same point too I reckon. Having said all this, maybe a seperate rod would be better although they would need to be at least a couple of rod lengths apart I think to prevent a potential on one ending up on the other one,

Dave.
 26 January 2013 09:48 AM
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perspicacious

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"Is it OK for two flats in the same converted property to share the same rod, presuming it's well labelled as such and there is agreement by the two flat owners?"

What is written in the agreement with their freeholder?

Regards

BOD
 26 January 2013 11:22 AM
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BrucieBonus

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thanks for replies

Dave - yes thanks for clarifying. No RCD in ground floor. I ASSUME (but not able to check yet) that RCD in top flat if someone has taken the trouble to fit the rod - but I will check when I get access. As for a second rod, it really could be no more than 1 meter away, so I guess that that's the same essentially as having just one rod! Maybe this is the way to go. I understand about the possible double tripping problem and can warn them both

BOD - I would imagine nothing as it's in London where rods are not the norm. The rod looks quite recent.. I will make some enquiries though - thanks for pointing this out
 26 January 2013 05:03 PM
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peteTLM

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id say 1 rod would be the preferred way to go. 2 flats inside 1 structure would have to have the same earthing arrangement anyway, what better way then by tying them together.
Presumably they would have a shared path via metallic services in some shape or form which we wouldnt want to have 2 different potentials held together by gas or water pipes.

crack on.

P

-------------------------
----------------------------------------
Lack of planning on your part doesn't make it an emergency on mine....

Every man has to know his limitations- Dirty Harry
 26 January 2013 05:36 PM
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BrucieBonus

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Cheers Pete!

Crack on - yes once they've agreed to my price!!

BB
 26 January 2013 06:10 PM
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metsystem

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Gents,

The resistance between two earth rods only 1m apart in soil, (even if wet) will be in the 10-100s of Ohms range. Should you link your 16mm Main earth conductor of both flats to a common point, you have created a low resistance path between the two flats earthing system.

In reality there are probably fortuitous earth paths through the buildings anyway, however I would not recommend commoning up the main earth conductors, as certain fault conditions could cause problems.

One thing to consider is if the flat block takes a three phase common supply and earth from the network, in to the landlords ,and then splits off to seperate single phase supplys. Or if there are individual single phase supplies coming in from the network.

Have you checked BS7671 to see if this recommends anything.

Regards

Alex
 26 January 2013 06:30 PM
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BrucieBonus

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OK should have clarified - it's a single phase shared head in a Victorian conversion to 2 flats, not a block.

If it was a standard TNS system as is common for this set up (and indeed probably 'was' like this until the earth started to disintegrate.....) then there would be a single common point of earth anyway - so what is the difference? (I'm sure someone here will enlighten me)

I can't find anything in the Regs, but then that was the reason I asked on here
 26 January 2013 06:52 PM
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perspicacious

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"it's a single phase shared head in a Victorian conversion to 2 flats"

What is the earthing arrangement?

Are there two meters that are billed by the MO(s) or is it still a landlord meter that has private check meters, possibly only one for each flat with the difference between the summation being deduced as the landlord element?

They were converting houses into flats pre 1903?

Regards

BOD
 26 January 2013 07:08 PM
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metsystem

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Brucie,

From what your saying you believe the system used to be a TNS but is now a TT.

The supply is the same single phase incomer for both.

If you have converted it from TNS to TT then obviously the supply characteristics have changed. I would say that as long as both properties are protected by 100mA RCD's on their respective DBs then you'll be fine on the one earth rod

Cheers
 26 January 2013 07:26 PM
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BrucieBonus

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BOD - ha, ha! You know what I meant!

No landlord meter but sep meters for each flat (one will be covering the cost of the entrance hall light - big bucks!)

I've not done any work there (yet) but someone has seen fit to stick a rod in an covert the upstairs flat to TT The downstairs flat is.... well TNS/TT/???. They've asked me to upgrade the DB - and I would 30mA RCD protect all circuits.
 26 January 2013 07:51 PM
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daveparry1

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Are you sure it wasn't a gas "engineer" that put the rod in just to "earth" his gas pipe Brucie! I had this happen some time ago, i'd put a new supply in for boiler electrics including bonding and some time later a BG "engineer" went to do a service and told the customer that the gas pipe had to be connected to an earth rod! (tncs system) You have to laugh don't you!!

Dave.
 27 January 2013 06:55 AM
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ebee

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Yes Dave I see it all the time.
TNS system in for years then a brand new gas pipe with an earth rod attached!
LOL

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

Knotted cables cause Lumpy Lektrik
 27 January 2013 08:41 AM
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sparkiemike

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Originally posted by: BrucieBonus

...
If it was a standard TNS system as is common for this set up (and indeed probably 'was' like this until the earth started to disintegrate.....) ...


If there was s TNS arrangement don't the DNO have an obligation to maintain that earth?

Are you saying that there is currently no means of earthing for the downstairs flat?
 27 January 2013 10:36 AM
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OMS

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Brucie - take a look initially at 542.1.8 - decide if you have two "installations" and then decide if your protective conductors are common to both.

In this scenario, I would say you have two installations and that you have common conductors.

You also (presumably) have a MET at the supply intake position (ie on the end of that original TNS service.

I would take a look at:

1 - The means of earthing to each flat - is it via a 6.0mm2 CPC in a 16mm2 T&E for example

2 - The position at which main equipotential bonding is situated and connected (if not to the MET, then via what is effectively an earth marshalling bar in each flat)

3 - Where that earthing condutor to the electrode emanates from

Ideally you want a MET at the intake with a bond to gas and water incomers from it

You then want bonding to gas and water where it enters each flat from the flat DB earth bar

You want a means of earthing for that MET - that could be the TNS sheath or the TT electrode or both

You want overcurrent protection for the supply submains to each flat (which may need to incorporate RCD protection if the system goes TT or depending on the installation method or both)

The CPC in the sub mains ideally want's to be 16mm2

you also mentioned a landlords installation - same drill for that although you probably don't need the additional bonding as that at the origin will tend to suffice.

to answer your specific question - yes, one electrode will do - but see generally the above for the optimum arangement.

You could arrange some hybrid version of course - just try to ensure there is something reasonablly big in terms of CSA between each "installation" - it could be copper cable - but a combination of gas, water pipes and/or any common metallic elements would suffice if you can be resonably assured they are likley to remain in place over the installation lifecycle.

If you are working for the landlord then no drama, just advise what he needs to provide for the "common parts". If you are working for the GF tenant, then advise them to advise the landlord that a bit of remadials (maybe) are required.

If the first floor flat is TT'd from the flat then the original installer is a bit adrift if the solution was to address a common lack of earthing from the former TNS supply.

You could of course leave it that way and if the TNS earth is good, then use that for the GF flat.

On balance, first decide what the problem is (if any) and then come up with the solution - generally as above, if I've read your posts right.

Can you sketch out what's there and post that - always easier to answer by looking at a picture

regards

OMS

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Failure is always an option
 27 January 2013 10:52 AM
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Zs

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Originally posted by: BrucieBonus

At this stage I'm trying to establish if I have to fit another rod (which seems bonkers as they will be at most 1 m apart) or if I can use the existing one.


Hi Brucie,

1 m apart bothers me for two separate properties. As does one supply with two earthing arrangements. I think you are right that you need to have a look at both flats before deciding what to do. I'm seeing an unfused link in one of them in my head,

If the upstairs flat has been botched over to a TT there's no reason why you should do the same.

UKPN are brilliant these days, ever since they put trackers in their vans.

Here is a number for them 0800183 8838

I'd be inclined to give them a call and log a query about a disintegrated (?) TNS. Make it seem worse than it is. Then they'd come out and check it all from their side of the fence and if there is a reason for a TNS being faulty, it'll get fixed. I look upon a TT rod on an old TNS as an emergency measure.

Zs
 27 January 2013 11:31 AM
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OMS

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Just be minded that the DNO "repair" may well get you TN-C-S - and in that case you really do want robust copper between the both flats - hence my suggestion above of a minimum of 16mm2 in all parts of the common system.

There is no need to have seperate rods - it's an exercise in futility unless your keep fit regime requires you to hammer in an electrode a day. If the system is common, (as it almost certainly will be) then one electrode is fine - if you want seperation (and I can't possibly see why) then you need to be many many metres apart to avoid overlapping ground potentials.

regards

OMS

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 27 January 2013 06:16 PM
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BrucieBonus

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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.

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).

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.

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.

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.

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!


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

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).

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

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