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Discuss Exporting PME to outbuildings in the Electrical Forum area at ElectriciansForums.net

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Hi all,
Just wanted to run this past some people, its for a little job im doing (possibly by moonlight). The property has recently (yesterday) been converted to PME when the DNO changed the cut-out.
It is a farm and ususally around here they will not give PME to farms. The old service was TNS with direct bare earth overhead to the transformer (20 yards away) which came to the MET.
The cut-out started arcing and was replaced by a call out engineer in the evening, the callout engineer has put the bare earth in with neutral and then looped out to the MET.

No problem you may think, however, the supply feeds some switch fuses which feed 2x outbuildings, each with their own metallic services (water and structural steel). These buildings are protected by 300ma type s Rcd's at their origin and one has a 'supplementary' stake, but the supply earth from the 16mm2 T+E's is still connected.

Now, it may be that I should just get the DNO to reconnect the TNS earthing, however assuming that they won't, my plan was to run 2x 10mm G/Y singles alongside the T+E's feeding the buildings, thus continuing the PME, however if I was to do this, would I have to run a 10mm for every bonded service in each of the outbuildings or would one suffice?
 
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E

Engineer54

A Bare earth connected in with the neutral at the cut out!! Good job there is little chance of a neutral break if that bare earth is buried underground for any length.... lol!!!
 
K

KFH

I believe one would suffice for each building but I would be very tempted to look at TT for the outbuildings especially if livestock is involved. The structural steel may be an adequate earth.
 
G

Guest55

I believe one would suffice for each building but I would be very tempted to look at TT for the outbuildings especially if livestock is involved. The structural steel may be an adequate earth.
i agree.
This very scenario was recently discussed on the IET forum and was suggested that you can TT the outbuilding supplies and use the structural steel as your earth rod.
 

spark 68

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Arms
Esteemed
GN7 says although PME is not banned for installs with livestock, it is not recommended, but if PME is used a metal grid should be in the floor and needs to be included in the supplementary bonding to all extraneous and exposed conductive parts of the location.

There is quite a bit more to this see diagram 705 in the BGB.

It will be far simpler to segregate the earthing system and TT it.
 
  • Thread Starter Thread Starter
  • #6
The bare earth comes overhead from the transformer, there is only 1 pole between the transformer and cut-out.
I am reluctant to TT the buildings if I don't have to as they have metalclad boards, although they are already protected by a 300ma Rcd as the incomer on each of them, one has a 'supplementary stake' although this is a twig. The structural steel would suffice as an earth as it gives sub 5 ohm readings, as does the steel water piping (which is the customers not the water board) Both are bonded in 10mm2.
These buildings are an office and a workshop, the livestock buildings are already TT.

So you reckon 1x 10mm with each sub-main to continue the bonding would be ok, rather then one for each bonded service?
 
G

Guest55

One shared protective bond for each building would suffice if you dont want to go down the TT route.
 
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