Earths at outbuildings...

[platty1005]

Boberto
Read your OP the origin is a TNCS if you supply the outbuilding with a SWA with a CPC from the main DB you are not exporting the PME system it belongs to the DNO and stops at the service head so the SWA should be treated as just another final circuit best protected by an RCD and complying with all relevant Regs . An earth electrode could be fitted at the origin as extra safety especially if the supply cable is overhead just in case there is a break in the neutral supply cable .

 

[Engineer54]

Boberto
Read your OP the origin is a TNCS if you supply the outbuilding with a SWA with a CPC from the main DB you are not exporting the PME system it belongs to the DNO and stops at the service head so the SWA should be treated as just another final circuit best protected by an RCD and complying with all relevant Regs . An earth electrode could be fitted at the origin as extra safety especially if the supply cable is overhead just in case there is a break in the neutral supply cable .

Not exactly, ....If that were the case, then there wouldn't be any restrictions on the use of a PME supply to the likes of caravan installations and the like. But your right in some respect, your not ''Exporting'' anything, your Extending the equipotential zone, especially when there is extraneous parts within the outbuilding and a correctly sized bonding conductor is used bond it to the MET at the origin.

 

[platty1005]

Eng 54 read 708.411.4 TNCS OK to permanent buildings but not to caravans or similar construction. So the Origin in a permanent building on a caravan site can be TNCS but all sub feeders must be TNS from Main DB. This supply to the outbuilding is actually a sub feeder and should be wired with a separate CPC.

 

[Engineer54]

Eng 54 read 708.411.4 TNCS OK to permanent buildings but not to caravans or similar construction. So the Origin in a permanent building on a caravan site can be TNCS but all sub feeders must be TNS from Main DB. This supply to the outbuilding is actually a sub feeder and should be wired with a separate CPC.

Sorry, not quite sure what your saying here. How can you derive a TN-S sub-feeder/sub-main from a TNC-S supply ?? All circuits or sub-feeders should always carry a CPC no matter what the system earthing arrangement is.

 

[platty1005]

The DNO supply cable is a PEN conductor to incoming service head. Separate Earthing conductor and neutral to DB Separate Line(s) neutral conductors and CPC's to final circuits and sub feeder circuits. So OK technically not a TNS supply from DNO but the sub feeders should all have separate neutral and earth and so are not PME supplies I admit I could have worded it slightly better

 

[La Poste]

An outbuilding is a permanent structure, the cable will not be overhead, there is no likely hood the outbuilding will move at any time soon so there's not much chance of losing the neutral IMHO.
TNC-S should be fine.

 

[Engineer54]

The DNO supply cable is a PEN conductor to incoming service head. Separate Earthing conductor and neutral to DB Separate Line(s) neutral conductors and CPC's to final circuits and sub feeder circuits. So OK technically not a TNS supply from DNO but the sub feeders should all have separate neutral and earth and so are not PME supplies I admit I could have worded it slightly better

I'm sorry but they are PME!! Just because the neutral and earthing conductors are separated at the point of distribution, the nature of the earthing system remains very much a PME system throughout the installation. And why a break in the supply neutral cable is hazardous to all connected earthed metalwork throughout the installation.

 

[platty1005]

Eng 54 A break in the neutral on the supply side is dangerous hence the PME. DNO earthing rods at various points. If the cable is underground the risk of a neutral break is minimal
Overhead to an installation much greater risk of losing the PEN hence the reason an electrode is often installed at the service head alongside the TNCS incomer
A PME TNCs supply is the realm of DNO's only. This means the installation after the DNO cutout must have separate protective conductors. The rules for caravan sites are to stop installers trying to save a bit of cash by running concentric supply cables with a PEN to caravan outlets. The OP is about a 30m run to an outbuilding likely from a permanent building with a TNCs supply. Exporting a PME I understand is running a supply cable with a PEN conductor and no separate CPC which is not allowed. If he runs a cable with a separate CPC he is NOT exporting the TNCS which was the original question

 

[Engineer54]

Eng 54 A break in the neutral on the supply side is dangerous hence the PME. DNO earthing rods at various points. If the cable is underground the risk of a neutral break is minimal I think i know what and how a PME system is derived
Overhead to an installation much greater risk of losing the PEN hence the reason an electrode is often installed at the service head alongside the TNCS incomer Rarely do the DNO provide an earth rod at the cut-out/service head. They may ask the homeowner to provide a ground rod on the consumer side. But then that rod would be connected to the MET ...Not to the service head CNE connection...

A PME TNCs supply is the realm of DNO's only. This means the installation after the DNO cutout must have separate protective conductors. ''Agreed'' The rules for caravan sites are to stop installers trying to save a bit of cash by running concentric supply cables with a PEN to caravan outlets. It has nothing to do with saving cash. So if they ran a CPC into the Caravan on PME derived supply, it is ok?? The OP is about a 30m run to an outbuilding likely from a permanent building with a TNCs supply. Exporting a PME I understand is running a supply cable with a PEN conductor and no separate CPC which is not allowed ''I don't think I've ever heard of anyone even thinking of running a PEN conductor or any cable without a CPC to an out building on a PME supply''. If he runs a cable with a separate CPC he is NOT exporting the TNCS which was the original question '' Are you saying that once this supply is in the outbuilding and supplying sockets and light it is no longer a PME system?? If you are , ...you couldn't be more Wrong!!''

My reply in RED!!

 

[platty1005]

Eng 54 please read the OP
He is running a SWA to an outbuilding and thinks he needs an earth electrode at the outbuilding. Providing he runs a separate CPC then there should be no problem. Just like any other Final circuit. Exporting the TNCS in my understanding is running a 2 core supply line and PEN AFTER the DNO service head. Which then makes the installation TNC which is not allowed. I know you know your stuff and I like to think I do. TNCs is for the supply companies Sparkies should always run a separate CPC that complies with the Regs. In the original OP he asked does he need to install a rod at the outbuilding and I think not!

 

[ackbarthestar]

As ive always understood it if you were to run a SWA to a garage / shed, lets say 30m away, and if the earthing system at the origin (house) is tncs then a earth rod would need to be installed at the outbuilding because tncs/pme cannot be exported.

Where the earthing system at the origin is a tt system am I right in saying that an earth rod can be installed at the out building especially if Ze readings on the rod at the house are particularly high. If this is the case would a 2 core cable suffice as a supply to the outbuilding or would a 3rd core (earth) be advisable (belt and braces)???

What is the norm with a tns system? Export the earth or install earth electrode at outbuilding?

Ive just read this back to myself and I havent worded it brilliantly, and its late, and i've been on the Coors so apologies for being a thicko.

Exporting the PEN or the earth part it has been viewed with concern in this country primarily because the rise in voltage potentials on the neutral (unbalanced 3-phase loading under normal conditions) and hence on the earth. Under supply fault conditions this potential will become very high (supply potential on single phase systems .
In the UK the IET have recommended that an earth electrode is provided at the MET to protect the installation to clamp the fault voltage to earth potential but can give rise to another problem such as high circulating currents. However, this was part of a wiring Matters article written a few years ago and is not explicitly stated as part of the regulations.

So IMO there are two concerns
1/ If you export the cpc it must first be able to cope with the possible fault currents (adiabatic)

It is unlikely that the armour of the SWA cable will be able to provide a low enough Zs value on its own, but might be Ok with the use of an RCD.
However, if you separated the earthing arrangements, by bonding the armour at the MET only and used an earth electrode at the point of utilization (outbuilding) Your main method of protection will be by the RCD and will allow you to have a much higher Zs in which case you would only need a 2-core cable

2/ Exporting the cpc away from the equipotential bonding zone to structures containing extraneous conductive parts.

It would be unwise to export any cpc where there is the possibility of external extraneous metalwork because of unaccountable or 'unknown' network faults which would include high resistance or loss of neutrals . In this case your earthing system would carry all of a proportion of the lost neutral currents from your neighbour's installation downstream of the network fault .
If you choose this method then I would recommend that you size the cpc to the same size as stated in BS7671 table 54.8 with a minimum of 10mm2. In which case you would be wise to use a 3-core cable equal to or larger than 10mm2

 

[Engineer54]

Exporting the PEN or the earth part it has been viewed with concern in this country primarily because the rise in voltage potentials on the neutral (unbalanced 3-phase loading under normal conditions) and hence on the earth. Under supply fault conditions this potential will become very high (supply potential on single phase systems .

In the UK the IET have recommended that an earth electrode is provided at the MET to protect the installation to clamp the fault voltage to earth potential but can give rise to another problem such as high circulating currents. However, this was part of a wiring Matters article written a few years ago and is not explicitly stated as part of the regulations.

This was a very common occurance on even TN-S systems in the early to late 50's.
There is never anything stopping you from adding a rod on a PME system now either, but it must be connected to the MET and not to the neutral /earth link at the service head...

So IMO there are two concerns
1/ If you export the cpc it must first be able to cope with the possible fault currents (adiabatic)

It is unlikely that the armour of the SWA cable will be able to provide a low enough Zs value on its own, but might be Ok with the use of an RCD. You will generally find that a SWA cables armour WILL comply upto 95mm and even further, ...check the links in the sticky's on this subject!!

However, if you separated the earthing arrangements, by bonding the armour at the MET only and used an earth electrode at the point of utilization (outbuilding) Your main method of protection will be by the RCD and will allow you to have a much higher Zs in which case you would only need a 2-core cable Why would you put all your faith in a RCD to protect you from earth faults when you can take a far superior earthing system to the out building??

2/ Exporting the cpc away from the equipotential bonding zone to structures containing extraneous conductive parts.

It would be unwise to export any cpc where there is the possibility of external extraneous metalwork because of unaccountable or 'unknown' network faults which would include high resistance or loss of neutrals . In this case your earthing system would carry all of a proportion of the lost neutral currents from your neighbour's installation downstream of the network fault . If there is extraneous metalwork at the outbuilding then you MUST run a bonding conductor not less than 10mm, Nothing to stop you still using the SWA armour as your CPC!!!


If you choose this method then I would recommend that you size the cpc to the same size as stated in BS7671 table 54.8 with a minimum of 10mm2. In which case you would be wise to use a 3-core cable equal to or larger than 10mm2
The bonding conductor can be incorporated within the SWA cable or run separately, it just makes more sense to incorporate within the cable if the cable is sized for 10mm or larger!!

I take it that you are of the opinion, that you should always leave PME/TNC-S at the supplying building and use TT systems at outbuildings etc. Thereby totally relying on an RCD for your earth fault protection, even though you read of RCD failures every single day on these forums, ...rather you than me!! lol!!!

 

[ackbarthestar]

I take it that you are of the opinion, that you should always leave PME/TNC-S at the supplying building and use TT systems at outbuildings etc. Thereby totally relying on an RCD for your earth fault protection, even though you read of RCD failures every single day on these forums, ...rather you than me!! lol!!![/COLOR]


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Not always, but depends on the soil resistivity, number of properties, location of TXs etc.

Thank you for your red outlined comments, I'm in agreement with most but for an outbuilding with extraneous conductive parts. You do not have to run a main protective bonding conductor back to the origin of the installation if the outbuilding is TT, because you have created a completely separate installation and all protective bonding will go to the new MET in the outbuilding.

I agree about the sole reliance on poorly manufactured RCDs, but as our UK regs are changing so we as electricians should be aware of these new changes and take them on board.

Just a small point
XLPE SWAs are used now in the UK more and more, Any (most) sizes larger than 6mm2 will not comply with fault protection which means that the maximum size realistically would be 6mm2 and then you would have to run a separate protective bonding conductor .

 

[Engineer54]

Not always, but depends on the soil resistivity, number of properties, location of TXs etc.
Thank you for your red outlined comments, I'm in agreement with most but for an outbuilding with extraneous conductive parts. You do not have to run a main protective bonding conductor back to the origin of the installation if the outbuilding is TT, because you have created a completely separate installation and all protective bonding will go to the new MET in the outbuilding.
I agree about the sole reliance on poorly manufactured RCDs, but as our UK regs are changing so we as electricians should be aware of these new changes and take them on board.

What's wrong with what i wrote about outbuildings with extraneous conductive parts, that's basically what's stipulated in the Reg's. What i don't, and will never understand, is why would anyone ditch a superior earthing system, in favour of a TT system with an Ra of around 200 ohms. The ONLY time i would even consider doing that, is if the financial aspects made it literary non-viable

 

[ackbarthestar]

What's wrong with what i wrote about outbuildings with extraneous conductive parts, that's basically what's stipulated in the Reg's.
I can't see a problem here if you are saying regardless of where you position you MET (PME at main MET or TT at Outbuilding) MPB must be in position and sized according to 54.8

What i don't, and will never understand, is why would anyone ditch a superior earthing system, in favour of a TT system with an Ra of around 200 ohms. The ONLY time i would even consider doing that, is if the financial aspects made it literary non-viable
You maybe right but I would get a bit twitchy if I had to rely on a Ra of around 200 ohms and as far as costs are concerned it would most definitely be a financial saving to keep the amount of copper used to a minimum.

I must now write something here to get the editor to recognize that I am not posting thin air

I have just noticed that we are beginning to track a 'sticky' thread on the general regulations forum