Exporting TN-C-S to outbuilding - again!

[THRIPSTER]

Would welcome your views please. Please note that I have posted exactly the same question on another forum as I am keen to get as many views on this as possible - it seems to be a contentious issue!

Domestic property, TN-C-S described as PME feeding 10mm² to outside wall of house where it changes (inside IP rated box) to 3 core 6mm² SWA to outbuilding 25m away. SWA terminated in plastic box, no banjo fitted. Three cores carry on to plastic DB where only two cores are used as Line and Neutral. Outbuilding TT'd. MET of outbuilding DB connected to rod and to incoming copper water pipe. Other end of copper water pipe bonded to MET of house DB. In effect therefore, the outbuilding DB is connected to the house MET by the copper pipe and would suffer the same consequences as had the SWA and unused third core of the SWA been used to earth the outbuilding DB. How, under the regulations, should this situation be dealt with?

Thanks

 

[Spark123]

The biggy with TN-CS is what happens should the combined neutral and earth (CNE) conductor become broken (in DNO land). This is why there are minimum sizes of main protective bonding conductors which connect to extraneous conductive parts such as your water pipes from the house MET, largeish currents can flow down your main protective bonds to earth (and then through mother earth and back up into the source via the substation's earth electrode).
As your outhouse doesn't use the CNE for an earth then there isn't a path for current to flow in this situation hence normal size bonding can be used.

 

[RF Lighting]

The setup you have is almost fine.

Due to the cables used, you do not have a large enough CPC to allow you to use it for bonding, so you must TT the supply at the outbuilding and create a seperate equipotential zone for the out building as you have done.

You must not export the earthing system from the house without increasing the size of the earth all the way from the house MET to the outbuilding MET to at least 10.0mm² as there is extraneous metal work present in the out building.

The only slight concern I have with your existing setup it that whilst you are not using the 3rd core or the armouring of the SWA to supply an earth to your outbuilding, you MUST still earth these at the house end.

The armouring must be earthed as it is required for fault protection, and any unused core of a multi core cable must be tied down to earth to prevent any induced voltage being present on the conductor.

 

[Monkeh]

Is the combination of the third core and the armour not sufficiently large enough to export the earth?

 

[RF Lighting]

I've just read your posts on the other two forums.

All I can say is wow! There's some really clueless people out there when it comes to earthing and bonding!

connect in the usual way and add a rod and bond to water . Thus PMEing at the end of the run .

Speechless

 

[RF Lighting]

Is the combination of the third core and the armour not sufficiently large enough to export the earth?

No because the 10.0mm² T&E supply to the external joint only has a 4.0mm² CPC.

 

[Monkeh]

Is the combination of the third core and the armour not sufficiently large enough to export the earth?

No because the 10.0mm² T&E supply to the external joint only has a 4.0mm² CPC.

Ah, missed that. Shame.

 

[ericmark]

It is down to a risk assessment. There is no magic answer. With a TT system the question is can the TT and the TN-C-S earths cause two metallic items to have different voltages where the gradient is so steep that a shock can be received from the two different voltages.

Where a housing estate has gardens encircled by houses all fed from a PME supply the risk of using a different earth to that supplying the houses can be quite high. But where beyond the house you have open fields then likely the TT system would be better.

There is also the legal and regulation to consider where caravans and cars supplied with power from the house are required to be TT even when the vehicle is rather close to the house.

If ones work is to be scrutinised then there is far less chance of being criticised when using TT than with TN-C-S so many will go for the TT route so they can't be criticised latter for their actions.

However I looked at my father-in-laws motor caravan parked within feet of gas and electric meters which were accessed from outside and bonded to the TN-C-S supply and with a risk assessment I considered there was too much chance of some one touching metal from the gas and motor caravan at the same time to use a different earth system. However following the rules by the letter it should have been TT.

Across the road we have the same where an electric power van is charged from the house supply.

So it is down to the person designing the installation to use his warranty of skill and select the earthing method and to document why the selection was made and hope if anything goes wrong his insurance will cover and pay out any claims.

 

[JohnW2]

Are people not overlooking the 'complication'('interesting situation'), already highlighted by the OP, created by the water pipe?

Even if, as the OP has done, one religiously isolates the outhouse from the house's TN-C-S earth, and TTs the outhouse, as the OP pointed out, the copper water supply pipe is joining the house's MET to the outbuilding's MET. In effect, the PME earth is being exported to the outhouse (via the pipe), with the local TT rod as another 'M' of the PME.

However, if one is allowed to apply electrical common sense, there may be a saving grace. The effective copper CSA of 15mm copper pipe (15mm OD, 1mm wall thickness) is about 22.8mm². So, as well as being the means of exporting the PME earth, it is also providing more-than-adequate bonding back to the bonded pipework (hence MET) of the house.

Thoughts?

Kind Regards, John

 

[THRIPSTER]

Thanks for your replies RF, Eric & John,

but for the 4mm cpc on the 10mm cable and the fact that the copper pipe may be changed, I would PME the install. As it is, I'm going to TT and investigate a plastic section of pipe before it enters the outbuilding. Were it possible to run a 10mm eatthing conductor from the house MET to the joint of T/E and the SWA, I'd PME the install. Interesting situation though.

Thanks again.

 

[JohnW2]

Thanks for your replies RF, Eric & John, ... but for the 4mm cpc on the 10mm cable and the fact that the copper pipe may be changed, I would PME the install. As it is, I'm going to TT and investigate a plastic section of pipe before it enters the outbuilding. Were it possible to run a 10mm eatthing conductor from the house MET to the joint of T/E and the SWA, I'd PME the install. Interesting situation though.

Yes, it's an interesting situation. Inserting a plastic interuption in the water pipe will certainly make the situation more straightforward (aka 'less interesting ). However, as I said, the copper pipe effectively represents a 22mm²+ connection between the METs, so I personally would not be worried about the adequacy of bonding, per se.

However, there is another theoretical hazard of an exported PME earth, which arises if it's possible to stand on ground outside of the outhouse (maybe with bare, wet, feet) and touch a (PME) earthed part of the the electrical installation within the outbuilding (e.g. a light switch nearthe door, or maybe an outside light) - which, under very rare fault conditions, could be at a high potential relative to true earth. Some people regard that as a reason for not exporting a TN-C-S earth, even if bonding would be adequate.

Kind Regards, John

 

[RF Lighting]

Are people not overlooking the 'complication'('interesting situation'), already highlighted by the OP, created by the water pipe?

No. There is no complicated situation.

It's no different to a standard TT incommer from the DNO with a water or gas service which supplies a better earth than the installation earth electrode.

In both that and the OP's situation, you are relying on the rod for your earth, and the installation must be designed to suit this, such as the use of RCD(s) to provide automatic disconnection in the even of a fault.

It does not make any difference that a water pipe is entering the equipotential zone with probably a lower Z reading than the earth rod as long as they are bonded together.


Imagine a steel framed building with a PME supply. The steel frame is bolted into the ground and will provide an unintended earth into your equipotential zone. We don't worry about trying to insert insulating sections into the frame of the building. We just bond the two items together so no potential exists between them.

It doesn't matter which item actually provides the earth in the event of a fault, as long as your installation is designed to rely on the earth you have deliberately provided to be earth, and all other unintended earths are bonded together to maintain equipotential.

 

[JohnW2]

No. There is no complicated situation. It's no different to a standard TT incommer from the DNO with a water or gas service which supplies a better earth than the installation earth electrode.

It's surely very different, since (the vanishingly improbable aside) there is no risk that the TT electrode's potential will ever suddenly shoot up to a high potential above true earth as a consequence of a fault outside of the installation.

Whatever, my point surely remains. There's no point in going to pains to isolate an outbuilding from the house's PME earth if there is a length of copper pipe joining the METs of the two buildings.

Imagine a steel framed building with a PME supply. The steel frame is bolted into the ground and will provide an unintended earth into your equipotential zone. We don't worry about trying to insert insulating sections into the frame of the building. We just bond the two items together so no potential exists between them.

Sure - but you're talking about a situation in which a decision has been made to use a PME supply within that building (the fact that a DNO earth is provided doesn't mean that it has to be used) - so there is no alternative but to bond. The OP was talking about a situation in which he had done all he could (other than interupting the copper pipe) to isolate the outbuilding from, and not use, the PME earth within the outbuilding.

Kind Regards, John

 

[ban-all-sheds]

So, as well as being the means of exporting the PME earth, it is also providing more-than-adequate bonding back to the bonded pipework (hence MET) of the house.

Thoughts?

Yes.

You are not allowed to use that pipe as a conductor.

 

[bernardgreen]

I would put an isolating section in the water pipe and use TT with ground rod in the outbuilding and bond water pipe copper in the outbuilding to the ground rod.

Exporting the PME "earth" to garden tools is considered to present a hazard if there is a fault or abnormality in the supply network that bounces the neutral above ground potential.

Like wise exporting a PME "earth" to earthed items in an outbuilding where contact with true ground is possible will present a hazard when the neutral bounces.

When this bounce occurs a person using a PME "earthed" tool in the garden will have a PME "earth" in one hand and his or her feet on the ground. He or she will receive a shock the severity of which depends on how high above ground the neutral goes. As the shock current is on the "earth" wire it will not be detected by the RCD.

With metal thefts the risk of a prolonged neutral bounce is much higher than it was when the regulations / guide lines about PME were written.