Earths at outbuildings...

[Boberto]

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.

 

[Sintra]

There is nothing wrong with exporting PME / TNC-S especially if there are no extraeneous conductive parts in the out building.

 

[MarkieSparkie]

Hi Boberto, Not being able to export a TN-C-S (PME) earth is a widely accepted myth and the earthing of outbuildings is one the most frequent subjects on this forum. You really need to read the following sticky-thread which covers exported earthing in detail.
http://www.electriciansforums.net/e...159-good-guide-exported-pme-outbuildings.html

 

[Engineer54]

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.

You should always run a CPC with your TT supplied sub-main cable. Not only that, you should also link the separate earth electrodes. Having said that, it doesn't matter too much if all you ever intend installing is a single 3/8'' 1.2m rod for your TT system!!! Your overall Ra value will still remain crap, and totally worthless...

 

[Deleted member 9648]

You should always run a CPC with your TT supplied sub-main cable. Not only that, you should also link the separate earth electrodes. Having said that, it doesn't matter too much if all you ever intend installing is a single 3/8'' 1.2m rod for your TT system!!! Your overall Ra value will still remain crap, and totally worthless...

In which case there are vast numbers of crap and worthless installs in the UK which nevertheless comply with Bs7671.

 

[Engineer54]

And you would be exactly right .....in both cases unfortunately. Maybe because BS7671 has the lowest standard of TT earth electrode requirements in Western Europe..

 

[Deleted member 9648]

I wont argue with your knowledge of worldwide systems E54,but would point out that in my neck of the woods there are many,many single rod TT systems with Ra values typically around 100+ ohms. I maintain and have installed many of them ,and can honestly say I have never known or heard of a problem which could be attributed to the Ra value,and which a lower value would have prevented.
While we have long agreed to disagree on this,where I see posts rubbishing standard UK TT practices which I believe are perfectly safe,I will always endeavour to put across the other side for a balanced view.

 

[Engineer54]

That's why we have to agree to disagree wirepuller, The Ra values you state here are high in their own right. If you then add to that, the very real fact that that during the freezing winters you seem to now get every year, those 100 ohm Ra values will literary sore, on a single 1.2 m to next to nothing. Likewise, if you ever get an extended heatwave, where the ground could well dry out , leaving that poor excuse for a earth electrode (1.2m/3/8'') providing very little in terms of an Ra value....

I have nothing against single earth electrodes, so long as they go DEEP!! Now be honest, when was the last time you have seen a modern TT system in your area, or the UK, that used more than one extendable earth electrode on a domestic installation?? Going from this and other Forums on this subject, your answers going to be very few to None!!

 

[diditrain]

If this is such an issue, would it not be written somewhere that one rod is going to be completely useless when the ground freezes

 

[Engineer54]

If this is such an issue, would it not be written somewhere that one rod is going to be completely useless when the ground freezes

It is a know fact, that frozen ground soils will represent very high resistances to earth electrodes and is highlighted to one extent or another in every publication covering this subject that i know of. I'm sure you'll also find this mentioned in the PDF file that Lenny posted in the sticky section of this forum, ....it'll be there somewhere guaranteed!!! ...lol!!

EDIT: It's around the the first metre from FGL that will be affected, and this is the length that will be attributing very little in the way lowing resistance. only the lower 20 cm will be working relatively normally...

 

[diditrain]

So a specified minimum depth may be appropriate then

 

[La Poste]

What sort of depth are we talking about, just a ball park figure?

 

[Engineer54]

So a specified minimum depth may be appropriate then

Basically, i think 1.2 metre rods should be relegated to use as the last extendable section of a deep driven rod installation. and the 1.2m 3/8'' taken out of the market place altogether....

Very difficult to stipulate a minimum depth, due to the numourous types of ground conditions you can encounter. Much better to have a realistic minimum Ra level, one that would exclude the use 1.2m rods... lol!!

 

[La Poste]

I didn't realise frost affected soil resistance readings so much until reading this and a few other threads.

The deeper the better then.

Let's just hope nothing is underneath to drive your rod into like a big underground power cable. I mean how do you know what's down there when you really want to put a deep spike in?

 

[Engineer54]

I didn't realise frost affected soil resistance readings so much until reading this and a few other threads.

The deeper the better then.

Let's just hope nothing is underneath to drive your rod into like a big underground power cable. I mean how do you know what's down there when you really want to put a deep spike in?

You'll know all about THAT probably before you've gone the first metre!! lol!! Water and gas pipes however are a completely different kettle of fish!! lol!! But the chances of you hitting them with the small rod diameter are quite slim. Far more chance of hitting a bloody big rock or old builders rubble!! lol!!!

 

[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]


[/COLOR]

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

 

[IQ Electrical]

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

Hey ho, another week, another PME post-E54 is absolutely spot on in the post above-with the obvious exceptions listed in BS7671:2008 there is rarely a case (other than economic) for not using the suppliers earth connection.

The PME system has been adopted throughout the developed world and continues to be an economic and safe method of distribution.

On the other hand, a quick read through many posts on this forum indicate the lack of understanding and mystery surrounding the regulations and installation methods of a TT system!

I'm currently writing a FAQ sheet on TT system installations for a certain electrical publication, I'll post it up on the forum for comment before it's submitted.

 

[Engineer54]

Well for one thing, i would never walk away from any TT system that was only giving me an Ra of around 200 ohms, by the time i walked away it would be substantially less!! lol!! And talking of costs, ....It could well end up costing more to install a decent TT system than running that main bonding cable, in the time taken!!

By the way, there is nothing to stop you including a rod at the outbuilding, it will help clamp the touch voltage. But personally i wouldn't find it necessary to do so, i'd be quite happy with the PME/TNC-S at the outbuilding, whether it has extraneous parts or not!! lol!!

 

[Chr!s]

Im now worried about these extraneous conducive parts, my house is pme and i have lots of extraneous conductive parts inside, do i need to TT the installation ?

 

[Engineer54]

Id put a a roded earth ring round your house.... just to be on the safe side sort of thing!! lol!!

 

[IQ Electrical]

Im now worried about these extraneous conducive parts, my house is pme and i have lots of extraneous conductive parts inside, do i need to TT the installation ?

Ha, summed up perfectly!

 

[Knobhead]

I’ve finally got the solution to this. Jack the house up, have it mounted on glass insulators, an isolation transformer in the garden. Each of the front steps will have to be insulated from each other. Totally earth free!

 

[Chr!s]

I’ve finally got the solution to this. Jack the house up, have it mounted on glass insulators, an isolation transformer in the garden. Each of the front steps will have to be insulated from each other. Totally earth free!

Looks like we are in need of another amendment .

 

[Deleted member 9648]

Well for one thing, i would never walk away from any TT system that was only giving me an Ra of around 200 ohms, by the time i walked away it would be substantialy less

E54,I'm interested to know what value of Ra you would accept.given that a 30ma RCD will provide earth fault and additional protection up to 1667 ohms..(in theory!)...why is an Ra of 200 ohms not acceptable?
Unless you achieve an Ra of <5 ohms ,for example,the typical OCPD,s on final circuits will not reliably operate in an acceptable time under earth fault conditions. Therefore you are likely to be reliant on the RCD.
It seems to me that the RCD will operate in exactly the same way whether the Ra is 5 ohms,or 200 ohms.....so why bother with 5 ohms?

The question I am asking is what is your maximum Ra....and why that value?...discuss....:smilewinkgrin:

 

[ackbarthestar]

Im now worried about these extraneous conducive parts, my house is pme and i have lots of extraneous conductive parts inside, do i need to TT the installation ?

There's nothing to worry about, as long as
1/ Main Protective Bonding is in place and IS effective (Not connected to internal plastic pipework )
2/ the simultaneous touchable extraneous parts are connected together (usually now only bathrooms and the like)

You are now in a Faraday's Cage

Connecting an earth electrode to the MET will reduce the possible DNO's fault voltage and your Faraday's Cage to something around ground potential, bus as already stated, you might be supplementing the network return currents via your electrode.

 

[platty1005]

Am I losing the plot here or what
The earthing arrangement is at the ORIGIN in this case a TNCS
PME relates to multiple earthing on the DNO network
The S in TNCS stands for separate CPC after the service head
So why install an extra Electrode at an outbuilding if the sub feeder has a separate CPC that is correctly sized
I agree to ensure extra safety an electrode can be installed at the origin connected to the MET if there is an appreciable risk the incoming PEN may be broken IE overhead 2 core supply.