A good guide For 'exported PME' and outbuildings!

[IQ Electrical]

For anyone bored by the constant myths and rumours regarding this subject, have a read of this guide from the IEE on supplies to detached outbuildings.

It was published in 2005 but the important bits remain unchanged and might be of some help!

 

[Engineer54]

PME- protective multiple earthing, which means that the PEN conductor is grounded at many points along its path.

TNC-S, the PEN conductor is split at the service head, to from as neutral and an earth conductor. There is nothing to say that the PEN conductor is earthed on its way from the tranny.
Its good practice to install your own rod, to help stop diverted neutral currents appearing on your installation.

That's exactly what is required in most European countries, and in my opinion should also be in the UK too.


oldAL.

As for PME and TNC-S being different yes your correct, but you'll also find that many of the the DNO TN-S supplied systems are being converted to PME and many cases ARE PME but are still shown at the service head as TN-S.

TNC-S systems that are being supplied by DNO's are in the main TN-S systems that haven't been fully converted. All new cable services being provided by the DNO companies will be PME, where every cable joint will be roded as per PME regulations. Of course you have to take notice of a service cut-out that is labeled PME, you'll soon find out if it's not a full PME system when you do your Ze/KA tests. But in all honesty, i doubt if a DNO company will knowingly provide a earthing point that is not sufficiently safe. That could prove very costly for them, in more ways than one!!

Not sure what you mean by ''putting customs lives at risk'' ...How ?? So what measures do you make, to negate these risks your talking about??

 

[oldAL]

Engineer54, TNS is almost a PME in its very guise, as the sheath is in constant contact with earth, almost, as for actually changing TNCS to proper PME, do you honestly think that will happen overnight, or only as and when they are replacing joints and cables?
I really dont see any UK DNOs digging up streets to spike cables at every junction, which is required by PME.

Im a bit confused by your post about most TNCS are really TNS, thats really not the case in most that I have seen , older building perhaps (pre '70s), but definately for anything post 1980 they all seem to be simple tncs that almost everyone thinks is PME and its very unusual to find a 1990s tncs that is actually PME, in my experience anyway,

 

[Engineer54]

Engineer54, TNS is almost a PME in its very guise, as the sheath is in constant contact with earth, almost, as for actually changing TNCS to proper PME, do you honestly think that will happen overnight, or only as and when they are replacing joints and cables?
I really dont see any UK DNOs digging up streets to spike cables at every junction, which is required by PME.

Im a bit confused by your post about most TNCS are really TNS, thats really not the case in most that I have seen , older building perhaps (pre '70s), but definately for anything post 1980 they all seem to be simple tncs that almost everyone thinks is PME and its very unusual to find a 1990s tncs that is actually PME, in my experience anyway,

Over night?? They have been converting TN-S systems and installing new PME systems for well over 50 years now.

Ican assure you, from a very good good source that there are converted PME systems that are still being shown as TN-S. Probably in the more semi rural areas rather than in the towns and cities, but it's a fact. As and when these old cable sheaths break down to a point where they are no longer capable of providing a stable earth they are converted at the service head. So you can have service heads on the same local network cable showing TN-S and PME!!!


You still haven't told us of these risks your talking about, and what sort of measures you make to negate them??

 

[oldAL]

ok, so you are trying to say that simply by combining the neutral and sheath/earth at the head it is now PME?
that is so unreal it is almost believable, but yet again a statement from someone that doesnt understand how PME works.
how to negate this,?
simple, treat every installation as it is not PME.

 

[andyb]

So if you come across T-N-C-S system, what do you do different to the rest of us oldAL?

 

[IQ Electrical]

So if you come across T-N-C-S system, what do you do different to the rest of us oldAL?

Yes, I was interested in that point.

All this talk of 'additional risks' but I've yet to come across any documented cases to support that phrase and I DO understand earthing and supply systems of all types.

 

[johnboy6083]

if anything, isnt it prudent to assume PME/TNCS, so you can size your bonding conductors accordingly? To assume its TNS, would have you undersizing your bonding.

 

[Engineer54]

ok, so you are trying to say that simply by combining the neutral and sheath/earth at the head it is now PME?
that is so unreal it is almost believable, but yet again a statement from someone that doesnt understand how PME works.
how to negate this,?
simple, treat every installation as it is not PME.

Yes, it is unreal, and not what i said at all, more like you putting words in my mouth....

I think it is You that doesn't understand, it has nothing to do with the sheath. It's the sheath of these old PILC cables that has deteriorated, ...bit pointless connecting to the sheath in that instance isn't it??

TN-S systems that are being and/or have been converted to PME only Rod the Neutral at the joint positions, the sheath is left untouched so to speak, which is why they can still supply a TN-S service head. Have you ever seen a DNO PME'd joint on an old or new cable?? When the TN-S service head is converted to TNC-S/PME the sheath connection is normally removed or isolated....

I think we all know now, what you mean by removing customers risks!! Your one of these scaremongering fools that is convinced that death is just around the corner with a PME connection, ...Right?? You would prefer to put your customers at far greater risk by installing a buckshee single rod TT system with a 200 ohm Ra,because that's what BS7671 calls for. And then totally rely on RCDs remaining in good order, for your earth fault disconnection. And you have the cheek to to say we don't understand about earthing systems. lol, lol!!!

Please don't come back here and say that you know of several instances were a total breakdown of a supplying neutral has occurred, because you haven't!! Your listening to the hear-say of others, that also have probably never experienced one either....

What you Scaremongers forget, is that virtually all of Europe and beyond, have universal PME supplies and have never seen or known of TN-S system's, for general public distribution, that was basically a UK system. You DON'T get any of this PME scaremongering in those countries. So why do you think it should be any different in the UK??

 

[Engineer54]

if anything, isnt it prudent to assume PME/TNCS, so you can size your bonding conductors accordingly? To assume its TNS, would have you undersizing your bonding.

There is no reason to assume anything, you design your installation bonding arrangement according to the DNO supply presented to you...

 

[oldAL]

Yes, it is unreal, and not what i said at all, more like you putting words in my mouth....

I think it is You that doesn't understand, it has nothing to do with the sheath. It's the sheath of these old PILC cables that has deteriorated, ...bit pointless connecting to the sheath in that instance isn't it??

TN-S systems that are being and/or have been converted to PME only Rod the Neutral at the joint positions, the sheath is left untouched so to speak, which is why they can still supply a TN-S service head. Have you ever seen a DNO PME'd joint on an old or new cable?? When the TN-S service head is converted to TNC-S/PME the sheath connection is normally removed or isolated....

I think we all know now, what you mean by removing customers risks!! Your one of these scaremongering fools that is convinced that death is just around the corner with a PME connection, ...Right?? You would prefer to put your customers at far greater risk by installing a buckshee single rod TT system with a 200 ohm Ra,because that's what BS7671 calls for. And then totally rely on RCDs remaining in good order, for your earth fault disconnection. And you have the cheek to to say we don't understand about earthing systems. lol, lol!!!

Please don't come back here and say that you know of several instances were a total breakdown of a supplying neutral has occurred, because you haven't!! Your listening to the hear-say of others, that also have probably never experienced one either....

What you Scaremongers forget, is that virtually all of Europe and beyond, have universal PME supplies and have never seen or known of TN-S system's, for general public distribution, that was basically a UK system. You DON'T get any of this PME scaremongering in those countries. So why do you think it should be any different in the UK??

do they really,? none that I have seen actually do, they simply combine the N E at the head and I dont ever see the street getting dug up to rod the joints.
you are still getting confused and making out that TNCS and PME are the same thing, they are NOT
BS 7671 doesnt ask for 200 ohm Ra , so thats yet another myth you are perpetrating, and to simply bang in a single rod and rely on RCD protection, well thats the forte of all these so-called 'all the kit, full of it' done my time in 5 weeks sort of sparks that have destroyed a proper trade we used to have.
you obviously dont understand earthing by a few of the points I just mentioned, 200 ohm for a TT system, where did you actually pick that number from?
yes, after seeing quite a few installs with either broken neutrals or reversed polarity from localised transformers I dont like TNCS,
because they actually have proper PME systems, unlike the UK which has a poorly planned TNCS system which they are now trying to claim is actually PME to comply with their statutory obligations that come into force next year.

 

[Engineer54]

do they really,? none that I have seen actually do, they simply combine the N E at the head and I dont ever see the street getting dug up to rod the joints.
you are still getting confused and making out that TNCS and PME are the same thing, they are NOT
BS 7671 doesnt ask for 200 ohm Ra , so thats yet another myth you are perpetrating, and to simply bang in a single rod and rely on RCD protection, well thats the forte of all these so-called 'all the kit, full of it' done my time in 5 weeks sort of sparks that have destroyed a proper trade we used to have.
you obviously dont understand earthing by a few of the points I just mentioned, 200 ohm for a TT system, where did you actually pick that number from?
yes, after seeing quite a few installs with either broken neutrals or reversed polarity from localised transformers I dont like TNCS,
because they actually have proper PME systems, unlike the UK which has a poorly planned TNCS system which they are now trying to claim is actually PME to comply with their statutory obligations that come into force next year.

I think we've had this argument before haven't we?? And we both know you don't like TNC-S!! ...lol!!

So you've never seen a DNO's PME'd joint , i thought not!! As i stated to you before TNC-S distribution is a PME system in progress, the suppliers will NOT purposely leave you with a permanent unsafe earth connection, regardless of what you may think, full stop!! It may well be, that a local TX system network under progress of being converted to PME, will have a higher value of Ze than 0.35 ohm But it will never be higher than the TN-S system it's replacing. I can't remember the actual max they allow in those circumstances, but certainly a good deal lower than 0.60 ohms... The figure will depend on the TX neutral bonding, lengths of the strings, the ground types and the amount of joints already converted. I also think that a earth electrode should always be included at a PME service head, as part of that conversion...


I don't know where you live, but where i came from, if a head was converted to a PME connection, only the bonding and CU earthing conductors were left in place, pretty dammed pointless connecting a failed or failing sheath that stops at the head!!! Far more prudent to insulate any short lengths of bared, exposed lead sheath... Though i have no problem with bonding it where necessary, as an extraneous source of earth.
They do ''NOT'' as you are suggesting, just use the failing or failed sheath of a supply cable as the means of earthing the neutral, that's just a pure nonsense!!


BS7671 uses 200 ohms as it's max Ra. (it's not a figure i just pulled out of the hat) So even the most experienced electrician can walk away from a TT system they have just created with that value Ra. So, though i will agree with you on the 5 week wonder apprenticeships, and to the demise of our industry that we once had, there are still plenty of old hands that take notice of that BS7671 200 ohm Ra, and think it's a good-un!!

I think i understand earthing systems (especially TT systems) far better than you obviously do, of all types and of all descriptions. I've also probably been responsible for putting more rods in the ground than the membership of this forum combined, or very close to it!! And anyone that would diss a domestic TN-S system, for a 50 ohm Ra TT system and an RCD, ...is clearly daft, and i would argue, that He certainly doesn't understand what he's doing!!!

I don't believe a word of your witnessing several full broken neutrals, if you see just one, you would be doing well!! Got any documented facts about all these broken network distribution neutrals, that happen according to you on a regular basis, have you?? Reversed polarity is another matter altogether!!

Haha!! I've heard that little gem before, about Europeans using ''proper'' PME systems!! I think you need to cross the channel and see for yourself some of the standards of there local distribution companies, then come back and tell us about it!! lol!!!

The DNO's can't claim anything, that doesn't comply with ''Statutory Regulations and obligations''!! They have an obligation when so supplied/provided, to maintain a working and safe earthing point that meets those regulations. Extending a PME zone of protection is by far, safer than isolating it, then banging a rod in the ground!! And that's not a Myth!! Nothing stopping you including a rod at the extended location either, if you so wished!!!

Seems quite ironic really, in one hand you state European countries have proper PME, and in the other, the fact that most of the DNO's in UK these days, are owned by the very same European companies that operate these distribution systems in these countries!!! And probably with more statute regulations to adhere too, than they have in those European countries!!

I have no time whatsoever for myths and scaremongers!! And if you are who i think you are, ...you are one of the biggest scaremongers out there. Keep your scaremongering to the other forum, where many actually believe what your telling them. Here we have no time for your (or anyone else's) blatant scaremongery, based purely on what you like and don't like and myths, they are just your own biases....

 

[oldAL]

have we? I dont think Ive worked with you before, as for other forums, I dont think there is much chat about electrical earthing systems on many travel forums, lol.
across the channel? what to England? Im in Belguim ATM, and most buildings are rodded at point of entry.
you keep saying the same thing, and this is what people need to understand, TNCS is NOT always PME,
I like PME, when its implemented properly, TNCS is another matter altogether, that is where the danger [imo] arises, folks assuming that because they have a TNCS supply then its also a PME supply,
almost all TNCS systems I have come across, (PME'd or not) has had a Ze of <0.35ohms anyway, so I dont see that as a reason.
I know the DNOs are supposed to be changing them over, but in reality I think we all know that they are not going to start digging up streets and spiking all the joints, are they?
your point about adding rods to a TNCS, very valid, and that is usually what I do when required, rod at incoming point to MET, then again if I am taking a supply to any outbuildings,
thereby what I would term a localised PME system, backing up whatever the DNO supply is, whether it be TNCS or actually proper PME,
the problem with this is when another electrician with a lesser understanding of earthing systems comes to work on the building and assumes for whatever reason that its TT, or simply doesnt know what to do.

 

[andyb]

How on earth can you have a 'localised PME system' What do you put on your certificate?

It sounds as if you have a T-N-C-S system with a superfluous and pointless earth rod. Waste of time & money.

By the way, I'm loving the word 'scaremongery'

 

[IQ Electrical]

How on earth can you have a 'localised PME system' What do you put on your certificate?

It sounds as if you have a T-N-C-S system with a superfluous and pointless earth rod. Waste of time & money.

By the way, I'm loving the word 'scaremongery'

I suppose any statement of doom without documented instances to reference too would be accurately described as scaremongery.

I've asked and E54 has asked for any documented cases of the dangers, even BS7671:2008 deems the broken neutral scenario so rare as not to warrant special consideration.

The 'discussion' is all a bit pointless now, Myths and Legend V Facts and Experience will always be a one-sided battle......

 

[Guest123]

BS 7671 doesnt ask for 200 ohm Ra, so thats yet another myth you are perpetrating

Not quite,

Table 41.5, note 2, suggests "the resistance of the installation earth electrode should be as low as practicable, a value exceeding 200ohms may not be stable".

This is where the 200ohm 'rule of thumb' eminates.

 

[oldAL]

is it, do you know you are supposed to ask the DNO for permission to take a TNCS or PME earth outside the building to the one they have supplied it to? and do you know what their standard answer is? it is "NO" , wonder why that is? btw, their is an enormous amount of documented proof about the dangers of 'exporting' TNCS earthing, makes you wonder why agricultural premises are prohibited from utilising TNCS or PME supplies. maybe you should have a look at the 'Farady cage' scenario again to understand the difference in potentials, the DNO supply an artificial earth, this can be at a different voltage potential than the general mass of earth that we stand on, this is why we use equipotential bonding, to bring everything to the same potential, I do hope you understand these basic principles of electrical systems.

 

[Silly Sausage]

...........there is an enormous amount of documented proof about the dangers of 'exporting' TNCS earthing, ...............

Please post some links to such references then. I, for one, would be interested in reading them.

 

[oldAL]

I'd agree with that Lenny, its merely a rule of thumb, BS7671 actually permits much greater Ra readings, 1666 comes to mind by calculation, dependant on circumstances obviously. so Engineer's quote of 200 ohms is yet another 'fact' that isnt actually a fact at all, much like a lot of other stuff he tries to baffle with, on reading a lot of posts here I think engineer54 is actually very good, but stuff like he is stating here as fact clearly isnt.

 

[oldAL]

Please post some links to such references then. I, for one, would be interested in reading them. Farady cage ? or you could just try asking the DNO if they will allow it.

 

[Engineer54]

is it, do you know you are supposed to ask the DNO for permission to take a TNCS or PME earth outside the building to the one they have supplied it to? and do you know what their standard answer is? it is "NO" , wonder why that is? btw, their is an enormous amount of documented proof about the dangers of 'exporting' TNCS earthing, makes you wonder why agricultural premises are prohibited from utilising TNCS or PME supplies. maybe you should have a look at the 'Farady cage' scenario again to understand the difference in potentials, the DNO supply an artificial earth, this can be at a different voltage potential than the general mass of earth that we stand on, this is why we use equipotential bonding, to bring everything to the same potential, I do hope you understand these basic principles of electrical systems.

Since When?? There are plenty of farms and other associated agricultural businesses supplied by a PME supply!! Myths again!!!

 

[Engineer54]

Haha!! i have a friend that has just retired from being a DNO regional manager, He has an old farm house with more than a few outbuildings of various sizes. ...And yes you guessed it a PME supply, ...that has been extended to most of these outbuildings!!

Your talking in and around myths again. Most DNO's don't want you extending any house supply, probably not that interested either unless you make a point in getting them involved!! But as far as i know, there is no regulation that i know of that you can't, or that you need written permission to do so either!! Anyone been prosecuted for this on a domestic premises?? Nope, ...i didn't think so either!!! lol!!

 

[andyb]

is it, do you know you are supposed to ask the DNO for permission to take a TNCS or PME earth outside the building to the one they have supplied it to? and do you know what their standard answer is? it is "NO" , wonder why that is? btw, their is an enormous amount of documented proof about the dangers of 'exporting' TNCS earthing, makes you wonder why agricultural premises are prohibited from utilising TNCS or PME supplies. maybe you should have a look at the 'Farady cage' scenario again to understand the difference in potentials, the DNO supply an artificial earth, this can be at a different voltage potential than the general mass of earth that we stand on, this is why we use equipotential bonding, to bring everything to the same potential, I do hope you understand these basic principles of electrical systems.

This is of course total piffle.

There is no problem in using the existing earth from a property supplied with a pme supply. All you need to bear in mind is the fact that any extraneous-conductive-parts are bonded back to the met with the correct size cable.
More often than not, when you have an outbuilding with extraneous-conductive-parts, maybe a metalic water pipe, then it is easier to install a rod and make a separate TT system.

Then you can use your redundant Faraday Cage to keep a pet rabbit or other similar pet in.

 

[andyb]

Since When?? There are plenty of farms and other associated agricultural businesses supplied by a PME supply!! Myths again!!!

You can't use T-N-C supplies at agricultural premises Engineer. Section 705.

 

[Guest123]

makes you wonder why agricultural premises are prohibited from utilising TNCS or PME supplies. maybe

I hate to disagree again but, 705.415.2.1 states only that "Unless a metal grid is laid in the floor, the use of a PME earthing facility as the means of earthing for the electrical installation is not recommended".

Obviously, as you have mentioned, this is to safeguard against any possible PD between the 'supplied earth' & the general mass of earth.

I hope no-one takes offence to my continual spouting of regs regarding this topic but as it is such a frequently debated one, I believe hard fact is beneficial.

 

[Engineer54]

I'd agree with that Lenny, its merely a rule of thumb, BS7671 actually permits much greater Ra readings, 1666 comes to mind by calculation, dependant on circumstances obviously. so Engineer's quote of 200 ohms is yet another 'fact' that isnt actually a fact at all, much like a lot of other stuff he tries to baffle with, on reading a lot of posts here I think engineer54 is actually very good, but stuff like he is stating here as fact clearly isnt.

We could play with words and phases till the cow's come home, the reality is, ...that the 200 ohms mentioned in BS7671 is taken by the majority of electricians as being the maximum allowed. Now if you don't believe that, just go through some of the threads relating to TT systems here, and see for yourself!!

The greater values are related to 30mA RCD's, which is all fine and dandy until they fail!! Then that 200 ohm or higher Ra is going to leave you high and dry, with no protection being afforded whatsoever... Hey, but that's got to be better than having PME supply with a 0.35 Ze available to disconnect the protective device?? Well maybe in your opinion but not in mine, .... and thankfully not for most other electricians with even half an once of commonsense!!!

 

[Engineer54]

I hate to disagree again but, 705.415.2.1 states only that "Unless a metal grid is laid in the floor, the use of a PME earthing facility as the means of earthing for the electrical installation is not recommended".

Obviously, as you have mentioned, this is to safeguard against any possible PD between the 'supplied earth' & the general mass of earth.

I hope no-one takes offence to my continual spouting of regs regarding this topic but as it is such a frequently debated one, I believe hard fact is beneficial.

I think that recommendation only applies to buildings where livestock are housed or use. I'm pretty sure it doesn't apply to storage and other use buildings on a farm. But correct me if i'm wrong...

 

[Guest123]

I think that recommendation only applies to buildings where livestock are housed or use. I'm pretty sure it doesn't apply to storage and other use buildings on a farm. But correct me if i'm wrong...

It does, yes. I was going with worse case scenario in this example though.

Some area's within an agricultural premises, depending on their use, are completely beyond the scope of 705 altogether.

 

[oldAL]

right, here you go again, continually stating PME, do you know the difference between PME and TNCS ? as for no-one being prosecuted for exporting it, how many people do you know have been prosecuted for not notifying part P work? the following is an excerpt from ESQCR Special consideration should be given to the earthing and protection arrangements for certain installations where reliance on the connection of the consumer’s protective conductor with the distributor’s combined neutral and protective conductor could result in more significant risks. For example, installations where it may prove difficult to attach and maintain all the necessary equipotential bonding connections (e.g. farms or building sites), installations at certain wet environments (e.g. swimming pools and petrol filling stations) and certain installations outside the equipotential zone of buildings

 

[oldAL]

agricultural and horticultural premises are define in BS7671 as,, rooms, locations or areas where - livestock are kept, -feed, fertilizers, vegetable and animal products are produced, stored or processed, - plants are grown, such as greenhouses. pretty much anywhere on a farm apart from the actual farmhouse

 

[Engineer54]

It does, yes. I was going with worse case scenario in this example though.

Some area's within an agricultural premises, depending on their use, are completely beyond the scope of 705 altogether.

I thought as much!! lol!!

Going back to breaks in neutral on PME systems. That same friend i mentioned above, has only known of 3 such instances in the whole time that he has been with the Electric companies, from the old leccy boards to the new DNOs. And each of them he reckons was tripped by the protection relays.
But this guy has witnessed numerous events, ...i know who i believe!! lol!!

He's not one to protect the DNO's either, he's been totally demoralised over that last years by the once high standards across the board, being ----ed aside in the name of cost cutting and profits...