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!

 

[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...