should you install a earth rod if running a feed to a shed which is built on timber frame and is all timber , no metal parts and will have no services ie gas or water in it so no need for bonding only a supply for a light and a couple of sockets or can you use a pme earth from fuseboard if the shed supply is run in 6mm armoured from the outside of the house ( inside house 6mm t+e to fuseboard ) protected by 32amp rcbo will be installing a 2 way board with rcd main switch in shed approx 25 meters away
D
Darkwood
@essex
If the fault is after the last earthing point on a PEN conductor then it is useless - hence we do not take this into account.
Faults take the path of least resistance, PME is strapped down at very regular intervals to ensure a lost concentric connection will track back to the nearest point which is either the next rod or the substation itself, if you are on about the customers end then a break just before the customer will also raise the potential of any earthed metalwork in the building regardless, the risk is deemed so low that this is not a concern, you will often find the first earth strapping is within a few meters of the property to lower any such risk.
You said you would not TT a metallic building outside when a PME earthing arrangement is supplied. This is the exact opposite of the Reg.
I was reflecting on the context this whole thread is in IE domestic and outbuildings, guidance notes clearly state you can earth rod the out building if it does have structural metalwork, or metallic incoming services, it also says this is one of a few options, I choose the other option of bringing in a suitably sized earth to bond such metal work.
In what you describe I would say you have extended the earthing. Not the zone. (1)The very fact that there is (and likely always will be) a PD between what you have bonded and true earth means that when outside it is virtually impossible to extend the EBZ. The only way to achieve this would be to ensure your earth bonded to the shipping containers is the same potential as true earth. Using anything other than an earth electrode then this is just not possible. In my opinion,(2) which I believe is clearly backed up by 7671 you should have never connected those containers to a PME supply. It is specifically stated this should not be done.
(3)Any issue with the supply neutral will give results precisely what you have described. If it would have been a complete breakage in the PEN then it would have been more than just a tingle. As for the other services I would have just supplementary bonded them so that everything was the same potential.
(1)-I agree but we are not talking any voltage of concern here, normally hard to measure though it is that low and well within SELV limits to be considered a concern.
(2)Where does it state this, I agree the regulations are specific for many installations about earthing to PME say to caravans, funfairs, temp' stands etc etc but these containers and the prefab offices are classed as permanent, they been there 20+yrs now also my comment was more in line to the domestic arena to which this thread is based, guidance notes that are linked to in a previous thread express earth rodding an out building is one of a few options available on a PME supplied property, with regards to the OP's query then I stand my position on this and say there is no reason to rod if you have PME supply, even if you have incoming metallic services, all you need to do is bring out a bonding cable to meet the requirements of the installation and its supply. Like I said before, why create a less reliable earth connection when you have a reliable one provided.
(3) The choice to do this was based on knowing the network layout, this was not a network fault and has been confirmed, we suspect it was from an adjacent site who had recently has a generator fitted and incorrectly installed, upon questioning the owner he said he would contact the company, the fault mysteriously disappeared after this but I cannot say it was definitely the cause. I understand your concern because regulations are strict on PME earthing to caravans, funfairs temp' stalls etc but this was weighed up on this particular site, again my original comment was in the context of this thread and domestic outbuildings (with incite I should have been clearer)
RCDs will not even work as the link from the neutral is before the RCD so the RCD is not detecting an imbalance. The current goes back down the neutral, through the RCD before it goes back down the earth connected to the main head and to anything earthed or bonded.
I agree - no disagreement there.
Most spas are fully plastic including all pipework and all electrical pumps etc should be Class 2. If they are getting a tingle something is seriously wrong.
I disagree, I have seen spa's, hot tubs etc which require earth connection to the pump supply, class 2 is great when everything is in good condition but seals failing, motor running hot etc all can lead to water ingress, then you still have the same risk on a class 2 install.
If the fault is after the last earthing point on a PEN conductor then it is useless - hence we do not take this into account.
Faults take the path of least resistance, PME is strapped down at very regular intervals to ensure a lost concentric connection will track back to the nearest point which is either the next rod or the substation itself, if you are on about the customers end then a break just before the customer will also raise the potential of any earthed metalwork in the building regardless, the risk is deemed so low that this is not a concern, you will often find the first earth strapping is within a few meters of the property to lower any such risk.
You said you would not TT a metallic building outside when a PME earthing arrangement is supplied. This is the exact opposite of the Reg.
I was reflecting on the context this whole thread is in IE domestic and outbuildings, guidance notes clearly state you can earth rod the out building if it does have structural metalwork, or metallic incoming services, it also says this is one of a few options, I choose the other option of bringing in a suitably sized earth to bond such metal work.
In what you describe I would say you have extended the earthing. Not the zone. (1)The very fact that there is (and likely always will be) a PD between what you have bonded and true earth means that when outside it is virtually impossible to extend the EBZ. The only way to achieve this would be to ensure your earth bonded to the shipping containers is the same potential as true earth. Using anything other than an earth electrode then this is just not possible. In my opinion,(2) which I believe is clearly backed up by 7671 you should have never connected those containers to a PME supply. It is specifically stated this should not be done.
(3)Any issue with the supply neutral will give results precisely what you have described. If it would have been a complete breakage in the PEN then it would have been more than just a tingle. As for the other services I would have just supplementary bonded them so that everything was the same potential.
(1)-I agree but we are not talking any voltage of concern here, normally hard to measure though it is that low and well within SELV limits to be considered a concern.
(2)Where does it state this, I agree the regulations are specific for many installations about earthing to PME say to caravans, funfairs, temp' stands etc etc but these containers and the prefab offices are classed as permanent, they been there 20+yrs now also my comment was more in line to the domestic arena to which this thread is based, guidance notes that are linked to in a previous thread express earth rodding an out building is one of a few options available on a PME supplied property, with regards to the OP's query then I stand my position on this and say there is no reason to rod if you have PME supply, even if you have incoming metallic services, all you need to do is bring out a bonding cable to meet the requirements of the installation and its supply. Like I said before, why create a less reliable earth connection when you have a reliable one provided.
(3) The choice to do this was based on knowing the network layout, this was not a network fault and has been confirmed, we suspect it was from an adjacent site who had recently has a generator fitted and incorrectly installed, upon questioning the owner he said he would contact the company, the fault mysteriously disappeared after this but I cannot say it was definitely the cause. I understand your concern because regulations are strict on PME earthing to caravans, funfairs temp' stalls etc but this was weighed up on this particular site, again my original comment was in the context of this thread and domestic outbuildings (with incite I should have been clearer)
RCDs will not even work as the link from the neutral is before the RCD so the RCD is not detecting an imbalance. The current goes back down the neutral, through the RCD before it goes back down the earth connected to the main head and to anything earthed or bonded.
I agree - no disagreement there.
Most spas are fully plastic including all pipework and all electrical pumps etc should be Class 2. If they are getting a tingle something is seriously wrong.
I disagree, I have seen spa's, hot tubs etc which require earth connection to the pump supply, class 2 is great when everything is in good condition but seals failing, motor running hot etc all can lead to water ingress, then you still have the same risk on a class 2 install.
It’s funny but I was talking to the guys from the DNO on a new build site a few weeks ago, all new supply’s to the homes are tncs.
I asked how far apart do you apply the earth electrodes for the pme connection, he said there’s a rod at the transformer end and a rod at the end of the cable , that’s it.
Now one guy was connecting up the street lighting and one guy applying a joint for 2 supplies to some properties but both said the same thing.
I asked how far apart do you apply the earth electrodes for the pme connection, he said there’s a rod at the transformer end and a rod at the end of the cable , that’s it.
Now one guy was connecting up the street lighting and one guy applying a joint for 2 supplies to some properties but both said the same thing.
I'd agree, if the building is not of metallic construction extend the existing earth whether the TN C S is PME'd or not. If it's PME you could also add a local rod for belt and braces, for pure TN C S certain external faults could result in very high earth currents so I probably wouldn't.
PME is connected to earth at several points on the run to a house from the transformer, TNCS is not connected to earth several times on it's journey to your home. Regarding the earth rod, forget it, extend the existing earthing arrangement, popdorn at the ready
I suspect most houses are supplied as TNCS not PME as per your definition, but the DNO still calls its supplies to a property as PME.
N
Nigel
Jes, is this still going on
I know. Sorry.
I think @darkwood has completly misunderstood PME, extending EBZs and metallic buildings but there is only so much you can discuss in text so I will move on.
N
Nigel
This is my point. We cannot know what is going on on the DNO side so it is irrelevant to the design process. All we need is the earthing arrangment supplied to us and BS7671 guides us from there.
N
Nigel
I think any joint put in by the DNO will have an earth tap installed as standard so probably easier for the DNO to call it PME as a TN-C-S could become a PME at any time.
I'm no expert here. I've relayed a story before, when I had an issue with a TNS supply. A DNO engineer turned up. I asked if I could have it converted to a TNC-S supply, he said whats that in English, do you mean PME? He said, it would take 6 weeks, do it yourself, and demonstrated how to do so (I didn't take his advice).
So does TN-C-S in reality exist, all is it just a load of PME bull?
N
Nigel
I think it probably does exist on supplies with no joints and that have had no work done. I bet the DNO have no idea what supplies are and what supplies are not so just say all TN-C-S supplies are PME.
Why do you keep quoting EBZ that went out with the Spice Girls.
D
Darkwood
I have TNCS at home, it is connected to a converted TT, this is now classed as a TNCS, a TNCS can be supplied in a concentric cable or conversion of a TT or TNS system where all they have done is strapped both the E/N together where the earth existed or made the N in a TT the N/E, the ability to do this is done on a case by case to ensure it is a safe option as oppose to running new supplies.
PME (Protective multiple Earthing) is exactly what it says on the tin, the concentric cable is earthed a regular intervals all of which can be inspected and tested but that is the DNO's side and duty.
PME/TNCS system stop at the cutout of the property where you are provided a common terminal or in some cases they split to 2 terminals to take a N and E from, within this thread and the title people discuss extending PME, this is wrong, that is a very different system and would mean you extended the combined N/E with a concentric cable and depending on length you may have to strap it down along it length, this has to be agreed with the DNO and cannot be done as an option otherwise. The E/N facility that the DNO supply at there cutout is where PME or TNCS stops and you tap you N and E to as separate cables, once you have separated them your installation is based on a TNS design thus no circuits past the DNO's facility can be called PME or TNCS hence my continuous repeated point that you cannot word it as extending the PME.
PME (Protective multiple Earthing) is exactly what it says on the tin, the concentric cable is earthed a regular intervals all of which can be inspected and tested but that is the DNO's side and duty.
PME/TNCS system stop at the cutout of the property where you are provided a common terminal or in some cases they split to 2 terminals to take a N and E from, within this thread and the title people discuss extending PME, this is wrong, that is a very different system and would mean you extended the combined N/E with a concentric cable and depending on length you may have to strap it down along it length, this has to be agreed with the DNO and cannot be done as an option otherwise. The E/N facility that the DNO supply at there cutout is where PME or TNCS stops and you tap you N and E to as separate cables, once you have separated them your installation is based on a TNS design thus no circuits past the DNO's facility can be called PME or TNCS hence my continuous repeated point that you cannot word it as extending the PME.
Last edited by a moderator:
D
Darkwood
I also was only using the Term because essex introduced it into the conversation and been old hat I continued its use, it still exists but under a new guise that is all, the actually application and practice hasn't changed but EBZ was deemed confusing if I recall so they changed it by name only.
N
Nigel
Everyone knows what is meant when they say 'extending the PME'. Nobody means physically extending it and taking over the DNO's job.
It is like correcting someone for saying 'sub-main' or 'ring main'. We all know what is meant and no real need to correct it.
No I am an idiot.
D
Darkwood
I disagree, the forum is full of people from different stages from fresh faced apprentices to seasoned sparks, read the last several posts, it is clear members still have trouble understanding the systems and the differences 'by their own admission', I disagree also that this is somehow a similar comparison to ring and sub 'main' ... they were accepted and taught terminology it is only that they have been reworded that changed anything, PME and the term 'extending PME' with regards to a consumers side is wrong and has always been wrong, the fact it is a completely different system is why it shouldn't be casually used like it is, I have seen N/E linked together at a Distribution boards before, I expect they weren't sparkies who installed it but when incorrect terminology is banded around it wouldn't surprise me if they were...like you said, everyone says it but we know what we mean..Yes we do but not everyone knows is my point.
Either way, I'll break bread here and lets agree to disagree
Well done for admitting it, that's the first step on the road to recovery...
N
Nigel
Then your comment about it being removed is irrelevant.
A pure TN C S supply network and a PME'd TN C S network behave very differently under fault conditions. I know the DNO's can be a law unto themselves but surely they've got a national standard that dictates the multiple earthing point requirements a supply network must achieve to qualify as PME. Just calling a system PME when it's not can be dangerous for those designing end user installations on that system; you really don't want to locally rod a supply that's not PME'd whereas there's advantages to a local rod if it's a correctly multiple earthed TN C S supply.
As you have stated previously you do not export the PME as this stops at the service head, you are exporting for want of a better word TN-C-S. The earthing system is judged at the service intake point and not what is occurring previous to it.
I will let you in on a secret with the niceic and elecsa if you know the regstration number from a member, its on most websites and similar then you enter it on the voice prompt menu on the phone, then they put you through. never asked for anything more!
I never in 40 years of my working life asked for advice from any scheme.
M
MDJ
It must be your moustach putting me off my posting lol, apologies then
To be honest who needs to ask a schemer we have all we need here and more
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Whilst I value the knowledge & experience members on this forum have, if you have a question thats pertinent to a design or install on a working job, I don't see that you can rely on an response from a forum member. If it all goes horribly Pete Tong, I wouldn't like to be using 'the advice I received from Mr Miggins, on the electrical forum was' in my defence. Hence when I did seek guidance from my schemes technical, it was always in an email format, copy secured away.
I would use and have used this forum for advice, but always knowing the answer almost, and just seeking confirmation or agreement, like you would from any other working colleague.
I have read quite often the response, 'why ask some faceless individual on a forum, ask your scheme'.
I would use and have used this forum for advice, but always knowing the answer almost, and just seeking confirmation or agreement, like you would from any other working colleague.
I have read quite often the response, 'why ask some faceless individual on a forum, ask your scheme'.
As you said you need confirmation that your thoughts are on par with the trusted members on here you know who to heed and who to just ignore as some like a windup
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