Discuss Earth rod needed for shed supply or pme allowed ???? in the Electrical Forum area at ElectriciansForums.net

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@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.
 
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.
 
Because at the cut out is a sticker saying that it is a pme supply.
They could of course be wrong but that is what they said when I asked.
 
How do you know you have PME? you may have TNCS instead.

I have no idea what the difference is? I only say that, because at a job I said to a DNO engineer about a TNC-S supply, he said do you mean PME.
 
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:D
 
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:D

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.
 
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.

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.
 
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.

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 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?
 
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?

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.
 
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.
Why do you keep quoting EBZ that went out with the Spice Girls.
 
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.
 
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Why do you keep quoting EBZ that went out with the Spice Girls.

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.
 
I have TNCS at home, it is connected to a converted TT, this is 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 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.

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.
 

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