Earth rod needed for shed supply or pme allowed ????

[Tim collins]

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

 

[Nigel]

Let's just word this correctly, we are not using PME or suggesting it for the outbuilding which was the initial point in my post, we are extending the EBZ where the incoming supply is PME (noted you did say about EBZ earlier)
Also PEN conductors in a PME setup have multiple points of earthing along their length to protect against such network faults, it is TNCS that has an increased risk of ground faults but if bonded correctly then on these rare events the risks are minimised.

What can occur though is a PD between supplied earth and actually ground if you are in an isolated metal framed building where you could touch it and bridge between true ground and supplied earth, in these cases where the structural is exposed metal and it is raised up on something insulating where TT is not used then I have in the past implemented a dual arrangement of extending the BZ and adding Earthing rods. The strange thing is the BS7671 or guidance does not cover this latter scenario although it is a very real example and does pose a risk even from small network leakage through damaged cables of mainly the older kind where any steel ducting, armouring etc has corroded over time.

PS - if it is in guidance notes then this at most is just a simplified guide for most situations and often uses a hammer to crack a nut approach.

You keep saying about wording it correctly but throughout 7671 it is worded as using the PME earthing facility. I think it is best to utilise the terminology of the book that everyone should refer to to keep things uniform and clear.

As for where the PEN conductor is earthed, it is impossible to determine either if it is earthed, where it is earthed, how effective the earth is and also where the fault is on the PEN conductor. As such it is completely pointless bringing this up. It brings nothing to the design process.

I have utilised both before also. Although it is impossible to extend the EBZ in a situation where someone can touch a bonded metallic part and true earth. This is just not possible. So I utilised a perfectly good earth in the PME and installed everything to a TT so that in the event of a break in the PEN the installation would the TT.

Regulation 717.411.4 specifically states a 'PME earthing facility shall not be used as the means of earthing for an installation falling within the scope of this section'. There are some exceptions but certainly not to be used in a domestic setup.

Section 717 covers 'Mobile or Transportable Units' but also covers 'workshops' and 'offices' (detailed in 717.1).

I would be really interested in how you extended an EBZ but also determined that you needed earth electrodes??

 

[suffolkspark]

I dont see why you dont run the swa all the way back to the ccu?

 

[telectrix]

maybe it's difficult to run SWA inside? if thew T/E is in void and/or surface then there's no need for RCD protection.

 

[suffolkspark]

He said its going up in a cupboard then through a loft and dropping into the board?

 

[Darkwood]

@essex

'You keep saying about wording it correctly but throughout 7671 it is worded as using the PME earthing facility.'

It is worded correctly in the BS7671.. the PME earthing facility is the incoming earth/neutral connection into the cutout, after that its is no longer classed as PME and changes to normally TNS or TT, so when people talk about taking PME out to a shed this is a very different thing and would mean you are keeping the combine N/E set upwhich would require special permissions.

'As for where the PEN conductor is earthed, it is impossible to determine either if it is earthed, where it is earthed, how effective the earth is and also where the fault is on the PEN conductor. As such it is completely pointless bringing this up. It brings nothing to the design process.'
The debate is regarding a PME supply which will likely have a PEN conductor, been PME then it does have multiple earthing grounding straps/rods along the supply cable length so it is a valid point to bring it up as we were discussing the need to TT a metal cabin on a building site and I provided my opinion why this cannot be based on a PME system when you describe a damaged cable.

Regulation 717.411.4 - this is in agreement with everything I have said so far.

'I would be really interested in how you extended an EBZ but also determined that you needed earth electrodes??'
It was done on a stone yard site where they used steel shipping containers for a canteen and locker rooms, these were sat on railway sleepers just to avoid them corroding in the mud and had a step to the metal door of the cabin, it was simple to touch the container while been on the true ground, I initially just extended the EBZ as this was a reliable earthing source but comments of occasional tingles came back, now I tested it all and inspected with a fine tooth brush, rcd's operated etc etc couldn't find anything until I got called out in the rain and found they were touching the cabin outside while on true ground, it turns out there was about 55v PD between the wet ground and earthed cabin... Instinct would have been to remove the EBZ at entry and rod it but we had other earthed equipment and pipes within close proximity so it was decided to do a combination.
The leakage couldn't be identified as part of any of the installation and the Supplier says they couldn't identify any issues on their system but said it may not show up or could be a remote issue in someones else's property.
In such scenarios now I tend to belt and brace regardless as it is clear the BS7671 doesn't account for all circumstances hence it is only a guidance.

This particular subject has been discussed at length on the forum numerous occasions, the one I see on the internet the most but not on here too much is garden hot-tubs where people are getting shocks when climbing in or out of the spar, they are fully earthed and rcd functions are fine.. it too comes down to a PD between the supply earth and the actual ground from a possible local fault be it grid or dodgy garden electrics possible even a neighbours, in such cases the advice is the same as well as trying to identify the fault which isn't always possible.

 

[Nigel]

Let's just word this correctly, we are not using PME or suggesting it for the outbuilding which was the initial point in my post, we are extending the EBZ where the incoming supply is PME (noted you did say about EBZ earlier)
Also PEN conductors in a PME setup have multiple points of earthing along their length to protect against such network faults, it is TNCS that has an increased risk of ground faults but if bonded correctly then on these rare events the risks are minimised.

What can occur though is a PD between supplied earth and actually ground if you are in an isolated metal framed building where you could touch it and bridge between true ground and supplied earth, in these cases where the structural is exposed metal and it is raised up on something insulating where TT is not used then I have in the past implemented a dual arrangement of extending the BZ and adding Earthing rods. The strange thing is the BS7671 or guidance does not cover this latter scenario although it is a very real example and does pose a risk even from small network leakage through damaged cables of mainly the older kind where any steel ducting, armouring etc has corroded over time.

PS - if it is in guidance notes then this at most is just a simplified guide for most situations and often uses a hammer to crack a nut approach.

You keep saying about wording it correctly but throughout 7671 it is worded as using the PME earthing facility. I think it is best to utilise the terminology of the book that everyone should refer to to keep things uniform and clear.

As for where the PEN conductor is earthed, it is impossible to determine either if it is earthed, where it is earthed, how effective the earth is and also where the fault is on the PEN conductor. As such it is completely pointless bringing this up. It brings nothing to the design process.

I have utilised both before also. Although it is impossible to extend the EBZ in a situation where someone can touch a bonded metallic part and true earth. This is just not possible. So I utilised a perfectly good earth in the PME and installed everything to a TT so that in the event of a break in the PEN the installation would the PME.

Regulation 717.411.4 specifically states a 'PME earthing facility shall not be used as the means of earthing for an installation falling within the scope of this section'. There are some exceptions but certainly not to be used in a domestic setup.

Section 717 covers 'Mobile or Transportable Units' but also covers 'workshops' and 'offices' (detailed in 717.1).

I would be really interested in how you extended an EBZ but also determined that you needed earth electrodes??

@essex

'You keep saying about wording it correctly but throughout 7671 it is worded as using the PME earthing facility.'

It is worded correctly in the BS7671.. the PME earthing facility is the incoming earth/neutral connection into the cutout, after that its is no longer classed as PME and changes to normally TNS or TT, so when people talk about taking PME out to a shed this is a very different thing and would mean you are keeping the combine N/E set upwhich would require special permissions.

'As for where the PEN conductor is earthed, it is impossible to determine either if it is earthed, where it is earthed, how effective the earth is and also where the fault is on the PEN conductor. As such it is completely pointless bringing this up. It brings nothing to the design process.'
The debate is regarding a PME supply which will likely have a PEN conductor, been PME then it does have multiple earthing grounding straps/rods along the supply cable length so it is a valid point to bring it up as we were discussing the need to TT a metal cabin on a building site and I provided my opinion why this cannot be based on a PME system when you describe a damaged cable.

Regulation 717.411.4 - this is in agreement with everything I have said so far.

'I would be really interested in how you extended an EBZ but also determined that you needed earth electrodes??'
It was done on a stone yard site where they used steel shipping containers for a canteen and locker rooms, these were sat on railway sleepers just to avoid them corroding in the mud and had a step to the metal door of the cabin, it was simple to touch the container while been on the true ground, I initially just extended the EBZ as this was a reliable earthing source but comments of occasional tingles came back, now I tested it all and inspected with a fine tooth brush, rcd's operated etc etc couldn't find anything until I got called out in the rain and found they were touching the cabin outside while on true ground, it turns out there was about 55v PD between the wet ground and earthed cabin... Instinct would have been to remove the EBZ at entry and rod it but we had other earthed equipment and pipes within close proximity so it was decided to do a combination.
The leakage couldn't be identified as part of any of the installation and the Supplier says they couldn't identify any issues on their system but said it may not show up or could be a remote issue in someones else's property.
In such scenarios now I tend to belt and brace regardless as it is clear the BS7671 doesn't account for all circumstances hence it is only a guidance.

This particular subject has been discussed at length on the forum numerous occasions, the one I see on the internet the most but not on here too much is garden hot-tubs where people are getting shocks when climbing in or out of the spar, they are fully earthed and rcd functions are fine.. it too comes down to a PD between the supply earth and the actual ground from a possible local fault be it grid or dodgy garden electrics possible even a neighbours, in such cases the advice is the same as well as trying to identify the fault which isn't always possible.

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.

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.

In what you describe I would say you have extended the earthing. Not the zone. 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, 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. 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.

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.

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.

 

[Midwest]

Just thinking of taking them to the job for my initial assessment don’t want them to fail me if I don’t install a earth rod

I mean, don't tell them your not a paid up member yet. Once you've handed over payment, your entitled to tech support, regardless of whether you've passed your assessment or not.

 

[Midwest]

Whilst you two are discussing the finer points of exporting, extending PME etc etc, I think you might be confusing the OP, in his rather more simple set up.

In the resources section OP, you might find this download worth a read;

Exporting Pme - https://www.electriciansforums.co.uk/resources/exporting-pme.101/

 

[Tim collins]

Whilst you two are discussing the finer points of exporting, extending PME etc etc, I think you might be confusing the OP, in his rather more simple set up.

In the resources section OP, you might find this download worth a read;

Exporting Pme - https://www.electriciansforums.co.uk/resources/exporting-pme.101/

Thanks

 

[westward10]

EBZ, now there is an obsolete statement. When did that get wiped from the Regs.

 

[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.

 

[Midwest]

Jes, is this still going on

 

[Ian1981]

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.

 

[MDJ]

How do you know you have PME? you may have TNCS instead.

 

[Ian1981]

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.