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

Wilko said:

Hi Tim- The means of earthing at the installation origin can be used at outbuildings, but it does have to be designed. IET Guide Note 8 has some useful "how to" examples.

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Thanks I’ll have a look into this

Tim collins said:

Stroma

I’m running swa outside from the house to shed terminating into a wiska box then t+e in cupboard in trunking up to ceiling then will be clipped in ceiling void , before dropping back down to fuseboard which is cupboard under the stairs client doesn’t want any chasing

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Sorry yes just read post 10 .no nead for the upfront Rcd then and with no extraneous conductive part i cant see a problem using the pme

Tim collins said:

Stroma

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Just for future reference, if they don't ask, don't give.

Midwest said:

Just for future reference, if they don't ask, don't give.

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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

The 2 RCDs in series will still work but it's best avoided. Often both will trip on a fault, or when you're trying to test. On the plus, it will allow you the opportunity to show how to test each separately if you want .

Just a note as the title is somewhat ambiguous, the PME arrangement stops at the cutout, we do not export or extend PME normally as that would need to be agreed with the National Grid guys, I sense you didn't mean this, the PME or other arrangement at source can effect though your own earthing arrangments which will be a TNS format or a TT or a mixture.
If you ever have a PME or TNCS at source then you have a very sound reliable earthing system and regardless how the outbuilding is arranged I would always use the provided earth and if needed then bring an equipotential bond out... I would only consider TTing the out building to reduce costs on a bigger scale but there would need to be a very large distance to be covered to justify this IMHO. TT systems are inherently variable so getting a constant reliable good earthing reading is difficult as your readings are easily effected by the conditions on the day.

darkwood said:

Just a note as the title is somewhat ambiguous, the PME arrangement stops at the cutout, we do not export or extend PME normally as that would need to be agreed with the National Grid guys, I sense you didn't mean this, the PME or other arrangement at source can effect though your own earthing arrangments which will be a TNS format or a TT or a mixture.
If you ever have a PME or TNCS at source then you have a very sound reliable earthing system and regardless how the outbuilding is arranged I would always use the provided earth and if needed then bring an equipotential bond out... I would only consider TTing the out building to reduce costs on a bigger scale but there would need to be a very large distance to be covered to justify this IMHO. TT systems are inherently variable so getting a constant reliable good earthing reading is difficult as your readings are easily effected by the conditions on the day.

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There are a few situations when utilising the PME earthing arrangement on installations outside of the EBZ is not permitted due to safety, not cost.

For example is it was a metal outbuilding then it would almost be a certainty that an earth electrode would have to be used. Building sites is another example.

I would like to see a reference for your 'not permitted' claim, I haven't worked on building sites for over 20yrs so maybe a bit out of touch and will happily stand corrected.
I agree that it may be easier to TT on a building site but we are taking my comment a little out of context, we are discussing a permanent install here not a temp', the cost issue is the opposite if it is temp' and there in my mind is no other reason than reducing costs as to why you cannot bond the metal outbuilding as would be done in a permanent install other that running a separate earthing conductor may leave it vulnerable to damage if say the other cables are SWA.

darkwood said:

I would like to see a reference for your 'not permitted' claim, I haven't worked on building sites for over 20yrs so maybe a bit out of touch and will happily stand corrected.
I agree that it may be easier to TT on a building site but we are taking my comment a little out of context, we are discussing a permanent install here not a temp', the cost issue is the opposite if it is temp' and there in my mind is no other reason than reducing costs as to why you cannot bond the metal outbuilding as would be done in a permanent install other that running a separate earthing conductor may leave it vulnerable to damage if say the other cables are SWA.

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There is no dispensation in the Regs for temp install. All installs regardless of their length of service need to meet BS7671 in full.

I will open the BBB sometime this morning and have a look.

The reason we do not use a PME for a metallic outbuilding is if a PEN conductor fails on the DNO side it will bring the neutral, and therefor the earth in the installation up to 230 volts. I would not fancy opening the door to the metal outbuilding while standing on the grass would you?

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.

darkwood said:

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.

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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??

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

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.

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

@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.

darkwood said:

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.

Click to expand...

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??

darkwood said:

@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.

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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.

Tim collins said:

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

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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.

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/

Midwest said:

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/

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Thanks

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