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

[Tim collins]

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.

Thanks I’ll have a look into this

 

[Flanders]

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

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

 

[Midwest]

Stroma

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

 

[Tim collins]

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

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

 

[Wilko]

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 .

 

[Darkwood]

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.

 

[Nigel]

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.

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.

 

[Darkwood]

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.

 

[Nigel]

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.

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?

 

[Darkwood]

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.