Supply to garage.

[DPG]

That's half the problem - the phrase 'exporting PME'. Can we just abolish it here and now??!!

 

[TJ Anderson]

You are not 'exporting PME', you are simply extending an existing earthing system.
I've never ever heard anyone say they are 'exporting TNS....or TT'.

It's not a special location.

The car charger comes under Special installations (722). That's what I was getting at when asking regarding why going for TT. He may be still ok with PME still if nothing is extraneous and also that the charger meets criteria of 722.411.4.1 (iii).

 

[radiohead]

The car charger comes under Special installations (722). That's what I was getting at when asking regarding why going for TT. He may be still ok with PME still if nothing is extraneous and also that the charger meets criteria of 722.411.4.1 (iii).

Yes absolutely right, when I posted I had in mind my general point about sheds/garages etc and completely forgot the OP's job includes a car charger.

 

[davesparks]

If the remote building has a concrete earth floor, or conductive floor, or the building is of metal construction or metal outside skin, the shock risk is increased. It’s therefore recommended that the remote building has a TT earthing system.

I've not seen anything in the regulations, or in the rules for the application of PME preventing its use in concrete floored buildings, not have I ever heard of a DNO refusing to provide a PME connection due to a building having a concrete floor.
Yes for some metal structures they will refuse to provide a PME connection due to the increased risk.

Do you have any reasonable evidence to explain and support this, regulations, guides to the regulations, technical publications etc?

When meter tails are over 3m an isolator is normally needed to satisfy the dno.

The discussion was if the new tails absolutely have to have a fused switch then why is just an isolator acceptable for every other property in the uk with meter tails over 3m.

The reason for the TT is its safer.

A fuse or circuit breaker is required to satisfy the DNO as they do not allow their fuse to be used to protect tails over 3m long (in most cases although some do allow 4m and some say the length must include the tails from cutout to meter)

Just an isolator is not acceptable for every other property in the UK with tails over 3m.

TT is not safer to the best of my knowledge, unless you can get a very good resistance to earth from the earth electrodes you install, then it may become equally safe.
PME provides a very low resistance earth directly back to the transformer, TT generally provides a much higher resistance path which can vary with seasonal changes and disturbance of the ground.
The last set of earth rods I installed had a combined Ra of 18 ohms, that's over 50 times the design value of a PME earthing connection for a 100A service.

 

[Edward67]

That's half the problem - the phrase 'exporting PME'. Can we just abolish it here and now??!!

How would you like to explain it so we can avoid the endless circle.

I've not seen anything in the regulations, or in the rules for the application of PME preventing its use in concrete floored buildings, not have I ever heard of a DNO refusing to provide a PME connection due to a building having a concrete floor.
Yes for some metal structures they will refuse to provide a PME connection due to the increased risk.

Do you have any reasonable evidence to explain and support this, regulations, guides to the regulations, technical publications etc?


A fuse or circuit breaker is required to satisfy the DNO as they do not allow their fuse to be used to protect tails over 3m long (in most cases although some do allow 4m and some say the length must include the tails from cutout to meter)

Just an isolator is not acceptable for every other property in the UK with tails over 3m.

TT is not safer to the best of my knowledge, unless you can get a very good resistance to earth from the earth electrodes you install, then it may become equally safe.
PME provides a very low resistance earth directly back to the transformer, TT generally provides a much higher resistance path which can vary with seasonal changes and disturbance of the ground.
The last set of earth rods I installed had a combined Ra of 18 ohms, that's over 50 times the design value of a PME earthing connection for a 100A service.

I was referring to an article in electri

I've not seen anything in the regulations, or in the rules for the application of PME preventing its use in concrete floored buildings, not have I ever heard of a DNO refusing to provide a PME connection due to a building having a concrete floor.
Yes for some metal structures they will refuse to provide a PME connection due to the increased risk.

Do you have any reasonable evidence to explain and support this, regulations, guides to the regulations, technical publications etc?


A fuse or circuit breaker is required to satisfy the DNO as they do not allow their fuse to be used to protect tails over 3m long (in most cases although some do allow 4m and some say the length must include the tails from cutout to meter)

Just an isolator is not acceptable for every other property in the UK with tails over 3m.

TT is not safer to the best of my knowledge, unless you can get a very good resistance to earth from the earth electrodes you install, then it may become equally safe.
PME provides a very low resistance earth directly back to the transformer, TT generally provides a much higher resistance path which can vary with seasonal changes and disturbance of the ground.
The last set of earth rods I installed had a combined Ra of 18 ohms, that's over 50 times the design value of a PME earthing connection for a 100A service.

Exporting A PME Supply To Another Building

In this article the technical team at Stroma Certification offers expert guidance on exporting a PME supply to another building. One of the most common queries to the Stroma help desk concerns running

professional-electrician.com

Doesn't say it's not allowed but was referring back to this.

 

[davesparks]

I was referring to an article in electri

Exporting A PME Supply To Another Building

In this article the technical team at Stroma Certification offers expert guidance on exporting a PME supply to another building. One of the most common queries to the Stroma help desk concerns running

professional-electrician.com

Doesn't say it's not allowed but was referring back to this.

Ah yes, that well known source of verified technical information, the freebie mag from the wholesalers.

The article makes some good points but also makes some mistakes.
There is nothing, as far as I am aware, preventing the use of an earth from a PME supply being used in a building with a concrete floor.
I'm sure a lot of us have been to, and worked on, installations which have a PME supply and a concrete floor.

 

[radiohead]

I'm sure a lot of us have been to, and worked on, installations which have a PME supply and a concrete floor.

Probably a majority of buildings with a PME supply have a concrete floor.
My house does. And so does my attached garage, it's on the same PME as the house.
I do often wonder why the 'exporting' thing only seems to apply to PME??
Why dont we 'export' TT or TNS?
And why does it only apply to outbuildings? Why does the fear of PME seem only to apply to outbuildings?
I 'exported' the PME to a socket by a pond last week, never ever seen anyone TT a pond socket..

 

[davesparks]

I 'exported' the PME to a socket by a pond last week, never ever seen anyone TT a pond socket..

I'd keep quiet about that if I were you, otherwise someone will be along with an eath rod and drive it straight through your pond lining!

All that water leaking down the earth rod will be great for keeping the Ra down, but pretty terminal for the prize koi!

 

[TJ Anderson]

Probably a majority of buildings with a PME supply have a concrete floor.
My house does. And so does my attached garage, it's on the same PME as the house.
I do often wonder why the 'exporting' thing only seems to apply to PME??
Why dont we 'export' TT or TNS?
And why does it only apply to outbuildings? Why does the fear of PME seem only to apply to outbuildings?
I 'exported' the PME to a socket by a pond last week, never ever seen anyone TT a pond socket..

It only applies to PME because of the of the potential risk of exposed and extraneous conductive parts rising to mains potential if the PEN conductor suffered a broken earthed neutral and the line conductor stayed intact. This cannot occur on a TN-S or a TT as they have separate conductors.

 

[radiohead]

It only applies to PME because of the of the potential risk of exposed and extraneous conductive parts rising to mains potential if the PEN conductor suffered a broken earthed neutral and the line conductor stayed intact. This cannot occur on a TN-S or a TT as they have separate conductors.

I understand the potential dangers of PME supplies, I don't need that explaining.
But it does not explain why only PME is apparently 'exported', TNS and TT are also 'exported' where they feed a remote installation .
Neither does it explain why PME is only ever 'exported' to outbuildings. There are many other situations remote from the intake where 'exported' PME is never questioned. The pond socket I installed last week. External lighting/garden lighting. I recall a job a few years back where the installer had fed a string of drive light columns from the TNCS house supply, and run a distribution circuit to a summer house which had been separated and TT'd......so they couldn't export the PME to the shed but it was fine on the drive lights!
I just hate the term 'exported' and the lack of logic behind only applying it to outbuildings.

 

[TJ Anderson]

They only mean extending the earth outsider by the term exporting so I am not worried about the terminology. They could be more specific regarding outbuildings (as with Caravans) but it is really because the impact of a PEN failure is likely to be more severe with some outbuildings dependant on what is being supplied in them. Lots of class 1 metal cased equipment for example and also just having exposed ground or concrete. Also what is the likelyhood of someone being in contact with it. The way it is at the moment allows an electrician to apply his/her judgement and I prefer that.

My home has PME. My garage is remote from my house and I utilise the distribution circuit earth. I don’t have lots of metal cased class 1 equipment. If I did, I might consider changing it to TT.

Your pond pump (likely class 2) would not be any risk at all even under an PEN failure conditions.