A good guide For 'exported PME' and outbuildings!

[IQ Electrical]

For anyone bored by the constant myths and rumours regarding this subject, have a read of this guide from the IEE on supplies to detached outbuildings.

It was published in 2005 but the important bits remain unchanged and might be of some help!

 

[SW1970]

Thanks IQ. But what if user is wearing wellies and there's no RCD protection in garage (but is in house). User is holding failed tool and therefore a live part, hasn't got a shock because there's no earth path but then touches water pipe on side of garage. Will MCB or RCD in the consumer unit in the house provide any protection? If no, then shouldn't earthing be improved in the garage (e.g. bonding of this pipe and either TT or 10mm2 back to consumer unit). Thanks, SW.

 

[Deleted member 9648]

If there's RCD protection in the house and the garage is fed from the house...then the garage will be RCD protected.....it doesnt matter how far back up the system the rcd is...the current imbalance it see's will be the same.

 

[rfcses_com]

Thank you

 

[warrier]

:wink5:

For anyone bored by the constant myths and rumours regarding this subject, have a read of this guide from the IEE on supplies to detached outbuildings.

It was published in 2005 but the important bits remain unchanged and might be of some help!

An important omission is the fixing height of a socket outlet in a garage.
The 17th Edition of the IEE Regs require it to be fixed at a suitable height.
The 13 Edition stated a height of 4ft 6 inches. (1.5 metres)

This was to try to remove the risk of explosion / fire from petrol fumes.
This is a good guide line for today's installers

 

[Mr CJ]

thanks.

 

[ProsafeAsh]

Thanks - very usefull

 

[Gbroad]

Thanks, really helpful guide

 

[Edd]

Hi just a reply to the Thought that Neutral breakdowns in PME supplys are Rare,, Last week I was called to a house that had been recently rewired by another party and then tenants moved in..
Dueing the day they were shocked to see smoke from under kitchen sink. the flexy hose had melted and blown apart leaking water across the kitchen a plumber came out to fix it and got a shock everytime he tryed to reconect it, an electrician was called but couldnt find the problem!!
i was called and found a high resistance on the neutral supply and when the kettle was put on and the shower run (any high load) The main and supplementary bonding became the prefered return path!!! just a thought when running to external systems garages etc.. a nice metal framed garrage would make a lovley return path!!... also note this was a 17th edition board so completly RCD protected the RCD was never unbalanced because the fault was external of the RCD

 

[ecoelectric]

Good guide indeed!

Electrician Brisbane south west Jindalee

www.ecoelectric.com.au

 

[sinewave]

This subject has been done to death over on the S/F Sparks Forum.

A few years ago I posted what I called tongue in cheek thread about 'The Utimate PME/TNS Exportation'

The following is a Cut & Paste from said thread with suitable updates for the current Regs.

The Ultimate PME/TNS Exportation! by Sinewave.

Existing
PME/TNS supply at service position.


Run a minimum of 10mm 3 core SWA cable to your external location, following all approved cable installation
procedures.

Terminate in to a 17th Ed Metal Clad Dis-Brd or use RCBO's for all Circs depending on size of remote installation.

Link SWA Earthing Tag to DB-2's MET with 10mm, despite it being a Metal Clad Dis-Brd.
(Belts & Braces!)

Drop an Earth Rod in as close as possible to DB-2 and connect back to DB-2's MET with 10mm

Bond any metal services entering
the remote building with 10mm

Bond any exposed metal framework of the
building with 10mm


Resultant installation has an excellent Ze DB-2

Resultant installation is tied down to 'True Earth' at it's remote
location thus avoiding any 'potential' differentiation problems between the MET at the DNO service poistion/DB-1 and the remote installation at DB-2.


Most of the UK's MV Network is PME'd now lead or otherwise as all joints have a Rod dropped in as a matter of course.

 

[Deleted member 26818]

I noticed the other day, a post on the IET's forum linking to this sticky, which made me chuckle as the link here is to a guide on the IET's website.

 

[IQ Electrical]

I noticed the other day, a post on the IET's forum linking to this sticky, which made me chuckle as the link here is to a guide on the IET's website.

Yes, I saw the irony there!

 

[Chr!s]

The way i look at is, if i export PME am i taking it into a zone where an equal potential can be created. If it can then there's no more of a risk than anywhere else.

Personally if i cant create an equipotential zone i wont export, or ill go done the route of placing an electrode system of a suitable Ra to keep the touch voltage down.

With the country now becoming pretty much PME it will be interesting to see the rate of failures on the network to see how rare these broken pens are now.

Regards Chris

 

[IQ Electrical]

The way i look at is, if i export PME am i taking it into a zone where an equal potential can be created. If it can then there's no more of a risk than anywhere else.

Personally if i cant create an equipotential zone i wont export, or ill go done the route of placing an electrode system of a suitable Ra to keep the touch voltage down.
With the country now becoming pretty much PME it will be interesting to see the rate of failures on the network to see how rare these broken pens are now.

Regards Chris

And that's what it all boils down to....

 

[Engineer54]

The way i look at is, if i export PME am i taking it into a zone where an equal potential can be created. If it can then there's no more of a risk than anywhere else.

Personally if i cant create an equipotential zone i wont export, or ill go done the route of placing an electrode system of a suitable Ra to keep the touch voltage down.

With the country now becoming pretty much PME it will be interesting to see the rate of failures on the network to see how rare these broken pens are now.

Regards Chris

Why not take the PME to the outbuilding/shed or whatever, and connect a rod to the EMT at the small CU?? You then have best of both worlds relatively speaking. Personally i wouldn't feel the need to rod, i'd be more than happy with the PME...

The way things are in modern times, is profits Rule!!! and it's how much that maintaining PME is going to eat into those profits that will decide. That and substantial fines and/or damages for Not maintaining their LV network earthing systems...

 

[Chr!s]

Why not take the PME to the outbuilding/shed or whatever, and connect a rod to the EMT at the small CU?? You then have best of both worlds relatively speaking. Personally i wouldn't feel the need to rod, i'd be more than happy with the PME...

The way things are in modern times, is profits Rule!!! and it's how much that maintaining PME is going to eat into those profits that will decide. That and substantial fines and/or damages for Not maintaining their LV network earthing systems...

Well i think we are singing off the same hymn sheet, connecting an electrode system at the emt or met if at the correct Ra will maintain a safe touch voltage upon a broken pen, or do you mean take out the PME and the divorce the earth at entry?

Regards Chris

 

[Engineer54]

Well i think we are singing off the same hymn sheet, connecting an electrode system at the emt or met if at the correct Ra will maintain a safe touch voltage upon a broken pen, or do you mean take out the PME and the divorce the earth at entry?

Regards Chris

Come on Chris, why would i want to ditch a superior earth and replace it with a TT system!! ...lol!!!
As i said in my post above i wouldn't feel the need to add an earth electrode to the EMT of an outbuilding. But if there was an existing rod in place, near to the outbuilding, of course i'd certainly make use of it!!

 

[Chr!s]

Come on Chris, why would i want to ditch a superior earth and replace it with a TT system!! ...lol!!!
As i said in my post above i wouldn't feel the need to add an earth electrode to the EMT of an outbuilding. But if there was an existing rod in place, near to the outbuilding, of course i'd certainly make use of it!!

Lol, when i was replying i thought as much, just testing .

Regards Chris

 

[DanTheSparkyMan]

one thing this whole topic has made me think about is that when I happen to be taking a 10mm sub main or bigger to an out building I automatically have an earth wire large enough to cope with bonding, so or though I would normally make this area TT I could legitimately connect it to the PME incomer.

 

[Deleted member 26818]

one thing this whole topic has made me think about is that when I happen to be taking a 10mm sub main or bigger to an out building I automatically have an earth wire large enough to cope with bonding, so or though I would normally make this area TT I could legitimately connect it to the PME incomer.

Yes if the CPC of the sub main is 10mm² copper, then it could be used as a bonding conductor.
The problem being that the armour of 10mm² SWA being steel, does not have an equivelant conductance to 10mm² copper.
Thsi leaves you with the choices of 2 core SWA and a rod, 2 core SWA and a separate bonding conductor or 3 core SWA.

 

[cmac91000]

Thanks for this, very useful and I understand what has been explained about the myths of exporting PME.
I am connecting a garage 45m to the CU in the house on a TNC S and have decided to connect by means of 2 core armoured cable. I shall be adopting a TT system in the garage with an earth rod. The armour will of course only be connected at the house CU.
My question is about discrimination of the RCD's. The MCB for the garage, in the house is protected by a 30mA RCD and the socket circuit in the garage requires a 30mA RCD. What is the best way to achieve discrimination of the RCD's? If I replace the house RCD with a slow 30mA would this contravene the regs on the other circuits protected by it. thanks

 

[Engineer54]

What do you mean by ''Slow 30mA'', do you mean an S type RCD?? How are you going to do that then?? Any other circuits on that S type RCD will not meet BS 7671 requirements!!

Either a non protected MCB in the CU, or Henley Blocked tails with a 100mA S type RCD.

And if there isn't any extraneous services at this garage, then you would be much better off, taking the PME earthing with you from the house!!

 

[mark_]

Can someone please fill in the blanks for me as I'm really interested in this topic but strugling with a few bits.

If you have an outbuilding which you install Db2 with say lighting and power radial circuits as part of a PME system then any copper water bond would have to be connected in 10mm to the DB1 MET.

Now as you have sized your submain to DB2 as 4mm, rather than running a 10mm back to the supply origin you elect to make the out building a TT system. You connect your spike with 6mm and connect to DB2 (all going well) but then there is the copper water pipe. Minimum bonding conductor is 6mm (pretty sure half of line or neutral conductor or 6mm minimum for non PME) but does it matter that this is larger than our line and neutral conductor?


It just seems a bit odd with a 6mm earth and a smaller line and Neutral. If ive got this way wrong please dont bite my head off.

 

[IQ Electrical]

If you don't understand something, just ask, don't bother with the abusive comments.

 

[matt v]

A good guide, am pricing to install heat light and power to out building 35 m from main board. Installing new circuit on none rcd side of split board to 32amp mcb in 4mm armoured clipped and buried to 2nd rcd db in shed with 4 ways, 1 for heater, 1 for sockets, 1 for light and a spare for garden lights heat and power on 20a each and lights on 6a.
Luckily no metal work to bond to complicate matters!

 

[oldAL]

If you don't understand something, just ask, don't bother with the abusive comments.

actually I do,but it seems a lot of people do not understand the differences between TNCS and PME.
they would much rather give out what may well be potentially lethal advice!
it appears more and more frequently that earthing systems seem to be very much misunderstood by far too many people that claim to be competent electricians.

 

[Engineer54]

actually I do,but it seems a lot of people do not understand the differences between TNCS and PME.
they would much rather give out what may well be potentially lethal advice!

it appears more and more frequently that earthing systems seem to be very much misunderstood by far too many people that claim to be competent electricians.

Perhaps you could explain it to us then??

 

[oldAL]

a really easy point to make would be that PME and TNCS are NOT the same thing,
far too many so called 'competent' 'electricians' seem to think they are,
there is a world apart from the 2 systems described by TNCS and PME,
if you actually know what the letters stand for then you should also know what the differences are.
and its not always a good thing to believe what the DNO may tell you or label your cut-out up as,
I know we are supposed to take that as being correct, but I for one am not willing to put someones life at risk for the sake of a sticker on the incoming fuse.

 

[johnboy6083]

PME- protective multiple earthing, which means that the PEN conductor is grounded at many points along its path.

TNC-S, the PEN conductor is split at the service head, to from as neutral and an earth conductor. There is nothing to say that the PEN conductor is earthed on its way from the tranny.
Its good practice to install your own rod, to help stop diverted neutral currents appearing on your installation.

 

[oldAL]

it is, but without a proper knowledge of the systems involved and adequate labelling, it unfortunately very seldom happens,
and that is where a very inherent danger occurs from a very substantial number of sparks.