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!

 

[james king]

cant c it on my phone sorry!! as far as i am aware u can take a supply from a existing house CU via a MCB then to another sub CU ran in SWA. if the supply to the house is PME why would u need to put a earth rod in? can u not run a 3core cable and take the earth from the MET in the house??? i think i have miss understood

 

[IQ Electrical]

You can but there are reasons (often economic) why TT might be used, the point is, you have a choice.

Have a look at the guide when you''re at a pc.

 

[james king]

ok i am guessing a rod would be used for earth metal parts eg framework? could you use a rod conbined with the earthing from the house CU?

 

[james king]

can u please suggest some reason why u would use both ?

 

[IQ Electrical]

Perhaps where main protective bonding conductor sizing results in a completely uneconomic CSA of swa cable for the distribution circuit.

 

[Engineer54]

A lot of these myths stem from the numerous ''Scaremongers'' of the PME system!! In that ''IF'' (and it's a very big IF) a break in the neutral occurs all connected metal parts will have a potentially dangerous voltage. In reality, the chances of that happening is remote to say the least. I would take a PME system earth out, to any out-building, wouldn't give it a second thought either

Again In reality, what would you rather have as your earth connection, a system that gives you 0.35ohms or a rod at maybe 200ohms, ...i know what i would rather have in my home.

There is no reason why you can't employ a rod on a PME system, in fact many countries insist on it. Many local authorities used to rod TNS supplies to there council houses in the 50s.

 

[Blue Whale]

Many thanks for posting this info.

 

[Engineer54]

Then there's 'if there are extraneous-conductive parts in the outbuilding, you can't export PME.

Then there's 'you must inform the DNO'
Then there's 'you must inform the DNO'

Seems you still haven't quite grasped the point have you. Yes you CAN take PME to an outbuilding with extraneous-conductive parts!! Your not exporting your extending the equipotential zone!! Please read ALL of this thread, or you will be taking the thread back to the very start again with your post above!!

There is NO must inform the DNO, ...about the majority of installations undertaken by electricians, and certainly not in the case of shed in the back garden. Get your facts sorted out please!!!

 

[ElectricalAsh]

thanks

 

[IQ Electrical]

I am an electronics engineering graduate and I know a little about electrical topics.

Oh good, for a moment I thought you were a spammer...

 

[WayneL]

Let's get the terminology right, shall we chaps.

You cannot 'export PME' to outbuildings.

Combining the Neutral and Earth on the consumer side of the installation is not allowed in this country.

You can, however, run an Earth, or Bonding Conductor, from your installation MET to an outbuilding - regardless of the supplied earthing system.
You must though, as Designer and Installer, take into account 'Extraneous Conductive Parts' in the outbuilding.
If it's a PME supplied Earthing System, then you would need to bond these back to the MET, as you would any other Extraneous Conductive Part.
This could mean running a 10mm Bonding to the shed to accompany a 2.5 mm socket/lighting feed.


The other thing that needs to be taken into consideration is
uninsulated flooring in the shed/outbuilding.
This could mean that the floor is at 'true earth' potential - bit like being outside.
This could be dangerous if there was a loss of supply neutral, (or a high resistance joint on the supply neutral), as all bonded metalwork will rise in potential compared to true earth.

So, it's not a case of myths, it's a case of people only grasping bits of information and repeating them.

There isn't a 'yes you can' or a 'no you can't' - it's up to you, the designer/electrician, to make your decision based on the characteristics of the installation and the outbuilding.

Happy Days

 

[Engineer54]

Your not exporting anything, but rather Extending an Extraneous Conductive Zone, in cases where there are extraneous conductive parts at outbuildings, whether it be TN-S or TNC-S (PME)....

As for uninsulated floors, well there's not much difference in this instance, to housing built prior to i think the 60's where there was no slab damp proofing layer required or laid under solid concrete floor slabs and with tiled finishes...

As for the dangers, the same dangers exist at any PME installation, but the chances of that ever happening is to say the very least is slim indeed.... If there was to be a catastrophic neutral breakdown on a PME supply line, the chances are, it will be tripped out of service by the DNOs protective devices at the supply source. In truth, there are as many myths to PME neutral failures, as to being able to extend a PME system!!!

 

[WayneL]

Your not exporting anything, but rather Extending an Extraneous Conductive Zone, in cases where there are extraneous conductive parts at outbuildings, whether it be TN-S or TNC-S (PME)....

As for uninsulated floors, well there's not much difference in this instance, to housing built prior to i think the 60's where there was no slab damp proofing layer required or laid under solid concrete floor slabs and with tiled finishes...

As for the dangers, the same dangers exist at any PME installation, but the chances of that ever happening is to say the very least is slim indeed.... If there was to be a catastrophic neutral breakdown on a PME supply line, the chances are, it will be tripped out of service by the DNOs protective devices at the supply source. In truth, there are as many myths to PME neutral failures, as to being able to extend a PME system!!!

Being an 'Engineer', perhaps you could explain how a broken neutral, or (more likely), a high resistance neutral joint in the street outside your property would trip a DNO protective device.

Let's take a scenario:

If you had a rising copper water main in your outbuilding and this was bonded back to the MET in 10mm to satisfy PME supply conditions.
If, for whatever reason, you then lost the supply neutral (which is your earth), or the impedance of this neutral rose undetected due to a dodgy joint etc, then some of the 'normal operating current' from your property (and possibly some of your neighbours' as well), will find an alternative path back to the transformer.

This path could be the rising main in your outbuilding, which would mean that the 'rising main' and any other metalwork will rise in potential, depending on the amount of current taking this path. (don't forget, the RCD won't trip).

If this happens and you touch any of this metalwork in the outbuilding, you should be ok, because it's all bonded together, so all at the same potential, BUT, if you are in contact with 'true earth' (e.g. through the floor) then you could get a shock.

This could also happen under normal operating conditions, although the current taking this alternative path would be very low, unless you had a fantasticly low Ra on the rising water main.

Like I said in my post, it's up to the designer/installer to weigh up the pros and cons of running earths to seperate buildings - it all depends on the circumstances.

 

[Engineer54]

Being an 'Engineer', perhaps you could explain how a broken neutral, or (more likely), a high resistance neutral joint in the street outside your property would trip a DNO protective device.

Let's take a scenario:

If you had a rising copper water main in your outbuilding and this was bonded back to the MET in 10mm to satisfy PME supply conditions.
If, for whatever reason, you then lost the supply neutral (which is your earth), or the impedance of this neutral rose undetected due to a dodgy joint etc, then some of the 'normal operating current' from your property (and possibly some of your neighbours' as well), will find an alternative path back to the transformer.

This path could be the rising main in your outbuilding, which would mean that the 'rising main' and any other metalwork will rise in potential, depending on the amount of current taking this path. (don't forget, the RCD won't trip).

If this happens and you touch any of this metalwork in the outbuilding, you should be ok, because it's all bonded together, so all at the same potential, BUT, if you are in contact with 'true earth' (e.g. through the floor) then you could get a shock.

This could also happen under normal operating conditions, although the current taking this alternative path would be very low, unless you had a fantastically low Ra on the rising water main.

Like I said in my post, it's up to the designer/installer to weigh up the pros and cons of running earths to separate buildings - it all depends on the circumstances.

DNOs tend to take great care in there PME supplies, employing pretty sophisticated monitoring/protection relay devices. I will in due course enquire what such monitoring devices are normally provided, when i next speak to my DNO friend (Regional Manager) and relay that information back to this thread.... I would imagine it would be in the form of a VIP module (multi function monitoring/protection relay) set to the individual parameters of the system it's connected too..

Don't knock a good rod system, lol!!! ...Depending on it's installation and the type of ground soil, it's not particularly uncommon to be re-paid with a sub 1ohm or thereabouts readings. But very doubtful with the use of these now common short thin rods... And would be totally pointless from an economical view point for a domestic outbuilding/shed!! ...lol!!!!

Yes i do agree with you, it most certainly is upto the designer/installer to decide what is the best solution/method of running PME, or any other type of electrical supply to out buildings.

 

[Deleted member 9648]

It always baffles me why the outbuilding PME handwringers only seem to apply their logic to sheds and garages....it never seems to occur to them that the same perceived hazards presumably will apply to external lights and sockets connected to the PME earth on the main building.