Exporting earths

[Deleted member 9648]

This old chestnut has surfaced yet again on a thread.....can someone please explain to me why otherwise sensible and knowledgable sparkies seem to go into a panic as soon as 'garage' or 'shed' is mentioned and suggest a rod?...and yet those same sparks will happily install outside lighting,outside sockets and other stuff back to the house CU without even a mere suggestion of the dreaded 'exported earth' and a rod......I've never been able to understand the thinking behind it,can someone enlighten me please?

 

[Jimmy Boy]

This subject was raised on probably one of my first posts on here I think Sintra nailed it, I still can't sleep at night though with a crap Ze..

http://www.electriciansforums.net/e...ns/21131-detached-garage-regs.html?highlight=

 

[Des 56]

There is the tooth fairy and the Easter bunny,thats two to start off
They can be grouped in the same category as the exporting pme myths

Fear spreads much easier than truth, and the rumours that fear generate, they eventually get touted as realism

Export or dont export,make the decision on the circumstances at hand,its a decision thats available to the installer and thats the myth dispeller
Make your own choice,nothing prevents you from doing so

 

[La Poste]

And what about the dreaded outside garden tap?
I've heard the Regs were meant for outbuildings that were a seriouly long way from the main PME point like farms etc.
A long distance would result in a potentially large potential difference between the PME earth and real Terra earth of 0 volts. (PME earth can fluctuate, electric currents flow like water back and forth, PME earth is rarely 0V)
And so exporting earths can result in one hand holding a PME earth lead at who knows what voltage and one un-insulated foot placed in the zero voltage of earth (Terra = 0 volts.)
One hand holding a cable at PME earth voltage and one foot in the real solid earth.
Difference in voltage = electric shock.

 

[Sparky Ninja]

And what about the dreaded outside garden tap?
I've heard the Regs were meant for outbuildings that were a seriouly long way from the main PME point like farms etc.
A long distance would result in a potentially large potential difference between the PME earth and real Terra earth of 0 volts. (PME earth can fluctuate, electric currents flow like water back and forth, PME earth is rarely 0V)
And so exporting earths can result in one hand holding a PME earth lead at who knows what voltage and one un-insulated foot placed in the zero voltage of earth (Terra = 0 volts.)
One hand holding a cable at PME earth voltage and one foot in the real solid earth.
Difference in voltage = electric shock.

And it's reasons like that, that places like farm and other agricultural premises are no longer considered acceptable for TNC systems.

 

[pushrod]

And it's reasons like that, that places like farm and other agricultural premises are no longer considered acceptable for TNC systems.

Didn't know that TNC was ever much used in the UK.

 

[malcolmsanford]

Like a lot of things electrical exporting an earth as grown to myth status over the years, the bottom line to it is that in most cases it is the DNO that does not want you to do it. In a TNC-S the earth is theirs and what they say goes.

It is like all things it should be done on an individual basis. A seperate garage 3 metres from the house would hold no problems provide it is installed correctly, and correctly bonded. A seperate garage 60 metres from the house will cause problems and so rather than dealing with each individual case, most DNO's use a blanket no export policy.

So I found that rather than go down the road of asking and getting rejected I would automatically TT the system unless the customer wanted to export and then I would get them to enquire, knowing the likely outcome.

 

[pushrod]

Isn't it much to do with economics as well - if you have to put in 25+ metres of 10mm bonding it is cheaper to put a rod in?

 

[Sparky Ninja]

Didn't know that TNC was ever much used in the UK.

TNC systems

 

[pushrod]

TNC systems

Not being awkward just not sure what you mean? There are 3 TN systems that i am aware of TN-C, TN-S and TN-C-S. Which of these earthing arrangements are you saying are not recommended for farms?

 

[IQ Electrical]

Not being awkward just not sure what you mean? There are 3 TN systems that i am aware of TN-C, TN-S and TN-C-S. Which of these earthing arrangements are you saying are not recommended for farms?

TN-C-S if you check out the post from widdler above your previous post, you'll see the missing 'S' in red!

 

[Deleted member 9648]

Like a lot of things electrical exporting an earth as grown to myth status over the years, the bottom line to it is that in most cases it is the DNO that does not want you to do it. In a TNC-S the earth is theirs and what they say goes.

It is like all things it should be done on an individual basis. A seperate garage 3 metres from the house would hold no problems provide it is installed correctly, and correctly bonded. A seperate garage 60 metres from the house will cause problems and so rather than dealing with each individual case, most DNO's use a blanket no export policy.

So I found that rather than go down the road of asking and getting rejected I would automatically TT the system unless the customer wanted to export and then I would get them to enquire, knowing the likely outcome.

Most DNO's simply ask for a cert....in my experience they dont give a stuff what you do providing there is a cert from an approved contractor....and as long as the install complies with Bs 7671..(which makes no mention of any problems with exporting earths) i cant see what it has to do with the DNO....I accept that opinions vary on this one and mine isnt necessarily the right one though!!......My main issue with the rodders though is that they insist on rodding a shed 2m from the house and yet will install other outdoor equipment straight off the house earth....exporting it!.....When was the last time someone came on here and said..."Ive been asked to install a couple of lamp posts up a long drive ,and a socket next to the pond,should I separate the earth and TT those 2 items?".......and yet we get the shed/garage rodders about 3 times a week!

 

[J@mes]

Not being awkward just not sure what you mean? There are 3 TN systems that i am aware of TN-C, TN-S and TN-C-S. Which of these earthing arrangements are you saying are not recommended for farms?

TNCS should not be used as a lost neutral on the supply side can cause the bonding to be used as the return path - cattle grids and pens could become live as a result, and as the big stupid beasts that inhabit farm yards are very sensitive to voltage it can kill them quite easily (I mean cows by the way, not farmers or their wives!)

 

[La Poste]

I'm considering rodding our garden shed.
It's 30 metres from the main house and at a much lower elevation with a stream running at the end of it.
The earth is very damp with clay soil.
All the garden machinery, strimmers, hedge cutters etc is fed from this shed.
It would make a nice little project.
I just hope it will also make things safer.

 

[Deleted member 9648]

La Poste..........if there was a danger with using the main house earth for your garden shed Bs 7671 would not permit it to be used,there is no such regulation.