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!

 

[JosephHenree]

The gr8 article. Thanx for sharing!

 

[Mark42]

Please excuse a newbie butting in to this fascinating exchange of views.

I read somewhere years ago that the main reason why it is good practice to bond the reinforcing matting in concrete floors in livestock buildings is that cows are extraordinarily sensitive to potential differences through their feet.

I’m not making this up!

It can apparently be difficult to drive stock into a building across the PD that could be created by an unbonded floor. This could be a long-forgotten reason why livestock buildings are selected for special treatment in guidance notes.

Farmers have apparently blamed supernatural influences at work, when the real reason was a volt or two between a building’s floor and the ground outside

Otherwise what’s so special about ‘livestock’ buildings? Don’t forget that most houses are occupied by primates.

 

[IQ Electrical]

Please excuse a newbie butting in to this fascinating exchange of views.

I read somewhere years ago that the main reason why it is good practice to bond the reinforcing matting in concrete floors in livestock buildings is that cows are extraordinarily sensitive to potential differences through their feet.

I’m not making this up!

It can apparently be difficult to drive stock into a building across the PD that could be created by an unbonded floor. This could be a long-forgotten reason why livestock buildings are selected for special treatment in guidance notes.

Farmers have apparently blamed supernatural influences at work, when the real reason was a volt or two between a building’s floor and the ground outside

Otherwise what’s so special about ‘livestock’ buildings? Don’t forget that most houses are occupied by primates.

You're correct in what you say regarding the PD but what is special about livestock is the distance between front and hind legs, that makes a massive difference in PD difference susceptibility.

 

[IQ Electrical]

Duplicate post, can a mod delete please?

 

[mateen]

good knowledge in pdf guide....thanks

 

[DNO]

"you are not exporting, you are extending the equipotential zone"
in fact you are exporting a pme method of earthing and then creating another
zone.
--------garden sheds?

 

[La Poste]

Well for a relative novice like myself it's hard to know where to draw the line.

This quote is from another thread:

In that case you may have a problem using your PME earth in this building, especially if your kids are going to be playing with hoses bare footed. There could well be a potential difference between true earth and the PME earth in this instance. If you did have a grid in the foundation floor that could be bonded, then that would have been acceptable. Looks like your going to have to go the TT route.

http://www.electriciansforums.net/e...lding-design-considerations-2.html#post443510

So is it fair to say that any building with a concrete floor that might become wet and have people with bare feet standing on it should not be PME'd?
Or perhaps the length of the distribution cable will effect the decision.
Where do you draw the line?

 

[Engineer54]

Where farm animals and people running round with bare feet it can be/maybe a problem, otherwise no problem!! If re-bar mesh, or grid incorporated in the the concrete floor foundations, even better ...lol!!!

 

[Guitarist]

The problem with "what-if" scenarios is that they are limitless. If you lived like that then you would never get out of bed. We must take all reasonable care but we also have to employ some common sense.

 

[Jimmy Boy]

What another cracking thread. So in summary so I can go forward,

1) We can export if practicable, if not TT it with all the usual requirements.

2)If we do export then a 10mm Main bonding cable must be provided, if services enter the location

3) If the CSA of the supply cable is <10mm we must provide a separate Main bonding conductor of 10mm

4)If the supply cable CPC CSA is > 10mm or + we can use said CPC as our 10mm main bond

Have I missed anything ?

 

[spark 68]

Have I missed anything ?

A look at GN8 suggests you may need a bigger CSA than 10mm, if the earthing conductor serves a dual purpose ie. as a CPC and main bonding conductor as it has to be large enough to cover the worst case scenario for both, this is why it is suggested to run a seperate 10mm for the bonding.

Edit: when using the armour as the CPC combined with a core also as a CPC, the combined CSA often covers the most onerous conditions.

 

[Jimmy Boy]

Cheers spark 68 I have ordered GN8 and will get my nose in it as soon as I can

ATB S

 

[ASL]

do it right follow BS7671, cover your ar*e

 

[Old School1]

Good stuff

 

[beechboy]

Must admit I fell for the contact DNO one a minute ago.
I've never understood where this 'can't export PME' stuff has come from, it's all pretty straightforward if you know your stuff.
If exporting the earth bond back to the MET, if not TT and use your MET in the DB board.
I know there can be more to it, but that's the basis.

you can export on a TNS SYSTEM IF TNCS TT ONLY OPTION