A good guide For 'exported PME' and outbuildings!

[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

 

[malcolmsanford]

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

Surreal man ........................right on

 

[IQ Electrical]

Surreal man ........................right on

Why do I bother?

 

[swifts]

The last guidance on high protective conductor current was published in 2005 and even Guidance Note 8 has still not been revised to reflect 17th edition changes.
​

 

[Engineer54]

This really needs to go back in the ''Sticky section'' above!! It seems that there are still far too many here, that have been brainwashed by the scaremongers into believing you can't extend a PME equipotential zone, or take a PME CPC with you to an outbuilding or shed!!

 

[Guitarist]

The last guidance on high protective conductor current was published in 2005 and even Guidance Note 8 has still not been revised to reflect 17th edition changes.
​

There is a new edition out. Check Amazon mate.

 

[sparkess]

Hi, great thread - quick question... I'm installing power and light to a workroom in the back of a detached barn. Supply is off a spare way in the house and will feed a small consumer unit in the barn. The barn cu will have 30ma RCDs so my question is: Am I right to use a 100ma RCBO at the house so the whole thing doesn't automatically trip?

 

[malcolmsanford]

You can do that yes ..........................

But you may not need to, it will depend on what your earthing system is Ie TNC-S TN-S or TT and what your Zs will be for the distribution circuit

 

[Guitarist]

Hi, great thread - quick question... I'm installing power and light to a workroom in the back of a detached barn. Supply is off a spare way in the house and will feed a small consumer unit in the barn. The barn cu will have 30ma RCDs so my question is: Am I right to use a 100ma RCBO at the house so the whole thing doesn't automatically trip?

Also, there is a strong possibility that the 100mA at the house will trip at the same time as to 30mA in the barn. You will need a time-delayed (s-type) RCD in the house if you want to provide discrimination whilst protecting the supply cable. Not sure whether you are an electrician or not, so just to let you know, this job would be notifiable to your LBC.

 

[SW1970]

And what cable type and which installation method.

 

[IQ Electrical]

As Guitarist said, the 100mA device will almost certainly trip simultaneously with a 30mA device downstream-does this 'supply cable' actually REQUIRE RCD protection?