Earthing requirements for a field?

[HappyHippyDad]

I am soon to install some weatherproof sockets and a commando socket in a field. This will be around 80m from an existing consumer unit in a barn. Earthing is PME.

The commando socket will be for a cooler. The weatherproof socket(s) will be to plug in heating mats and cables (for soil). These will be inside a polytunnel.

I was just wondering if there is any reason why I should not use the PME? Section 705 mentions not using PME in relation to livestock, but it doesn't relate this to horticultural premises. Just wondered what your thoughts were?

Cheers

 

[buzzlightyear]

it states unless a metel grid is laid in to the floor the use of a pme earthing is not recommended .neural lost dead cow ,steak any body .

 

[buzzlightyear]

This will be around 80m from an existing consumer unit in a barn. Earthing is PME.

and what kind of a barn is it .

 

[ImpededLoop]

There's several things to consider and the simple answer is to TT the external supplies.

Firstly, is any part of the installation in the field likely to be extraneous? Perhaps the chiller will have metal legs onto the soil or the heat mats may have exposed conductive parts in contact with the soil? If you use the TNCS you'd need a suitable bonding conductor back to the MET.

Secondly, the TNCS cpc may not have the same potential as true earth so there's a risk of tingles for those people stood in a wet field who touch the exposed conductive parts of the chiller etc.

Finally, in the unlikely event of a lost PEN conductor, the exposed conductive parts would have an increased potential vs the people standing on the soil.

If you go TT, you'd be likely to need a submain out to the field and then a small DB in some form of cabinet where you can devorce the TNCS earth and go TT. This would also give some flexibility for alterations or additions in the future.

 

[HappyHippyDad]

There's several things to consider and the simple answer is to TT the external supplies.

Firstly, is any part of the installation in the field likely to be extraneous? Perhaps the chiller will have metal legs onto the soil or the heat mats may have exposed conductive parts in contact with the soil? If you use the TNCS you'd need a suitable bonding conductor back to the MET.

Secondly, the TNCS cpc may not have the same potential as true earth so there's a risk of tingles for those people stood in a wet field who touch the exposed conductive parts of the chiller etc.

Finally, in the unlikely event of a lost PEN conductor, the exposed conductive parts would have an increased potential vs the people standing on the soil.

If you go TT, you'd be likely to need a submain out to the field and then a small DB in some form of cabinet where you can devorce the TNCS earth and go TT. This would also give some flexibility for alterations or additions in the future.

Thanks for this post. I had thought of all of those points apart from the first one which is probably the most important as it's the most likely to occur! So thankyou
The simple option does seem to be TT.

Just thinking further. If I TT it, then what happens to any extraneous conductive parts then? In a house if it is TT, we still have main bonding to extraneous. Would I still have to bond extraneous in this case? I think this is a silly question and the answer is 'yes', as under fault conditions there will be a potential between exposed conductive parts and extraneous? It's the fact it's in a field, it's throwing me a little. it's outside of my happy domestic comfort zone.

 

[buzzlightyear]

simple option does seem to be TT.

There's your option.

 

[buzzlightyear]

Just thinking further. If I TT it, then what happens to any extraneous conductive parts then? In a house if it is TT, we still have main bonding to

Then you would use the SWA earthing back to the scouse.wheres pc 99 with his diagrams.

 

[DPG]

Then you would use the SWA earthing back to the scouse.wheres pc 99 with is diagrams.

Always make sure to bond your scouse.

 

[ImpededLoop]

Firstly, is any part of the installation in the field likely to be extraneous? Perhaps the chiller will have metal legs onto the soil or the heat mats may have exposed conductive parts in contact with the soil? If you use the TNCS you'd need a suitable bonding conductor back to the MET.

Actually, just read GN8 and parts of the electrical installation aren't considered to be extraneous, so please ignore that paragraph. Everyday's a school day!

 

[HappyHippyDad]

Actually, just read GN8 and parts of the electrical installation aren't considered to be extraneous, so please ignore that paragraph. Everyday's a school day!

Can you expand a little ImpededLoop? "parts of the electrical installation aren't considered to be extraneous"? What does 'part' mean? The coolers legs stuck in the ground?
ps.. where abouts in GN8 are you reading? I'll have a look.
pps.. I've had a look, I can't find it.

 

[ImpededLoop]

Part 2 of 7671 page 30.

Section 6.1 in GN8 (it's a 17th edition version though...)

 

[HappyHippyDad]

Part 2 of 7671 page 30.

Section 6.1 in GN8 (it's a 17th edition version though...)

Same as mine then
I see, you are just talking about the definition of an Ext CP.
I think a cooler with metal legs in the ground would be be classed as extraneous and need bonding (assuming it tested <23K).

 

[westward10]

Metalwork of electrical appliances are exposed conductive parts and are earthed to the electrical installation they are not extraneous.

 

[HappyHippyDad]

Metalwork of electrical appliances are exposed conductive parts and are earthed to the electrical installation they are not extraneous.

Thanks Westward.
edit.. even though I have time to edit my clearly wrong previous post, I shall leave it there to highlight my ignorance.