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Topic Title: Outside lights on PME?
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Created On: 06 March 2013 08:15 AM
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 06 March 2013 08:15 AM
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misterben

Posts: 416
Joined: 11 June 2007

Hi all,

I know the reasoning for having external supplies i.e a detached garage on its own earth when supplied from a PME system, however is it necessary to put garden lighting on a similar system?
I think it should be, but have seen installs that export the PME to outside lighting scattered around gardens.
What do people think?

regards
Misterben
 06 March 2013 09:13 AM
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Parsley

Posts: 1040
Joined: 04 November 2004

There's no requirement in 7671 to TT the garden lighting but you should consider the risks of a lost neutral on the DNO's supply and perceived electric shock as the kids etc in the garden may be bare footed and in contact with true earth.

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 06 March 2013 09:24 AM
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AJJewsbury

Posts: 11587
Joined: 13 August 2003

My preference is not to use PME earth for class 1 equipment outside, especially if it's likely to be touched/handled. That said, a class 1 outside light, high up on the side of a house, is probably pretty low risk. Even one within reach of someone stood on the ground is unlikely to be actually touched 99.99% of the time. Probably less a of a problem in reality than the outside tap connected by copper pipe.

BS 7671 doesn't have any prohibition, although for myself, my refurb - as there'll be quite a bit of outdoor electrics, including sockets that'll undoubtedly be used for class 1 equipment outdoors, is getting a separate insulated CU with a TT earth for the outdoor circuits.

- Andy.
 06 March 2013 10:04 AM
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Parsley

Posts: 1040
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Yes Andy, the outside tap and the bonded copper pipe, I bet there's a lot of properties that were converted to PME and an insulating insert wasn't fitted.

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 06 March 2013 10:18 AM
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AJJewsbury

Posts: 11587
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Humm. I wonder what's generally meant by an "insulating insert" - in my head I imagined something similar to a couple of inches of plastic pipe or one of those plastic push fittings.

Going by the figures in http://www.plasticpipesgroup.com/pdfs/earthbonding.pdf - i.e. the water in 1m of 15mm plastic pipe has a resistance of 115k Ohms, then 50mm might yield about 5.75k Ohms - which seems a bit low when we're usually looking for around 22k Ohms to protect against dangerous shocks (10mA). Around 200mm of 15mm plastic pipe would seem to be a minimum.

- Andy.
 06 March 2013 10:48 AM
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Parsley

Posts: 1040
Joined: 04 November 2004

Originally posted by: AJJewsbury

Humm. I wonder what's generally meant by an "insulating insert" - in my head I imagined something similar to a couple of inches of plastic pipe or one of those plastic push fittings.



Going by the figures in http://www.plasticpipesgroup.com/pdfs/earthbonding.pdf - i.e. the water in 1m of 15mm plastic pipe has a resistance of 115k Ohms, then 50mm might yield about 5.75k Ohms - which seems a bit low when we're usually looking for around 22k Ohms to protect against dangerous shocks (10mA). Around 200mm of 15mm plastic pipe would seem to be a minimum.



- Andy.


Yes Andy that seems about right.
I wonder what would be classed as an insert, it also states plumbed in plastic though.

From GN5

Under an open circuit supply neutral condition, the potential of an outside water tap will rise above Earth potential. A person coming into contact with the tap could receive an electric shock, which could be severe if that person were barefooted. The probability
of these two conditions occurring together is considered to be small so that the use of PME where a metal outside tap exists is not precluded.
It is recommended, however, that a plastic insert be provided in the pipe to the outside water tap or the tap be plumbed with plastic pipe.


Regards
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