Talk:Taking electricity outside

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concrete resistance

"A concrete floor in a garage that has no extraneous metal parts in contact with earth and no other services like water pipes entering the building will offer a good isolation from earth,"

Experience with ufer earthing seems to indicate the opposite. NT 23:26, 4 June 2007 (BST)

Not been able to find much on this. Normal practice in new builds seems to suggest that a concrete floor is considered to pose no risk with a PME installation. Ufer earthing also seems to depend on quite a high rebar content in the foundation slab.

The conductivity of concrete also seems hard to pin down - but it is influenced by its moisture content. I have seen figures of 25K ohm / cm - which if true would place more than enough impedance in the earth reference to make this a non issue.

I might post another invitation to comment on the group since the first one seemed to generate somewhat less than the normal level of response.

--John Rumm 11:17, 7 June 2007 (BST)


Some comments on Ufer concrete earth resistance:

"When structural steel is bonded to the earth electrode system, the results can be quite dramatic. The author participated in one mountain-top project where the measured resistance-to-earth of a 20-rod, building earth electrode system — prior to bonding to foundation steel — was 150 ohms. After bonding foundation steel to the earth electrode system (all below grade), the resistance-to-earth was 1.0 ohm"

more here: http://www.psihq.com/iread/ufergrnd.htm NT 22:45, 7 June 2007 (BST)

I posted a question on the group on this, it will be interesting to see what is said.

A couple of points to note is that the ufer earth does require the slab to have both high moisture content and rebar. Most floor slabs will have a couple of inches of dry screed, isolated from the sub base by a DPC membrane among other things.

--John Rumm 23:45, 7 June 2007 (BST)

supply from indoor circuit

"If it already has RCD protection, then this may make it difficult to meet your usage requirements since it might then be difficult to provide a lighting system that is not vulnerable to interruption in the even of another fault (i.e. you would lose "discrimination")."

a 2 pole fcu can help with this to some degree, in that if (or when) the outdoor circuit becomes faulty, it can easily be isolated by any householder, allowing the indoor stuff to work fine. Not a good solution, but IRL people do connect outdoor lectrics to other circuits, in which case this does help NT 11:50, 13 November 2007 (GMT)

Ah, yup, I see that was not clear enough - you have actually got the wrong lighting circuit in mind! I meant the one in the outbuilding. What I was getting at is that if you feed the whole outbuilding a RCD protected supply, and then run power and lighting from it, you you have no easy way of keeping the lights on (other than battery backup) should you do something that trips the head end RCD. Your point is good practice though, in that having a way of isolating any outside stuff is a very good idea. I tweaked the words - see if that reads better. --John Rumm 15:34, 13 November 2007 (GMT)

Store the PVC SWA table for the mo. BS5467:1997 does not include adequate data

For 70° C PVC clad SWA - Steel Armour

Phase Conductor CSA (mm²) CSA of armour (mm²)

2 Core SWA

CSA of armour (mm²)

3 Core SWA

CSA of armour (mm²)

4 Core SWA

1.5 15 16 17
2.5 17 19 20
4 21 23 35
6 24 36 40
10 41 44 49
16 46 50 72
25 60 66 76

Twin and Earth wire is NOT suitable for use outside unless it is within conduit. PVC does not provide sufficient UV stability to prevent the insulation from breaking-down over a short period of time and becoming dangerous.

This is VERY bad advice and should be avoided in all circumstances except as cited above.

This recommendation is disapointing especially as the rest of the article is so good!

ECS Electrical, Essex http://www.electriciansouthend.com/


"Twin and Earth wire is NOT suitable for use outside unless it is within conduit. PVC does not provide sufficient UV stability to prevent the insulation from breaking-down over a short period of time and becoming dangerous."

There is plenty of it in use outdoors around the country, some in direct sunlight, and it simply doesnt break down over a short period. Surface damage occurs over decades, and is in no way a safety hazard.

If you think that's not so, why not come to uk.d-i-y and explain the issue. NT 21:45, 9 September 2009 (BST)


I am slightly confused by this comment anyway, since the section on T&E basically says much the same anyway - i.e. is usually used in conduit, is not UV safe etc.

I would say it is important that T&E is mentioned however since it will often from a link between CU and the exit point of a sub main from a building. Personally I would not use it outside for anything substantial, but would not object to say a couple of exposed inches running under a soffit to a bulkhead lamp or similar.

--John Rumm 21:26, 11 September 2009 (BST)