While titting about with my wiki article on outside electrics:

http://wiki.diyfaq.org.uk/index.php?...ricity_outside

NT raised a question about the comment I made on the into section for
selection of appropriate equipment for outbuildings, and about choice of
earthing system.

I said:

"Know your environment

To make a sensible selection we need knowledge of the local environment.
The phrase "outbuilding" can encompass anything from a damp draughty
shed, to something as civilised as your front room.

Hence establish:

* How damp is the interior likely to get?
* Is it easy to make a good contact with earth? (e.g. 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, whereas a greenhouse sat on damp
ground would offer none. "

NT raised the question of ufer earthing (i.e. connecting the rebar in a
reinforced concrete slabs, and using it as an earth), and is it really
true that a concrete slab will offer good isolation from ground?

http://www.psihq.com/iread/ufergrnd.htm

ISTM that concrete slabs in new builds, and PME supplies are quite
common. Also ufer seems to require the concrete to absorb water -
something a floor screed at least (being above a DPC etc), is unlikely
to do.

Anyone have a take on this?



--
Cheers,

John.

/================================================== ===============\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/

On 7 Jun, 23:42, John Rumm wrote:
While titting about with my wiki article on outside electrics:

http://wiki.diyfaq.org.uk/index.php?...ricity_outside

NT raised a question about the comment I made on the into section for
selection of appropriate equipment for outbuildings, and about choice of
earthing system.

I said:

"Know your environment

To make a sensible selection we need knowledge of the local environment.
The phrase "outbuilding" can encompass anything from a damp draughty
shed, to something as civilised as your front room.

Hence establish:

* How damp is the interior likely to get?
* Is it easy to make a good contact with earth? (e.g. 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, whereas a greenhouse sat on damp
ground would offer none. "

NT raised the question of ufer earthing (i.e. connecting the rebar in a
reinforced concrete slabs, and using it as an earth), and is it really
true that a concrete slab will offer good isolation from ground?

http://www.psihq.com/iread/ufergrnd.htm

ISTM that concrete slabs in new builds, and PME supplies are quite
common. Also ufer seems to require the concrete to absorb water -
something a floor screed at least (being above a DPC etc), is unlikely
to do.

Anyone have a take on this?


AIUI Ufer earthing produces very low earth R for 3 reasons:
concrete slabs absorb & store water, staying wet
it provides ions in solution
it chemically dopes the soil around it, enhancing conduction further

So R of floor surface to ground is going to depend entirely on water
content of the slab. If you have a working dpc and no leaks, then R
should be high. But that is by no means always so with house crete
floors, thre are still lots about that harbour some damp and thus
conduct electrically.


NT