Hi,

A while back I asked for advice on a CU and circuit design. I've noe
bought the CU and have decided on the number/location of sockets,
switches etc.

I've been over both the Electricans Guide and the On-Site Guide to
BS7671 and require a sanity check on my wiring design. I'd also like
some advice on channeling, so without futher ado :-)

Wiring:

1) The ground floor has concrete floors so I'll have to do all my
cabling for the ground floor from above. Lots of channeling. My question
relates to how I route the first floor cables. Can I also route the
cable for the socket ring under the first floor floorboards and create a
smaller channel up to the socket? AIUI this seems to be acceptable as it
will be in a vertical line from under the floor to the socket itself. Am
I correct in this thinking.

2) In the living room I intend having five double sockets close together
to support my AV system. Do I have to channel and cable up and down for
each one or can I have the cable come down in a channel to the leftmost
socket, pass through to the other sockets then have the cable return up
in a channel at the rightmost socket?

Channeling:

1) I have seen a channeling chisel at Axminster tools which you can use
to create 30mm wide channels. Has any one any experience of this or
similar chisel? [http://tinyurl.com/m2c4q]

2) Axminster also do an SDS set for sinking electrical boxes for
single/double sockets. Has anybody any experience of these. I have
approx 45 double sockets to install and I get the feeling this will make
my job a lot easier. [http://tinyurl.com/o6ubr]

TIA.

--
Organoman
xj600s

On Tue, 18 Apr 2006 21:25:01 GMT, Organoman
wrote:

Wiring:

1) The ground floor has concrete floors so I'll have to do all my
cabling for the ground floor from above. Lots of channeling. My question
relates to how I route the first floor cables. Can I also route the
cable for the socket ring under the first floor floorboards and create a
smaller channel up to the socket? AIUI this seems to be acceptable as it
will be in a vertical line from under the floor to the socket itself. Am
I correct in this thinking.

Yes you are correct.

2) In the living room I intend having five double sockets close together
to support my AV system. Do I have to channel and cable up and down for
each one or can I have the cable come down in a channel to the leftmost
socket, pass through to the other sockets then have the cable return up
in a channel at the rightmost socket?

Yes, you can run the cable horizontally between sockets on the same wall
and that's exactly what I'd do for a group of very closely-positioned
sockets like that.
--
Chris Cowley

Organoman wrote:

I intend having five double sockets close together
Do I have to channel and cable up and down for
each one or can I have the cable come down in a channel to the leftmost
socket, pass through to the other sockets then have the cable return up
in a channel at the rightmost socket?

Of course you can (indeed should) do the latter, always better to ask I
suppose, but having to ask such a question doesn't bode well ...

Hi

Not an electrician but...

Organoman wrote:

Hi,

A while back I asked for advice on a CU and circuit design. I've noe
bought the CU and have decided on the number/location of sockets,
switches etc.

I've been over both the Electricans Guide and the On-Site Guide to
BS7671 and require a sanity check on my wiring design. I'd also like
some advice on channeling, so without futher ado :-)

Wiring:

1) The ground floor has concrete floors so I'll have to do all my
cabling for the ground floor from above. Lots of channeling. My question
relates to how I route the first floor cables. Can I also route the
cable for the socket ring under the first floor floorboards and create a
smaller channel up to the socket?

This is a perfectly normal way to do it. Watch the routes where you need to
go through a joist - there is a correct range of positions that you should
drill holes in the middle of a joist to avoid weakening it unnecessarily -
certain fraction of the span to another fraction, from the supported end.
But I cannot recall what the range is - someone else will be along no doubt
to answer that.

AIUI this seems to be acceptable as it
will be in a vertical line from under the floor to the socket itself. Am
I correct in this thinking.

2) In the living room I intend having five double sockets close together
to support my AV system. Do I have to channel and cable up and down for
each one or can I have the cable come down in a channel to the leftmost
socket, pass through to the other sockets then have the cable return up
in a channel at the rightmost socket?

Whitfield Electrician's Guide Ed. 7 Page 72 Fig 4.14 "Acceptable
installation zones..." explains all.

Essentially, yes - cables may be run vertically or horizontally from the
fitting, or within 150mm of an internal corner to ceiling or another wall.

Other routes require the cable to be protected, which may include using
specialist types of cable. In my unqualified opinion, you are better to go
horizontally between all sockets in a room (doors not withstanding) or you
run the risk of excessive cable length with all the ups and downs and thus
voltage drops going out of spec.

Don't take my word for it - but I think the diagram I cited is fairly clear.

If the plaster depth allows, have you considered PVC oval conduit? Cheap as
chips and you'll get the cables out again if you ever need to.

Channeling:

1) I have seen a channeling chisel at Axminster tools which you can use
to create 30mm wide channels. Has any one any experience of this or
similar chisel? [http://tinyurl.com/m2c4q]

Now that looks *very* useful. Please let us know how well it works.

2) Axminster also do an SDS set for sinking electrical boxes for
single/double sockets. Has anybody any experience of these. I have
approx 45 double sockets to install and I get the feeling this will make
my job a lot easier. [http://tinyurl.com/o6ubr]

TIA.


HTH

Tim

Organoman wrote:

A while back I asked for advice on a CU and circuit design. I've noe
bought the CU and have decided on the number/location of sockets,
switches etc.

I've been over both the Electricans Guide and the On-Site Guide to
BS7671 and require a sanity check on my wiring design. I'd also like
some advice on channeling, so without futher ado :-)

Wiring:

1) The ground floor has concrete floors so I'll have to do all my
cabling for the ground floor from above. Lots of channeling. My question
relates to how I route the first floor cables. Can I also route the
cable for the socket ring under the first floor floorboards and create a
smaller channel up to the socket? AIUI this seems to be acceptable as it
will be in a vertical line from under the floor to the socket itself. Am
I correct in this thinking.

Yes. As long as the cables are running vertically above or below the
sockets then you do not need to take any special precautions to
protect them. You could also put the ground and first floor sockets on
the same ring as this would reduce the amount of wiring you would need
to thread through the joists.

If you need more than one ring then splitting the house vertically
rather than horizontally can save a lot of extra work and materials.

2) In the living room I intend having five double sockets close together
to support my AV system. Do I have to channel and cable up and down for
each one or can I have the cable come down in a channel to the leftmost
socket, pass through to the other sockets then have the cable return up
in a channel at the rightmost socket?

It is also permitted to run cables in a horizontal line from the
socket outlets, without any special protection. So your scheme meets
with the regulations.

Channeling:

1) I have seen a channeling chisel at Axminster tools which you can use
to create 30mm wide channels. Has any one any experience of this or
similar chisel? [http://tinyurl.com/m2c4q]

Just my personal opinion, but I found a similar chisel removed rather
more than just a 30mm channel, especially where the plaster was a bit
flaky anyway. My preferred technique on long channels is to cut two
slots in the plaster with an old tenon saw, and then use a SDS drill
and conventional chisel bit to remove the plaster between them.

I may however be somewhat old-fashioned in this as it's the same
technique I used with a lump hammer and bolster chisel.

2) Axminster also do an SDS set for sinking electrical boxes for
single/double sockets. Has anybody any experience of these. I have
approx 45 double sockets to install and I get the feeling this will make
my job a lot easier. [http://tinyurl.com/o6ubr]

Again I found the one I had rather disappointing. The first few
sockets went well in good plaster over block, but after that the tool
became blunt rather quickly. It also had difficulties with hard or
uneven walls and made a LOT of dust. Perhaps in block walls, (as in
the illustration) they might last longer.

I find that a good chisel bit in a light-weight SDS drill gets the job
done just as quickly.

John
--
John White,
Electrical Contractor

In article ,
Organoman writes:
1) The ground floor has concrete floors so I'll have to do all my
cabling for the ground floor from above. Lots of channeling. My question
relates to how I route the first floor cables. Can I also route the
cable for the socket ring under the first floor floorboards and create a

Yes (need to be passed through joist centres, not immediately
under the floorboards).

smaller channel up to the socket? AIUI this seems to be acceptable as it

You lost me there.

will be in a vertical line from under the floor to the socket itself. Am
I correct in this thinking.

2) In the living room I intend having five double sockets close together
to support my AV system. Do I have to channel and cable up and down for
each one

No

or can I have the cable come down in a channel to the leftmost
socket, pass through to the other sockets then have the cable return up
in a channel at the rightmost socket?

Yes, providing they are at the same height.

Channeling:

1) I have seen a channeling chisel at Axminster tools which you can use
to create 30mm wide channels. Has any one any experience of this or
similar chisel? [http://tinyurl.com/m2c4q]

If you're doing a whole house, I would recommend buying a cheap
wall chaser instead, such as
http://www.screwfix.com/app/sfd/cat/...81200&ts=98707
although I haven't seen this particular make (I have a Ferm
one). You can adjust slot width for different trunking sizes.

2) Axminster also do an SDS set for sinking electrical boxes for
single/double sockets. Has anybody any experience of these. I have
approx 45 double sockets to install and I get the feeling this will make
my job a lot easier. [http://tinyurl.com/o6ubr]

In brick (commons), the circular cutter will do about 25 holes
before going blunt IME. The square cutter is useless -- much
easier to square up the circular hole afterwards with an SDS
chisel bit. In thermal blocks, the tool will last much longer
(and the square cutter might work -- I didn't try it).

--
Andrew Gabriel

In article ,
(Andrew Gabriel) writes:
Channeling:

If you're doing a whole house, I would recommend buying a cheap
wall chaser instead, such as
http://www.screwfix.com/app/sfd/cat/...81200&ts=98707
although I haven't seen this particular make (I have a Ferm
one). You can adjust slot width for different trunking sizes.

Just to add -- you can hire them from tool hire places too.

--
Andrew Gabriel

Organoman wrote:

1) The ground floor has concrete floors so I'll have to do all my
cabling for the ground floor from above. Lots of channeling. My question
relates to how I route the first floor cables. Can I also route the
cable for the socket ring under the first floor floorboards and create a
smaller channel up to the socket? AIUI this seems to be acceptable as it
will be in a vertical line from under the floor to the socket itself. Am
I correct in this thinking.

Yup.

2) In the living room I intend having five double sockets close together
to support my AV system. Do I have to channel and cable up and down for
each one or can I have the cable come down in a channel to the leftmost
socket, pass through to the other sockets then have the cable return up
in a channel at the rightmost socket?

Yup, cables can be unprotected in plaster if the are in a vertical or
horizontal line with a visible accessory. Note that this also extends to
the reverse of a wall if it is easy to see if from the location (i.e. by
a doorway). You can also use the 6" margin at the corners and tops of
walls. Anywhere else and the cable needs to be buried more than 2" deep
or protected by mechanical means.

Channeling:

1) I have seen a channeling chisel at Axminster tools which you can use
to create 30mm wide channels. Has any one any experience of this or
similar chisel? [http://tinyurl.com/m2c4q]

I have one of these, and to be fair it is handy in some circumstances.
On modern plaster they will cut a nice neat channel. On older flaky
stuff they will tend to pull off large areas of skim in the area of the
cut. They are quite handy for getting behind smaller covings and
skirtings however. (an alternative can be a very long drill bit user
almost parallel to the wall into the chase behind the obstruction).

For older walls I tend to simply use a 40mm wide chisel on the SDS to
cut a couple of parallel slots and break out the middle. Make sure you
have a drill where you can lock the chisel bit into a fixed position
though else you will have a hard time cutting a straight line!

If you go for a wall chaser then make sure it has *very* good dust
extraction. Don't even dream of using an angle grinder!

2) Axminster also do an SDS set for sinking electrical boxes for
single/double sockets. Has anybody any experience of these. I have
approx 45 double sockets to install and I get the feeling this will make
my job a lot easier. [http://tinyurl.com/o6ubr]

Not used these, however I find I can chop out a double box with a plain
chisel bit in an SDS in about 5 to 10 mins anyway. (offer box to wall,
draw round it, chisel a little inside the line all round the perimeter
(mark the depth of the box on the chisel bit!), and finally chop out the
middle with a few oblique cuts with the chisel.

--
Cheers,

John.

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

Organoman wrote:

2) In the living room I intend having five double sockets close together
to support my AV system. Do I have to channel and cable up and down for
each one or can I have the cable come down in a channel to the leftmost
socket, pass through to the other sockets then have the cable return up
in a channel at the rightmost socket?

Assuming your sockets are in a horizontal row, channel down to the left
one, wire horzontally to each, return the wire to the left inside the
pattreses, then route it back up the original channel. No sense making
2 channels.

While youre doing it, good opportunity to put in cat 5 and coax. Cat5
has a slew of uses, and costs s all.


NT

In message , John
Rumm writes
Organoman wrote:

1) The ground floor has concrete floors so I'll have to do all my
cabling for the ground floor from above. Lots of channeling.


Channeling:
1) I have seen a channeling chisel at Axminster tools which you can
use to create 30mm wide channels. Has any one any experience of this
or similar chisel? [http://tinyurl.com/m2c4q]


For older walls I tend to simply use a 40mm wide chisel on the SDS to
cut a couple of parallel slots and break out the middle.

In our old house (1930's semi) the skim layer was relatively hard but he
underlaying plaster would come away from the wall quite easily, taking
even a hammer and chisel to it would have great chunks falling off.

For their the trick was to score through the plaster at the sides of the
channel with a Stanley knife, then you could remove most of the plaster
v. easily with a chisel
--
Chris French

John Rumm wrote:
For older walls I tend to simply use a 40mm wide chisel on the SDS to
cut a couple of parallel slots and break out the middle. Make sure you
have a drill where you can lock the chisel bit into a fixed position
though else you will have a hard time cutting a straight line!

If you go for a wall chaser then make sure it has *very* good dust
extraction. Don't even dream of using an angle grinder!


OTOH, if the area is already in a mess, two narrow slits with a thin
disc won't add much.

For minimum dust, score two parallel lines through the skim coat, and
then gently score/cut through the plaster by hand using a large hacksaw
blade. Once you're down to brick, you can prise most of the plaster
strip away in clean chunks.


--
Ian White

Andy Burns wrote:



Of course you can (indeed should) do the latter, always better to ask I
suppose, but having to ask such a question doesn't bode well ...

Why? Everybody has to start somewhere. Surely it's better to ask a silly
question than to plough ahead and make a mistake. I figured it would be
sensible to route them as I suggested but I was a bit worried about
possibly overloading the cable.

Cheers.

--
Organoman
xj600s

wrote:



Assuming your sockets are in a horizontal row, channel down to the left
one, wire horzontally to each, return the wire to the left inside the
pattreses, then route it back up the original channel. No sense making
2 channels.

That sounds like a much better idea :-)

While youre doing it, good opportunity to put in cat 5 and coax. Cat5
has a slew of uses, and costs s all.


NT


That was the plan, though I'm a bit concerned about having power,
network and television cables all bundled together. Won't there be the
capacity for lots of interference?

Thanks.

--
Organoman
xj600s

Thanks for all your advice. Especially regards channeling. I'll try some
of the suggestions before forking out on the box cutter I think.

Thanks again.

--
Organoman
xj600s

Organoman wrote:



That was the plan, though I'm a bit concerned about having power,
network and television cables all bundled together. Won't there be the
capacity for lots of interference?

Thanks.


They shouldn't be bundled together unless the data cables (etc) are rated
to mains voltage too - just run a separate conduit or chase down for aerial,
data, phones etc.

The interference may be a problem on the phones, but CAT5(e) carrying
ethernet packets will generally be OK.

Cheers

Tim

Organoman wrote:

wrote:



Assuming your sockets are in a horizontal row, channel down to the left
one, wire horzontally to each, return the wire to the left inside the
pattreses, then route it back up the original channel. No sense making
2 channels.


That sounds like a much better idea :-)

The only downside being that you then have to wire the return cable from
the last socket back through all of the other socket back boxes. This
can make it harder to fit the face plates of the sockets and makes the
wiring take longer. So you may not gain any time doing it this way and
you make adding further wires in future harder.

While youre doing it, good opportunity to put in cat 5 and coax. Cat5
has a slew of uses, and costs s all.

Only keep em away from the mains (unless you like hum)!

That was the plan, though I'm a bit concerned about having power,
network and television cables all bundled together. Won't there be the
capacity for lots of interference?

Yup, hence why 50mm separation between services is recommended.

--
Cheers,

John.

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

John Rumm wrote:
Organoman wrote:
wrote:

Assuming your sockets are in a horizontal row, channel down to the left
one, wire horzontally to each, return the wire to the left inside the
pattreses, then route it back up the original channel. No sense making
2 channels.

That sounds like a much better idea :-)

The only downside being that you then have to wire the return cable from
the last socket back through all of the other socket back boxes. This
can make it harder to fit the face plates of the sockets and makes the
wiring take longer. So you may not gain any time doing it this way and
you make adding further wires in future harder.

You can strip off the grey cover while its inside the pattresses if
space gets tight. I've not found it a problem myself, I'd sooner do
that than cut and plaster another channel, but I may be biased.


While youre doing it, good opportunity to put in cat 5 and coax. Cat5
has a slew of uses, and costs s all.

Only keep em away from the mains (unless you like hum)!

That was the plan, though I'm a bit concerned about having power,
network and television cables all bundled together. Won't there be the
capacity for lots of interference?

Yup, hence why 50mm separation between services is recommended.


Cat 5 is twisted pair and uses differential input, so hum is a
non-issue there. TV coax is screened plus the equipment is all C
coupled and wouldnt notice any hum as its way out of band, less than a
millionth the transmission frequency. Even if you connected mains
direct to coax (dont of course) the tvs would still work fine. This has
unintentionally happened before.

For possible as yet unknown future apps, we dont know what is and isnt
hum sensitive. However if you've got the channels it makes a lot more
sense to put wire in than not. Most apps are fine like this, including
audio distribution, there are some that arent, but future apps designed
to use round the house wiring are most likely to be humproofed.


NT

Organoman wrote:

Why? Everybody has to start somewhere. Surely it's better to ask a silly
question than to plough ahead and make a mistake.

Agreed, I did say better to ask, but the question made me wonder if a
re-wire was your best starting point that's all

I figured it would be
sensible to route them as I suggested but I was a bit worried about
possibly overloading the cable.

How would the cable be any less loaded by taking multiple trips back up
to the ceiling, topologically it's still a ring ...

wrote:

The only downside being that you then have to wire the return cable from
the last socket back through all of the other socket back boxes. This
can make it harder to fit the face plates of the sockets and makes the
wiring take longer. So you may not gain any time doing it this way and
you make adding further wires in future harder.


You can strip off the grey cover while its inside the pattresses if
space gets tight. I've not found it a problem myself, I'd sooner do
that than cut and plaster another channel, but I may be biased.

Fair point. If I was chasing up to a socket from floor level I would
probably go with the extra chase, however down from a ceiling to socket
level might encourage me to re-use the chase.

Only keep em away from the mains (unless you like hum)!

That was the plan, though I'm a bit concerned about having power,

network and television cables all bundled together. Won't there be the
capacity for lots of interference?

Yup, hence why 50mm separation between services is recommended.



Cat 5 is twisted pair and uses differential input, so hum is a
non-issue there. TV coax is screened plus the equipment is all C

Except when using cat5 for distribution of telephone signals - the input
is not differential, and worse, not even balanced (only requiring one
and a half pairs per line)

coupled and wouldnt notice any hum as its way out of band, less than a
millionth the transmission frequency. Even if you connected mains
direct to coax (dont of course) the tvs would still work fine. This has
unintentionally happened before.

Hum is not the only issue either - capacitive coupling over long runs
adjacent to mains wiring can also cause problems with it driving input
voltages out of spec for some applications.

For possible as yet unknown future apps, we dont know what is and isnt
hum sensitive. However if you've got the channels it makes a lot more
sense to put wire in than not. Most apps are fine like this, including
audio distribution, there are some that arent, but future apps designed
to use round the house wiring are most likely to be humproofed.

There are a bunch of regs that apply here, again hum is not always the
main issue (although BS EN 50081 and 50082 do concern electromagnetic
adn electrostatic issues with regard to telecoms circuits). There are
also requirements for segregation of high and low voltage circuits (or
provision of adequate earthed screening. See 528-01-01, and 02

--
Cheers,

John.

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

John Rumm wrote:
wrote:

Cat 5 is twisted pair and uses differential input, so hum is a
non-issue there. TV coax is screened plus the equipment is all C

Except when using cat5 for distribution of telephone signals - the input
is not differential, and worse, not even balanced (only requiring one
and a half pairs per line)

yes. OTOH phone signals travel on untwisted pairs for far greater
distances than around the house, and seem quite happy. Maybe 600 ohms
is low enough to avoid the problem.


coupled and wouldnt notice any hum as its way out of band, less than a
millionth the transmission frequency. Even if you connected mains
direct to coax (dont of course) the tvs would still work fine. This has
unintentionally happened before.

Hum is not the only issue either - capacitive coupling over long runs
adjacent to mains wiring can also cause problems with it driving input
voltages out of spec for some applications.

yes, though not any app thats designed for this. Low impedance is all
thats needed to vanish the hum.

As an example I sent audio at 1v level around a building on untwisted
unscreened 2 core, no differential inputs, simply relying on low source
impedance to kill the hum (apx 1 ohm). It worked perfectly, not a trace
of hum.


For possible as yet unknown future apps, we dont know what is and isnt
hum sensitive. However if you've got the channels it makes a lot more
sense to put wire in than not. Most apps are fine like this, including
audio distribution, there are some that arent, but future apps designed
to use round the house wiring are most likely to be humproofed.

There are a bunch of regs that apply here, again hum is not always the
main issue (although BS EN 50081 and 50082 do concern electromagnetic
adn electrostatic issues with regard to telecoms circuits). There are
also requirements for segregation of high and low voltage circuits (or
provision of adequate earthed screening. See 528-01-01, and 02

its kind of odd this one. Even though you've got 4 layers of insulation
between mains and LV ccts, 2 of which withstand over 1kV apiece, and
the other 2 of which will withstand mains if pushed to it, its still
not accepted. I understand the idea is that if the 2 services are
terminated in one box theres plenty of scope for safety problems, but
when they use different termination boxes, there really isnt. Triple
insulation is more than good enough.

Even in case of massive overload and melting (which again has at least
2 layers of protection to prevent that) the plaster over the wires keep
the 2 apart. So no less than 3 layers of protection there as well. You
can run mains wires over all the unearthed metalwork you like, but not
over LV circuits. I have to question it.


NT

Andy Burns wrote:



How would the cable be any less loaded by taking multiple trips back up
to the ceiling, topologically it's still a ring ...

*shrug* That's why I asked :-) I think I was more concerned about any
heat generated. I just want to get it as right as I can.

--
Organoman
xj600s

Organoman wrote:

*shrug* That's why I asked :-)

To be clear, my original comment was along the lines of "if you have to
ask *that* there could be a lot else you don't even realise you need to
ask/do/avoid"

wrote:

yes. OTOH phone signals travel on untwisted pairs for far greater
distances than around the house

I know the old "stiff type" overhead drop wire isn't twisted pair, but a
lot of the run is, mine is underground wired which is, and BT's cable
from where the underground wire is routed to the master socket is too.

With ADSL this is getting more important within the house, a friend who
works for BT (actually OpenReach) often complains about in-house and
underground cables installed by builders which use alarm cable instead
of phone cable.

Andy Burns wrote:

Organoman wrote:

*shrug* That's why I asked :-)

To be clear, my original comment was along the lines of "if you have to
ask *that* there could be a lot else you don't even realise you need to
ask/do/avoid"

He won't have an overloading problem but he *might* have an impedance to
high to guarantee tripping times at the MCB problem,
with all that cable going in.

Cheers

Tim

Andy Burns wrote:
Organoman wrote:

*shrug* That's why I asked :-)


To be clear, my original comment was along the lines of "if you have to
ask *that* there could be a lot else you don't even realise you need to
ask/do/avoid"

Aah, right. With you now :-)

--

Organoman
xj600s

Tim S wrote:

He won't have an overloading problem but he *might* have an impedance to
high to guarantee tripping times at the MCB problem,
with all that cable going in.

Assuming 2.5 mm^2 T&E cable and that overcurrent protection is by means
of a 32A Type B MCB, the maximum cable length for a conventional ring
circuit is 84 metres, measured as the 'circumference' of the ring. This
may need to be reduced if there are any unfused spurs taken off near the
middle of the ring.

The above limit is set by voltage drop considerations, not earth fault
loop impedance. Max lengths for other circumstances are to be found in
Table 7.1 of the OSG (p.42).

--
Andy