Hi,

I've been discussing this on the IET forum and offline too. This is
complicated - if anyone has a thought but doesn't want to commit to it in
public, feel free to email me (address is valid).



I'm partly posting this here as the engineers over at the IET have been very
helpful, but are good at pointing out all sorts of interesting snags. They
did produce some interesting guidance but not really a definative answer.

So I'm here for a fresh dose of common sense and perhaps someone's done this
before...

As previously mentioned, I need my consumer unit 14m away from the
supply+meter. Moving the meter is out of the question.

------
So, my proposal has always been, which seems pretty standard is to:

Run 25mm2 3 core armoured cable from near the meter to new CU. The third
core will be parallelled with the armour to function as the main earth
(circuit CPC *and* earthing conductor). My calculations for disconnection
times, R1+R2 and volt drops look perfectly OK, relative to the worst case
final circuits.


In line with EDFs rules, I must provide overcurrent protection and means of
isolation with 3m of the meter, so I was going to use a 100A DP switch
fuse. Seems normal so far.

-----

However, some of the folk at the IET pointed out some theoretical snags,
including:

Where does the MET (main earth terminal) and main bonding go?: next to the
meter or next to the CU at the other end. Conclusion was the MET is better
next to the CU, as during a fault on the distribution circuit, voltages on
the "earth" conductors all round the house may easily hit 100+V for upto 5
seconds, so it's better the water pipes etc come up with them in the spirit
of the equipotential zone that the house is supposed to be.


However, that leaves the reverse problem at the other end, eg big fault,
pipes go up to 100+V, exposed conductive parts like the switch-fuse case
don't. Ditto pipes outside and in shower see the 100+V potential difference
with respect to the floor slab.


Having read the whole of chapter 54 in the IEE Wiring Regs (17th) (twice!)
and 2 guide books (ECA guide and NICEIC Earthing Snags) and been all over
google, it appears that there's nothing wrong with my cable - it's CPC
oversized if anything.

But there's nothing prescriptive about multiple main bonding.

*** So should I bond gas and water to the means of earth back at the meter
as well, directly to EDF's earth terminal? BTW - this is a TN-S setup, Ze
of 0.19 Ohms

I came up with another way round: put a Type S 100mA DP RCCD in place of the
main switch in the switch fuse enclosure, and thus reduce the disconnection
time of the distribution circuit to 0.15s which avoids all these issues.
Unconventional with a TN-S system, but harmless at worst...

*** Overkill? Or not?

Random thoughts gratefully received...

Cheers

Tim

"Tim S" wrote in message
...
Hi,

I've been discussing this on the IET forum and offline too. This is
complicated - if anyone has a thought but doesn't want to commit to it in
public, feel free to email me (address is valid).



I'm partly posting this here as the engineers over at the IET have been
very
helpful, but are good at pointing out all sorts of interesting snags. They
did produce some interesting guidance but not really a definative answer.

So I'm here for a fresh dose of common sense and perhaps someone's done
this
before...

As previously mentioned, I need my consumer unit 14m away from the
supply+meter. Moving the meter is out of the question.

------
So, my proposal has always been, which seems pretty standard is to:

Run 25mm2 3 core armoured cable from near the meter to new CU. The third
core will be parallelled with the armour to function as the main earth
(circuit CPC *and* earthing conductor). My calculations for disconnection
times, R1+R2 and volt drops look perfectly OK, relative to the worst case
final circuits.


In line with EDFs rules, I must provide overcurrent protection and means
of
isolation with 3m of the meter, so I was going to use a 100A DP switch
fuse. Seems normal so far.



What type of installation is currently at the supply head? Is there a
separate earth already there? What type of protection has the supplier
provided in the form of switching?



However, some of the folk at the IET pointed out some theoretical snags,
including:

Where does the MET (main earth terminal) and main bonding go?: next to the
meter or next to the CU at the other end. Conclusion was the MET is better
next to the CU, as during a fault on the distribution circuit, voltages on
the "earth" conductors all round the house may easily hit 100+V for upto 5
seconds, so it's better the water pipes etc come up with them in the
spirit
of the equipotential zone that the house is supposed to be.



You are best to make the consumer unit your final circuits safety bonding
and keep it separate from the your new supply arrangement. That can look
after itself if done properly. Linking to the earth protection should be
enough.



However, that leaves the reverse problem at the other end, eg big fault,
pipes go up to 100+V, exposed conductive parts like the switch-fuse case
don't. Ditto pipes outside and in shower see the 100+V potential
difference
with respect to the floor slab.



That's why I would say that the consumer unit should be the protection for
the final installation. A simple link to the new supply should take of any
fault between the CU and the supply head.



Having read the whole of chapter 54 in the IEE Wiring Regs (17th) (twice!)
and 2 guide books (ECA guide and NICEIC Earthing Snags) and been all over
google, it appears that there's nothing wrong with my cable - it's CPC
oversized if anything.

But there's nothing prescriptive about multiple main bonding.

*** So should I bond gas and water to the means of earth back at the meter
as well, directly to EDF's earth terminal? BTW - this is a TN-S setup, Ze
of 0.19 Ohms

I came up with another way round: put a Type S 100mA DP RCCD in place of
the
main switch in the switch fuse enclosure, and thus reduce the
disconnection
time of the distribution circuit to 0.15s which avoids all these issues.
Unconventional with a TN-S system, but harmless at worst...

*** Overkill? Or not?

Random thoughts gratefully received...

Cheers

Tim


You are thinking over kill on this, in my opinion. You new supply should be
just that. All you are really doing is extending the supply to a new
position closer to where you want it.

From the supply head, install a 100amp RCD DP Switch after the meter. Bring
your SWA to the point where you want it. Terminate in an appropriate box to
allow new tails to your consumer unit. The new supply is then protected by
its own RCD back at the original supply head. If a fault occurs along it,
the RCD is there to break it.

I'd also go for a 2 core SWA and use the braid to supply the safety earth.
No need to extend the earth bonding back to the meter. If you have an
earthing point at the supply head, use it to bond with the SWA and maybe a
link from the new CU, but then a fault in your final circuits may also cause
the RCD at the supply to trip out.

Your final installation will be bonded to create equalisation across the
full final circuits. Your protection is supplied by what you want to put in
the consumer unit. With some current installations, that is the only
protection that is provided, so yours is really no different.

I think you are trying to over engineer it all. When all you really want is
to extend the supply to your new CU position. Once you have provided
protection for the supply cable, the rest looks after itself at the new
consumer unit. Bonding them together may actually give you more problems.

You could also ask the supplier to reinforce your supply directly to the new
position for you. Once you've made your installation complete, with just a
temporary supply to keep you working.

Good luck with it.

Tim S wrote:

So, my proposal has always been, which seems pretty standard is to:

Run 25mm2 3 core armoured cable from near the meter to new CU. The third
core will be parallelled with the armour to function as the main earth
(circuit CPC *and* earthing conductor). My calculations for disconnection
times, R1+R2 and volt drops look perfectly OK, relative to the worst case
final circuits.

Yup

In line with EDFs rules, I must provide overcurrent protection and means of
isolation with 3m of the meter, so I was going to use a 100A DP switch
fuse. Seems normal so far.

Yup

-----

However, some of the folk at the IET pointed out some theoretical snags,
including:

Where does the MET (main earth terminal) and main bonding go?: next to the
meter or next to the CU at the other end. Conclusion was the MET is better
next to the CU, as during a fault on the distribution circuit, voltages on
the "earth" conductors all round the house may easily hit 100+V for upto 5
seconds, so it's better the water pipes etc come up with them in the spirit
of the equipotential zone that the house is supposed to be.

At (or for that matter in) the main CU seems sensible to me.

However, that leaves the reverse problem at the other end, eg big fault,
pipes go up to 100+V, exposed conductive parts like the switch-fuse case
don't. Ditto pipes outside and in shower see the 100+V potential difference
with respect to the floor slab.

Maybe I am missing something, but I am not sure I follow what the
problem is...

Your switch fuse and CU will be linked by something like 52mm^2 of
earthing conductor[1] so any voltage rise on the CU's earthing terminal
is going to be reflected on the casework of the switchfuse pretty
closely. (I guestimate somewhere just north of 5V for a fault current of
1kA)


[1] (assuming XLPE 3 core with armour area alone of 62mm^2 giving a
copper equivalent of that over 2.255 or 27.5mm^2 before you add the
25mm^2 of the core)

*** So should I bond gas and water to the means of earth back at the meter
as well, directly to EDF's earth terminal? BTW - this is a TN-S setup, Ze
of 0.19 Ohms

Can't see the need. There are quite a number of mitigating factors here
as I see it. Firstly the potential difference (in both senses) between
CU and EDF's terminal is small. The chances of being in contact with the
metalwork of the switchfuse while a fault is present is small. The
chances of any fault taking even a second to clear on a 17th edition
type install that is festooned with RCDs is pretty slim anyway, let
alone 5 secs.

I came up with another way round: put a Type S 100mA DP RCCD in place of the
main switch in the switch fuse enclosure, and thus reduce the disconnection
time of the distribution circuit to 0.15s which avoids all these issues.
Unconventional with a TN-S system, but harmless at worst...

*** Overkill? Or not?

Probably is overkill in this circumstance.

Random thoughts gratefully received...

Random, and worth what you paid for em ;-)

--
Cheers,

John.

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

Hi,

Thanks for taking the time to post

BigWallop coughed up some electrons that declared:


"Tim S" wrote in message
...
Hi,
snip

What type of installation is currently at the supply head? Is there a
separate earth already there? What type of protection has the supplier
provided in the form of switching?

It's a single phase 100A supply with a BS1361 cutout, TN-S 16mm2 earth,
Ze=0.19 (by my measurement) and well PME'd in the network (TN-C-S is an
optional upgrade in my area). No user switching available before current
CU - isolation by removal of cutout fuse only. I'm going to slip a Wylex DP
switch (REC type) into the meter cabinet when I book the cutout fuse to be
pulled as EDF/SEEBOARD don't offer the option of an isolator (I've asked).
Hopefully the bloke won't mind if I ask nicely - that'll avoid future
nonsense.

snip

You are best to make the consumer unit your final circuits safety bonding
and keep it separate from the your new supply arrangement. That can look
after itself if done properly. Linking to the earth protection should be
enough.

Good - I'm glad you agree - I feel comfortable with that as it keeps the
equipotential zone inside the house good.



However, that leaves the reverse problem at the other end, eg big fault,
pipes go up to 100+V, exposed conductive parts like the switch-fuse case
don't. Ditto pipes outside and in shower see the 100+V potential
difference
with respect to the floor slab.



That's why I would say that the consumer unit should be the protection for
the final installation. A simple link to the new supply should take of
any fault between the CU and the supply head.

OK - thanks.

snip

You are thinking over kill on this, in my opinion. You new supply should
be
just that. All you are really doing is extending the supply to a new
position closer to where you want it.

I agree I'm worrying too much. But when you're out of standard territory,
you always worry that you're not doing enough.

From the supply head, install a 100amp RCD DP Switch after the meter.
Bring

Question - only an RCD? That will sort out earth-fault disconnection in a
timely manner (0.15s which is good), but what about a L-N short circuit?
Ok, EDF's cutout will ultimately rupture, but it's EDF themselves who say
that their cutout fuse is only for the protection of the meter tails up to
3m. Funny and rather annoying rule but I suppose they have their reasons.

I read that as "user must provide overload protection". I know that a BS88-2
100A fuse will have no practical discrimination against their 100A BS1361,
but at least I can prove it's protecting my cable.

I was also considering one of the Hager DP 100A MCBs instead of a BS88-2 -
this will just about discriminate against the cutout fuse and Hager state
selectivity with their own final circuit MCBs (RCBOs to be confirmed) upto
In=50A and 600A fault current, which is a fairly good bet most of the time
by my figures.

your SWA to the point where you want it. Terminate in an appropriate box
to
allow new tails to your consumer unit.

I've got that bit sorted - using a Hager JK series industrial metal board
with enough space to terminate the SWA into directly. Also takes RCBOs
without being cramped

The new supply is then protected
by
its own RCD back at the original supply head. If a fault occurs along it,
the RCD is there to break it.

I'd also go for a 2 core SWA and use the braid to supply the safety earth.
No need to extend the earth bonding back to the meter. If you have an
earthing point at the supply head, use it to bond with the SWA and maybe a
link from the new CU, but then a fault in your final circuits may also
cause the RCD at the supply to trip out.

I think a 100mA Type S RCD should handle selectivity OK (it's designed to)
against final circuit 30mA jobbies, so I'm happy with that.

The original reason I discounted 2-core was that the copper equivalent CSA
of the armour is only about 12mm2 (for this new fangled XLPE cable) (ie
less than 16mm2). Perhaps that's moot with RCD protection??

Your final installation will be bonded to create equalisation across the
full final circuits. Your protection is supplied by what you want to put
in
the consumer unit. With some current installations, that is the only
protection that is provided, so yours is really no different.

I think you are trying to over engineer it all. When all you really want
is
to extend the supply to your new CU position. Once you have provided
protection for the supply cable, the rest looks after itself at the new
consumer unit. Bonding them together may actually give you more problems.

Yep - I'd been round and round this in my head. One of the IET folk
suggested (tongue in cheek I think, to make a point) that I could put in a
125mm2 earthing conductor, therefore the touch voltage would always be less
than 50V so intrinsically safe.

But, when you step back, it is rather silly. I think I'm coming round to the
idea of one MET and one point of bonding, next to the CU at the load end of
the submain, and as you say, use the original MET just as a termination for
the earthing conductor.

You could also ask the supplier to reinforce your supply directly to the
new
position for you. Once you've made your installation complete, with just
a temporary supply to keep you working.

Interesting - I've not heard of that. Do you mean they bring their own
earthing conductor round to the new position and guarantee it?

Reason I was avoiding moving the supply head (which would actually be the
obvious and elegant thing to do) is that the thieving wotsits want upto
£1000 to do it, and I get to dig all the trenches and fit a new meter box
for them. The submain is about 200 quid in parts and a 10 times less work.

Good luck with it.

Again, massive thanks. I'll be taking pictures of this.

Cheers

Tim

Hi John

Thanks for taking the time, once again

John Rumm coughed up some electrons that declared:

Tim S wrote:

snip general agreement

At (or for that matter in) the main CU seems sensible to me.


Splendid. Until I raised the subject on the IET forum, I was going to leave
the MET back at the supply origin (because that's where it always is). As
BT used to say, it's good to talk! I'm completely happy I understand why it
should be at the CU instead now.

However, that leaves the reverse problem at the other end, eg big fault,
pipes go up to 100+V, exposed conductive parts like the switch-fuse case
don't. Ditto pipes outside and in shower see the 100+V potential
difference with respect to the floor slab.

Maybe I am missing something, but I am not sure I follow what the
problem is...

Your switch fuse and CU will be linked by something like 52mm^2 of
earthing conductor[1] so any voltage rise on the CU's earthing terminal
is going to be reflected on the casework of the switchfuse pretty
closely. (I guestimate somewhere just north of 5V for a fault current of
1kA)


[1] (assuming XLPE 3 core with armour area alone of 62mm^2 giving a
copper equivalent of that over 2.255 or 27.5mm^2 before you add the
25mm^2 of the core)

You're using a k1/k2 conversion for Cu equiv CSA? I was taking my data from
Prysmian's BS5467 datasheet which says that (for 25mm2 3-core)
conductor@20C is 0.727 milliOhms per meter and armour@20C is 2.5
milliohms/m, but the end result is in the same ballpark.

Think I had a brain spasm. The R2 of my distribution circuit is about
9milliOhms, so I agree (now that I see) that the max PD end to end is 7.5V
at my max fault current (790A due to Ze mostly).

Someone else confused me by looking at R1/R2 like a potential divider with
230V across it, hence the 100 odd volts.

*** So should I bond gas and water to the means of earth back at the
meter as well, directly to EDF's earth terminal? BTW - this is a TN-S
setup, Ze of 0.19 Ohms

Can't see the need. There are quite a number of mitigating factors here
as I see it. Firstly the potential difference (in both senses) between
CU and EDF's terminal is small. The chances of being in contact with the
metalwork of the switchfuse while a fault is present is small. The
chances of any fault taking even a second to clear on a 17th edition
type install that is festooned with RCDs is pretty slim anyway, let
alone 5 secs.

Said like that, I agree. The only fault that could take 5s to clear is a
nail though the distribution cable - but that's an unlikely fault.

I came up with another way round: put a Type S 100mA DP RCCD in place of
the main switch in the switch fuse enclosure, and thus reduce the
disconnection time of the distribution circuit to 0.15s which avoids all
these issues. Unconventional with a TN-S system, but harmless at worst...

*** Overkill? Or not?

Probably is overkill in this circumstance.

Random thoughts gratefully received...

Random, and worth what you paid for em ;-)


Excellent - this and BigWallop's post is exactly the sort of clarity I was
after.

Amazing how easy it is to get all complicated and confused...

Many thanks indeed - I shall now order my parts. TLC will be pleased...

Cheers

Tim

"Tim S" wrote in message
...
Hi,

snipped

From the supply head, install a 100amp RCD DP Switch after the meter.
Bring

Question - only an RCD? That will sort out earth-fault disconnection in a
timely manner (0.15s which is good), but what about a L-N short circuit?
Ok, EDF's cutout will ultimately rupture, but it's EDF themselves who say
that their cutout fuse is only for the protection of the meter tails up to
3m. Funny and rather annoying rule but I suppose they have their reasons.


Putting anything else in line will cause problems. The head fuse will take
care of any phase short. When you consider that a short on the SWA will not
have adverse effect on anything passed the new CU, then it's fine the way it
is. The RCD will protect the rest, if anything does happen between your
consumer unit and the supply. If anything else were needed on that end,
would it not be there already? If you are so worried, then install another
cutout fuse after the meter and before the RCD. Now it's getting into silly
zone again. :-)



I read that as "user must provide overload protection". I know that a
BS88-2
100A fuse will have no practical discrimination against their 100A BS1361,
but at least I can prove it's protecting my cable.


All you have to provide after the supply head is a double pole switch to
isolate your new SWA, at the correct rating for the supply of course. The
cutout fuse on the head is enough protection from fault or they would have
to install further protection to the supply head already. Nothing else is
needed. All you are doing is extending the supply cable. Nothing else.



I was also considering one of the Hager DP 100A MCBs instead of a BS88-2 -
this will just about discriminate against the cutout fuse and Hager state
selectivity with their own final circuit MCBs (RCBOs to be confirmed) upto
In=50A and 600A fault current, which is a fairly good bet most of the time
by my figures.



Why? The supply is already protected by the RCD DP Switch and the cutout
fuse at the head. The supply is taken care of. DONE!!! Finished !!!!



your SWA to the point where you want it. Terminate in an appropriate
box
to
allow new tails to your consumer unit.

I've got that bit sorted - using a Hager JK series industrial metal board
with enough space to terminate the SWA into directly. Also takes RCBOs
without being cramped


This is the bit. I would advise you to make provision for a separate box to
terminate the SWA and allow connection of tails to the CU from there. An
ABS box with three CEFCO terminal blocks inside, allows you to extend to
another set of tails in future. That way you have flexibility to extend
the final circuit to other installations, without touching the consumer at
all.

Example: You want to install a separate CU for external lighting at a later
date. A box will allow you to take other tails off without disrupting
everything. But that's my opinion.



The new supply is then protected
by
its own RCD back at the original supply head. If a fault occurs along
it,
the RCD is there to break it.

I'd also go for a 2 core SWA and use the braid to supply the safety
earth.
No need to extend the earth bonding back to the meter. If you have an
earthing point at the supply head, use it to bond with the SWA and maybe
a
link from the new CU, but then a fault in your final circuits may also
cause the RCD at the supply to trip out.

I think a 100mA Type S RCD should handle selectivity OK (it's designed to)
against final circuit 30mA jobbies, so I'm happy with that.

The original reason I discounted 2-core was that the copper equivalent CSA
of the armour is only about 12mm2 (for this new fangled XLPE cable) (ie
less than 16mm2). Perhaps that's moot with RCD protection??



Yep. Just protect the new supply cable, not the whole lot from there. The
rest is going to have its own safety and protection built into it.



Your final installation will be bonded to create equalisation across the
full final circuits. Your protection is supplied by what you want to
put
in
the consumer unit. With some current installations, that is the only
protection that is provided, so yours is really no different.

I think you are trying to over engineer it all. When all you really
want
is
to extend the supply to your new CU position. Once you have provided
protection for the supply cable, the rest looks after itself at the new
consumer unit. Bonding them together may actually give you more
problems.

Yep - I'd been round and round this in my head. One of the IET folk
suggested (tongue in cheek I think, to make a point) that I could put in a
125mm2 earthing conductor, therefore the touch voltage would always be
less
than 50V so intrinsically safe.

But, when you step back, it is rather silly. I think I'm coming round to
the
idea of one MET and one point of bonding, next to the CU at the load end
of
the submain, and as you say, use the original MET just as a termination
for
the earthing conductor.


Yep. Again. Just protect the new supply and leave the rest to look after
itself.


You could also ask the supplier to reinforce your supply directly to the
new
position for you. Once you've made your installation complete, with
just
a temporary supply to keep you working.

Interesting - I've not heard of that. Do you mean they bring their own
earthing conductor round to the new position and guarantee it?



They have to. It has to be the same as the existing head. It can't be done
any differently. And it has to be as close to the final circuits as
possible for the sake of protection Etc. Just like it is now.



Reason I was avoiding moving the supply head (which would actually be the
obvious and elegant thing to do) is that the thieving wotsits want upto
£1000 to do it, and I get to dig all the trenches and fit a new meter box
for them. The submain is about 200 quid in parts and a 10 times less work.


But they have to reinforce the supply to the new position for you. Free of
charge. Your new installation is not fully up to standard with it being a
sub-main supply. Them installing the supply to the new position is better
for them in long run, as they know they have no comeback to worry about in
the future.

You can still do all the digging when you install your temporary supply, so
they come along and lift your stuff out and put their new stuff in. But
it's a whole new supply head at the new position for you.

You do have to have your full installation complete so they can test and
connect. Any failure on your side of the new consumer unit will mean you
pay for their return visit. That's why you have to have everything complete
and tested from your temporary supply, then call them in to reinforce it all
for you.

A sub-main supply is more hassle to the supplier than it's worth, that's why
they will gladly change it for you. I know, they do it for us all the time.
:-) As long as we know that our side is on and working correctly, and that
we've prepared everything for the guys to come in and install. Then it's
straight forward for everyone.



Good luck with it.

Again, massive thanks. I'll be taking pictures of this.

Cheers

Tim


Yes please. :-)

Just sitting here on night watch, and thinking about what you're going
through with this sub-mains cabling.

I'm now going back 30 years, to the time when we had to pull through a
galvinised pipe and connect up to an old knife and fork, side lever,
individually fused phase and neutral box. If the head end was 100 amps you
could only give 80 amps sub-supply, with cartridge fuses in the knife and
fork box. The earth provision was the gun metal pipe, with wrappings of
copper pulled tight around the outside of the pipe with a twist clamp and
your conductor tucked neatly underneath it. Then you had to make sure that
you made off the pipe ends to stop chaffing on the insulation, and the ends
had to be threaded back to allow for a ferrol and have two lock nuts before
the end ferrol, to stop it shaking off.

There was no SWA cable back then, and no screw on glands to make things
easy, so everything had to be piped and clamped and welded. What a lazy
effin lot you are now. :-)

Can still remember my first time in a new sub-station erection. Was
literally pooping meeself. Had to make off for a new build project, with 6
X 2 inch (50 mm) phase supplies to three housing estates. We had to provide
14 grounded points within the sub-station, and two grounded neutral
equalisers. Both equalisers alone where 2 foot diameter and had to be
buried to 12 feet. Stuffed if the ground was solid, but it had to be done.
They were tested with a hammer and stethoscope to see if they vibrated to
much.

If you've ever watched a 1 inch thick (25 mm) brass alloy bar, quite
literally, dissolve before your very eyes, because you didn't follow
instructions properly, then you know how difficult to balance all this was.
We didn't have regulated supplies to the stations back then. So things
could come in at all angles and phase potentials. Come to think about it,
it was bloody dangerous for us workers back then.

But, when your journey man tells you to throw the switches and everything
buzzes into life. What a feeling. I couldn't take the grin off my face for
weeks. All the hard work just melts into the ether and the jobs a good 'un.
I even got to peel the backs off those new fangled self adhesive signs. OK,
I bubbled a couple on the first go, but got to do them again, properly that
time.

Now I go equipped with a roll of SWA and a couple of boxes. Fix it all to
the wall and make the ends off. WOW!!! What a lazy barsteward I am as well
now. :-) The changes in just that short space of time are amazing. It is
safer and more efficient now-a-days, than it was then. It is more basic now
than then as well. With equalisation (potential bonding) done around the
final circuits instead of grounding to the supply. What a difference it is
in safety, and you new guys don't realise that.

Breakers that work with a little coil wrapped around a ferrite rod, that
activates another smaller coil wrapped in close quarters around the same
ferrite rod. Coils wrapped around ferrite rods were used as radio aerials,
way back when I was a lad. :-) Little bits of bendy alloy that push a
plunger to release a spring and cut the circuit? What ever happened to the
big mouse trap handle and melting wire? LOL

So, what's the problem with your sub-mains supply? :-)

Hi,

Thanks for your reply.

Which DNO do you do your work with? Wonder if their requirements differ from
mine?

BigWallop coughed up some electrons that declared:

snip

Putting anything else in line will cause problems. The head fuse will
take
care of any phase short.

That would be the common sense and technically sound theory.

It's when EDF/SEEBOARD tell you that they want something withing 3m of the
origin, but fail to clarify what "somethign" is... I've heard of people who
fit DP isolators and people who fit switch-fuse units.

As there is a disparity of opinion on this specific point, I've just emailed
EDF Technical requesting their specific requirements (if any). They usually
reply within a day. No disrespect intended ;- But there's been a lot of
different opinion on this specific point.

When you consider that a short on the SWA will
not have adverse effect on anything passed the new CU, then it's fine the
way it
is. The RCD will protect the rest, if anything does happen between your
consumer unit and the supply. If anything else were needed on that end,
would it not be there already? If you are so worried, then install
another
cutout fuse after the meter and before the RCD. Now it's getting into
silly
zone again. :-)

One thing I'm lost on, is why have an RCD (RCCB?) if we are satisfied that
the circuit will disconnect in less than 5 seconds (by my calculations, it
will disconnect in 3 seconds at worst) and if we are no longer concerned
about touch voltages.


I've got that bit sorted - using a Hager JK series industrial metal board
with enough space to terminate the SWA into directly. Also takes RCBOs
without being cramped


This is the bit. I would advise you to make provision for a separate box
to
terminate the SWA and allow connection of tails to the CU from there. An
ABS box with three CEFCO terminal blocks inside, allows you to extend to
another set of tails in future. That way you have flexibility to extend
the final circuit to other installations, without touching the consumer at
all.

Example: You want to install a separate CU for external lighting at a
later
date. A box will allow you to take other tails off without disrupting
everything. But that's my opinion.


It's a good idea, though I'm personally not keen on multiple adjacent CUs.
My leaning is to leave enough spare ways that it will never be aproblem

Cheers

Tim

John Rumm wrote:

Maybe I am missing something, but I am not sure I follow what the
problem is...

I agree - it's a non-issue in this case. Ze = 0.19 ohm, so the earth
fault level is about 1.3 kA. 14 m of 25 mm^2 conductor has a resistance
of around 10 milliohm (less with the armour in parallel) so the fault
voltage drop across the CPC of the distribution circuit is only about 13
volts. If Ze falls for any reason the disconnection time will fall
rapidly and I suspect you'll stay on the right side of the IEC touch
voltage curve.

IOW the main bonding can be connected at whichever end is more convenient.

--
Andy

"Tim S" wrote in message
...
Hi,

Thanks for your reply.

Which DNO do you do your work with? Wonder if their requirements differ
from
mine?


Take your pick
http://www.nationalgrid.com/uk/Elect...tionCompanies/
:-)

They all want to help out. Set up a temporary supply so you can work on the
new build. Then get the completed work tested and re-connected with a new
supply. The guys will be in and out in no time, if everything comes up to
scratch on their first visit. If you've done the grunt work ahead of them,
and chosen a route to the new position, then it's a breeze for them. That's
what they live for. :-)




BigWallop coughed up some electrons that declared:

snip

Putting anything else in line will cause problems. The head fuse will
take
care of any phase short.

That would be the common sense and technically sound theory.

It's when EDF/SEEBOARD tell you that they want something withing 3m of the
origin, but fail to clarify what "somethign" is... I've heard of people
who
fit DP isolators and people who fit switch-fuse units.

As there is a disparity of opinion on this specific point, I've just
emailed
EDF Technical requesting their specific requirements (if any). They
usually
reply within a day. No disrespect intended ;- But there's been a lot of
different opinion on this specific point.



Yeeeessss...... That "something" does need clarified, doesn't it? And
within 3 mtrs of? Is that 3 mtrs of the head or the meter, or the new
position of the CU? Are you taking the meter to the new position as well?



When you consider that a short on the SWA will
not have adverse effect on anything passed the new CU, then it's fine
the
way it
is. The RCD will protect the rest, if anything does happen between your
consumer unit and the supply. If anything else were needed on that end,

would it not be there already? If you are so worried, then install
another
cutout fuse after the meter and before the RCD. Now it's getting into
silly
zone again. :-)

One thing I'm lost on, is why have an RCD (RCCB?) if we are satisfied that
the circuit will disconnect in less than 5 seconds (by my calculations, it
will disconnect in 3 seconds at worst) and if we are no longer concerned
about touch voltages.


An RCD will break supply if anything does happen on the skin of the SWA. If
you are making a bond across from the CU to the SWA, any fault will break
supply back at the head and keep the new install a little safer. But you
are correct, the circuit should disconnect quick enough with just the
protection provided as is. I'm only thinking of the route the SWA is
taking, and what might happen if the skin is damaged Etc. But no more than
residual protection should be necessary.



I've got that bit sorted - using a Hager JK series industrial metal
board
with enough space to terminate the SWA into directly. Also takes RCBOs
without being cramped


This is the bit. I would advise you to make provision for a separate
box
to
terminate the SWA and allow connection of tails to the CU from there.
An
ABS box with three CEFCO terminal blocks inside, allows you to extend to
another set of tails in future. That way you have flexibility to
extend
the final circuit to other installations, without touching the consumer
at
all.

Example: You want to install a separate CU for external lighting at a
later
date. A box will allow you to take other tails off without disrupting
everything. But that's my opinion.


It's a good idea, though I'm personally not keen on multiple adjacent CUs.
My leaning is to leave enough spare ways that it will never be aproblem

Cheers

Tim


That's my old ways, and they're set like that. :-) Always allow for
expansion. lol

But, on this type of installation, don't go over the top with it. Protect
the sub-mains supply as best you can, with as little equipment as possible.
There is always something that makes this possible, and it's always in a
smaller box.

Head..........Meter...........Tails...........Prot ection.............SWA....
...............JOBDONE.

BigWallop coughed up some electrons that declared:



Yeeeessss...... That "something" does need clarified, doesn't it? And
within 3 mtrs of? Is that 3 mtrs of the head or the meter, or the new
position of the CU? Are you taking the meter to the new position as well?

I almost get the feeling that I'm the first person to bother asking in any
detail(!).

Actually, the previous reply from them said, and I quote:

"EDF policy = 3 Metres Max length between meter and consumer unit
Double insulated 25mm copper tails 16mm earth wire"

I've asked them to specifically clarify that again for a submain.

In the meantime, I rang Hager regarding their 100A switch fuse (IU44-11),
which is a BS1361 type. They confirmed that the 100A DP isolator may be
swapped for one of their Type S DP RCDs, in other words I might order one
of them up and adapt it depending on EDF's reply. Equally well, the fuse
component could be removed if necessary. I need something to terminate my
SWA into anyway and something to join it to the tails to the meter.

I've got enough information to proceed, knowing it can be tweaked if someone
gets arsey.



That's my old ways, and they're set like that. :-) Always allow for
expansion. lol

I'm like that too - I'm using a 20way board which will have at least 5 ways
spare (some modules are double width, I don't have 15 circuits)



But, on this type of installation, don't go over the top with it. Protect
the sub-mains supply as best you can, with as little equipment as
possible. There is always something that makes this possible, and it's
always in a smaller box.

Wise words... That's why I'm getting all the information up front. Nothing
worse than finding out the SWA isn't up to spec or what it's terminated
into isn;t either.


Head..........Meter...........Tails...........Prot ection.............SWA....
..............JOBDONE.

Yep - I think we're singing to the same sheet. It's only the "protection"
that's attracting differing views.

BTW, meter stays where it is. As I mentioned, EDF charge a fortune to move
their gear. 35 quid for a fuse pull is bad enough!

Cheers

Tim

"Andy Wade" wrote in message
...
John Rumm wrote:

Maybe I am missing something, but I am not sure I follow what the
problem is...

I agree - it's a non-issue in this case. Ze = 0.19 ohm, so the earth
fault level is about 1.3 kA. 14 m of 25 mm^2 conductor has a resistance
of around 10 milliohm (less with the armour in parallel) so the fault
voltage drop across the CPC of the distribution circuit is only about 13
volts. If Ze falls for any reason the disconnection time will fall
rapidly and I suspect you'll stay on the right side of the IEC touch
voltage curve.

IOW the main bonding can be connected at whichever end is more convenient.

Andy


I think you'll find in reality, it's more like 40 milliohms, but still falls
within disconnect times, at either end.

BigWallop wrote:

I think you'll find in reality, it's more like 40 milliohms,

No. About 0.7 milliohm per metre, times 14 metres (we're only
considering R2, not R1+R2).

Admittedly the resistance will rise with temperature during the fault,
but not by that much.

--
Andy

"Tim S" wrote in message
...
BigWallop coughed up some electrons that declared:

snipped


Head..........Meter...........Tails...........Prot ection.............SWA....
..............JOBDONE.

Yep - I think we're singing to the same sheet. It's only the "protection"
that's attracting differing views.

BTW, meter stays where it is. As I mentioned, EDF charge a fortune to move
their gear. 35 quid for a fuse pull is bad enough!

Cheers

Tim


WHAT???????? They want paid to remove the fuse? Bugger off!!!!!! Pull it
yourself and let them seal it when your finished with it. If they get
stroppey. Tell them you had to pull it for emergency reasons.

God Almighty!!!! What are we paying our bills for? You do know that a
KiloWattHour of supplied power is costed at 3 pence to the suppliers, and
that the rest of the charge on your bill is for maintenance and
administration. I'm sure you've paid enough already, for them to give you a
new supply to the new CU position.

Honestly, man, you're being to soft with them. Tell them you are installing
a temporary sub-main, and that you'd like the new final circuits tested and
the supply re-enforced to the new position, please. They'll tell you what
they need from you, and you tell them when you're ready for them to do the
work.

As I have said before, though. Make sure your new install is up to scratch
before calling them in to change it over. You will be charged for a failure
and re-visit to the site. But with the way you are going about designing
the sub-mains supply, I don't think you're going to make mistakes with your
new install to the final circuits. The length you're talking about is a
breeze for them. They should be in and out before the kettle boils for the
tea and biscuits you offered them when they arrived. :-)

It's worth a phone call to ask their procedures on such matters.

snip

As I have said before, though. Make sure your new install is up to
scratch
before calling them in to change it over. You will be charged for a
failure
and re-visit to the site.

Worse they may disconnect you and leave you disconnected (extreme cases
admittedly, but it does happen)

But with the way you are going about designing
the sub-mains supply, I don't think you're going to make mistakes with
your
new install to the final circuits. The length you're talking about is a
breeze for them. They should be in and out before the kettle boils for
the
tea and biscuits you offered them when they arrived. :-)

Always a good idea


It's worth a phone call to ask their procedures on such matters.


Ditto


--
Bob Mannix
(anti-spam is as easy as 1-2-3 - not)

BigWallop coughed up some electrons that declared:

snip

WHAT???????? They want paid to remove the fuse? Bugger off!!!!!! Pull
it
yourself and let them seal it when your finished with it. If they get
stroppey. Tell them you had to pull it for emergency reasons.

Did I mention the 1000 quid to move the meter...

Which is why it's not getting moved.

God Almighty!!!! What are we paying our bills for? You do know that a
KiloWattHour of supplied power is costed at 3 pence to the suppliers, and
that the rest of the charge on your bill is for maintenance and
administration. I'm sure you've paid enough already, for them to give you
a new supply to the new CU position.

Honestly, man, you're being to soft with them. Tell them you are
installing a temporary sub-main, and that you'd like the new final
circuits tested and
the supply re-enforced to the new position, please. They'll tell you what
they need from you, and you tell them when you're ready for them to do the
work.

As I have said before, though. Make sure your new install is up to
scratch
before calling them in to change it over. You will be charged for a
failure
and re-visit to the site. But with the way you are going about designing
the sub-mains supply, I don't think you're going to make mistakes with
your
new install to the final circuits. The length you're talking about is a
breeze for them. They should be in and out before the kettle boils for
the
tea and biscuits you offered them when they arrived. :-)

It's worth a phone call to ask their procedures on such matters.

Not dealt with EDF before, have you?... ;-

Takes a week of bouncing around call centres who feign ignorance and pass
you somewhere else. After that week, you find random internal numbers on
the IET web forum. You call these until you get lucky and hit the technical
dept.

Then you communicate with them via the technical department's email, that
usually turns around in a day...

The fuse pull procedure is: Phone up, book pull, fax in your EIC (doubt they
even read it). Bloke turns up AM and pulls fuse. Comes back PM to replace.
If I have the isolator fitted in their box, he'll probably hang around for
10 minutes while I move the tails over. Nice thing with the Wylex REC2S
isolator is that there's a separate cover for the load terminals so it's
very safe to work on.

But in general nothing's easy with EDF.

Cheers

Tim

"Tim S" wrote in message
...
BigWallop coughed up some electrons that declared:

snip

WHAT???????? They want paid to remove the fuse? Bugger off!!!!!!
Pull
it
yourself and let them seal it when your finished with it. If they get
stroppey. Tell them you had to pull it for emergency reasons.

Did I mention the 1000 quid to move the meter...

Which is why it's not getting moved.

God Almighty!!!! What are we paying our bills for? You do know that a
KiloWattHour of supplied power is costed at 3 pence to the suppliers,
and
that the rest of the charge on your bill is for maintenance and
administration. I'm sure you've paid enough already, for them to give
you
a new supply to the new CU position.

Honestly, man, you're being to soft with them. Tell them you are
installing a temporary sub-main, and that you'd like the new final
circuits tested and
the supply re-enforced to the new position, please. They'll tell you
what
they need from you, and you tell them when you're ready for them to do
the
work.

As I have said before, though. Make sure your new install is up to
scratch
before calling them in to change it over. You will be charged for a
failure
and re-visit to the site. But with the way you are going about
designing
the sub-mains supply, I don't think you're going to make mistakes with
your
new install to the final circuits. The length you're talking about is a
breeze for them. They should be in and out before the kettle boils for
the
tea and biscuits you offered them when they arrived. :-)

It's worth a phone call to ask their procedures on such matters.

Not dealt with EDF before, have you?... ;-

Takes a week of bouncing around call centres who feign ignorance and pass
you somewhere else. After that week, you find random internal numbers on
the IET web forum. You call these until you get lucky and hit the
technical
dept.

Then you communicate with them via the technical department's email, that
usually turns around in a day...

The fuse pull procedure is: Phone up, book pull, fax in your EIC (doubt
they
even read it). Bloke turns up AM and pulls fuse. Comes back PM to replace.
If I have the isolator fitted in their box, he'll probably hang around for
10 minutes while I move the tails over. Nice thing with the Wylex REC2S
isolator is that there's a separate cover for the load terminals so it's
very safe to work on.

But in general nothing's easy with EDF.

Cheers

Tim


That's where your bill payment goes. It goes to pay for a the call center
in Istanbul. Cheaper to get the plane over there and book it directly,
then. :-)

Who do you pay for the electricity supply? Go through them. Tell them you
are moving everything to the new position, and let them deal with the new
supply. I'm sure they know a bloke down't pub that'll do it for a tenner.
As for the fuse. Do it yourself. They seal it once the new supply is
complete and tested. Save yourself a few pennies on that, at least.

I don't believe the faffing about they make everyone go through now. I
think I'll start up as an electricity supplier and steal all their customers
from them. No joke.

Tim S wrote:

One thing I'm lost on, is why have an RCD (RCCB?) if we are satisfied that
the circuit will disconnect in less than 5 seconds (by my calculations, it
will disconnect in 3 seconds at worst) and if we are no longer concerned
about touch voltages.

I can't see the need for a RCD at the switchfuse position.


Example: You want to install a separate CU for external lighting at a
later
date. A box will allow you to take other tails off without disrupting
everything. But that's my opinion.


It's a good idea, though I'm personally not keen on multiple adjacent CUs.
My leaning is to leave enough spare ways that it will never be aproblem

Can be handy if you decide you want a totally separate one for
outside/garden circuits for example.


--
Cheers,

John.

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

I've taken the chance to contact EDF Technical Services by e-mail, with a
simple description of the move of supply to a new postion on the same
property. I'm going to wait for their reply and make comment on it at that
time.

I prefer e-mail in these cases, because you have the chance to paste the
whole reply into the group and let everyone see what's what.

Here's hoping they get back to me. (from their call center in Istanbul)
:-)

John Rumm coughed up some electrons that declared:

Tim S wrote:

One thing I'm lost on, is why have an RCD (RCCB?) if we are satisfied
that the circuit will disconnect in less than 5 seconds (by my
calculations, it will disconnect in 3 seconds at worst) and if we are no
longer concerned about touch voltages.

I can't see the need for a RCD at the switchfuse position.

Me neither, but BigWallop's in that field so I won't discount anything just
yet, with no disrespect to anyone else.

To be fair, this comes up on the IET forum from time to time and the answers
are different every time, with the only common factor being the "3m rule".

When (if) I get a reply from EDF Technical, I'll post a summary here.

Ignoring them for the minute, I can see no need for anything beyond a DP
isolator.

Their 100A BS1361 fuse is quite capable of disconnecting my submain in 5s -
though it would not if the max permitted Ze of 0.8 Ohms was apparant (need
630A to meet 5s).

I used a science principle in my calcs - my measured Ze was 0.19 +/-5%
+/-0.07 Ohm

That's the uncertainty in a Megger 1552 plus not knowing which pair of leads
from 2 sets the LI test was calibrated with (0.04 Ohms between them).

So I'm working to the max Ze with the worst case uncertainty applied, which
is 0.34. Even with this figure the disconnect times are fine. That gives a
bit of latitude for their supply varying a bit over time.

So I don't know why the DNOs have this rule.

Unless they merely want an isolator at that position... Or the right to prat
around with their cutout fuse whever the mood takes them, safe in the
knowledge it won't make the installation unsafe...

When you get articles written by the ECA saying "stick an MCB in" you tend
to assume there's a reason. Even the NICEIC Snags and Solutions books don't
cover a simple supply extension.

What I suspect happens in reality is noone gives a hoot and just does
something reasonable. Me, being me, wants to be able to justify stuff with
confidence. Silly rabbit, me...



Example: You want to install a separate CU for external lighting at a
later
date. A box will allow you to take other tails off without disrupting
everything. But that's my opinion.


It's a good idea, though I'm personally not keen on multiple adjacent
CUs. My leaning is to leave enough spare ways that it will never be
aproblem

Can be handy if you decide you want a totally separate one for
outside/garden circuits for example.



Just for context, here's my final circuit list so far (I'm pretty happy with
this):

2 x 32A ring circuits covering kitchen and utility area
1 x 32A ring for rest of ground floor
1 x 32A ring for first floor
1 x 10A lighting for left side of ground floor (inc kitchen/dining room)
1 x 10A lighting for right side plus upstairs[1]
1 x 40A cooker circuit (we use gas, but best have a circuit in place)
1 x 45A distribution circuit for boiler + backup thermal store heating (3 x
3kW)[2]
1 x 16A radial feeding two external 16A weatherproof commando sockets[3]
1 x 10A external lighting, including direct feed to shed lights[4]
1 x 32A (or 25A TBD) radial to shed sockets.

[1] This arrangement gives a good split of load.

[2] It's more practical to feed one big cable round to the heating cupboard
and split with MCBs (as part of the control panel maybe) there. Only need
one RCBO at the head end, less ways in the main CU too.

[3] I like commando sockets for external use. I'll make up a couple of long
16A - 13A socket trailing leads for those long summers in the garden with a
laptop Naturally we'll have a convenient inside isolator.

[4] Why not submain to the shed? Well, my calculations show it's impossible
to meet volt drop requirements on the lighting component without using
excessively oversized cable. I have a need for a few outside lights so it
seemed sensible to maintain separate circuits back the the main CU and lump
all the lights together. Shed will have a couple of local DP isolators for
safety.

I might use DP RCBOs for the outside circuits, or at least the sockets,
which are 2 module wide, so at the extreme the above occupies 14 ways from
a 20 way board.

So you see, I'm not too worried about expansion

Cheers

Tim

Tim S wrote:
John Rumm coughed up some electrons that declared:

Tim S wrote:

One thing I'm lost on, is why have an RCD (RCCB?) if we are satisfied
that the circuit will disconnect in less than 5 seconds (by my
calculations, it will disconnect in 3 seconds at worst) and if we are no
longer concerned about touch voltages.
I can't see the need for a RCD at the switchfuse position.

Me neither, but BigWallop's in that field so I won't discount anything just
yet, with no disrespect to anyone else.

To be fair, this comes up on the IET forum from time to time and the answers
are different every time, with the only common factor being the "3m rule".

2m and 3m come up quite often as the limit before a switch fuse is
required IME.

When (if) I get a reply from EDF Technical, I'll post a summary here.

Ta

Ignoring them for the minute, I can see no need for anything beyond a DP
isolator.

I can see the need for a fuse to offer fault protection, since a fault
on the submain may otherwise not be cleared in time, and would result in
taking out a fuse on the distributors side of the network.

The need for a RCD could only be driven by two possibilities, one being
risk of direct contact, and the second being a loop impedance that is
too high to reliably clear a fault. The chances of the former on a SWA
submain are to all intents and purposes nil, and the later we know not
to be true in this case.

Their 100A BS1361 fuse is quite capable of disconnecting my submain in 5s -
though it would not if the max permitted Ze of 0.8 Ohms was apparant (need
630A to meet 5s).

(Don't think I have actually met a TN-S system yet that was as bad as
the "notional" value).

A fuse is also probably better here in that it will discriminate with
just about any MCB you care to use.

I used a science principle in my calcs - my measured Ze was 0.19 +/-5%
+/-0.07 Ohm

That's the uncertainty in a Megger 1552 plus not knowing which pair of leads
from 2 sets the LI test was calibrated with (0.04 Ohms between them).

So I'm working to the max Ze with the worst case uncertainty applied, which
is 0.34. Even with this figure the disconnect times are fine. That gives a
bit of latitude for their supply varying a bit over time.

So I don't know why the DNOs have this rule.

Unless they merely want an isolator at that position... Or the right to prat
around with their cutout fuse whever the mood takes them, safe in the
knowledge it won't make the installation unsafe...

That is probably quite likely. They won't always be able to offer TN-S
with a very low impedance.

When you get articles written by the ECA saying "stick an MCB in" you tend
to assume there's a reason. Even the NICEIC Snags and Solutions books don't
cover a simple supply extension.

Quick bung from one of the MCB manufacturers perhaps ;-)

What I suspect happens in reality is noone gives a hoot and just does
something reasonable. Me, being me, wants to be able to justify stuff with
confidence. Silly rabbit, me...

Not really - I like to have all potential questions answered as well ;-)

Example: You want to install a separate CU for external lighting at a
later
date. A box will allow you to take other tails off without disrupting
everything. But that's my opinion.

It's a good idea, though I'm personally not keen on multiple adjacent
CUs. My leaning is to leave enough spare ways that it will never be
aproblem
Can be handy if you decide you want a totally separate one for
outside/garden circuits for example.



Just for context, here's my final circuit list so far (I'm pretty happy with
this):

2 x 32A ring circuits covering kitchen and utility area
1 x 32A ring for rest of ground floor
1 x 32A ring for first floor
1 x 10A lighting for left side of ground floor (inc kitchen/dining room)
1 x 10A lighting for right side plus upstairs[1]
1 x 40A cooker circuit (we use gas, but best have a circuit in place)
1 x 45A distribution circuit for boiler + backup thermal store heating (3 x
3kW)[2]
1 x 16A radial feeding two external 16A weatherproof commando sockets[3]
1 x 10A external lighting, including direct feed to shed lights[4]
1 x 32A (or 25A TBD) radial to shed sockets.

snip comments that seem fair enough

[4] Why not submain to the shed? Well, my calculations show it's impossible
to meet volt drop requirements on the lighting component without using
excessively oversized cable. I have a need for a few outside lights so it
seemed sensible to maintain separate circuits back the the main CU and lump
all the lights together. Shed will have a couple of local DP isolators for
safety.

One presumes the distance to the shed lights remains the same either
way, so house circuit fed shed lighting is unlikely to dim less than a
submain based installation (more in all probability!). This sounds like
an example of complying with the letter rather than the spirit of the
rule ;-)

I might use DP RCBOs for the outside circuits, or at least the sockets,
which are 2 module wide, so at the extreme the above occupies 14 ways from
a 20 way board.

I usually have a policy of making sure that anything outside does not
share a RCD with something inside. Since the outside circuits are a more
likely source of nuisance trips etc.

Here I have installed two split load CUs in the house[1] (preceded by
one main switch in a separate two module enclosure)). Both TT, the
second smaller CU is reserved for outside stuff. The 30mA RCD carries
outside sockets and lights, the 100mA type S RCD directly feeding
things like the garage and my workshop (via BS 88 cartridge fuse
carries). The garage/workshop has another split load CU with inside
lights on the main switch part, and sockets and outside lights on the
30mA RCD section.

[1] When we got here there was one CU with everything sharing a single
30mA trip RCD. Needless to say a horse farting would plunge the place
into darkness.

So you see, I'm not too worried about expansion

That's ok then!

--
Cheers,

John.

/================================================== ===============\
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John Rumm coughed up some electrons that declared:

Tim S wrote:

snip
A fuse is also probably better here in that it will discriminate with
just about any MCB you care to use.

Agreed, they'll generally discriminate for faults upto around 2-3kA + even
for the biggest final circuit MCB.

Hager do an interesting range of MCBs, the HMF series, type C in double pole
upto 100A which will discriminate with most of the standard Hager CU MCBs
(and presumably RCBOs, need to check, not in the data sheet).

Discrimination is close though, with the max fault current limited to 600A
in one common case. That's quite close. Chances are that a fault any
distance down a final circuit would be OK, but close to the CU might be
borderline. OTOH, despite Hager failing to quote a let-though energy figure
for the 100A device, I suspect it would, looking at data for the lower
current devices in the range.

One of those design choices that's not obvious, but a balance of
compromises...

Not really - I like to have all potential questions answered as well ;-)


One presumes the distance to the shed lights remains the same either
way, so house circuit fed shed lighting is unlikely to dim less than a
submain based installation (more in all probability!). This sounds like
an example of complying with the letter rather than the spirit of the
rule ;-)

I should perhaps clarify that the shed submain would originate at the CU
too, so distribution characterstics upto the CU are common to both
solutions.

What I found was that using separate circuits, one can take advantage of the
permissible volt drop being 5% of nominal supply voltage for non lighting
circuits (lighting being 3% of course). With lighting rolled in on a 38A
combined circuit, it meant a jump to the next size of cable (10mm2 in this
case, rather than 6mm2 which would be fine for a 32A socket circuit). I
have chosen 5A to be the design load on the leg of the lighting circuit
going to the shed - should be suitably overstated.

The "wastage" of running a second bit of 1.5mm2 is mitigated by the the fact
that there'll be a bit of 1.5mm2 running in that general direction from the
house for lighting on another tool shed and any other lighting requirments
in the garden. It's a very specific set of conditions I have, but teh twin
circuit solution does buy guaranteed discrimination of shed lights vs shed
angle grinder on shed socket, whereas that's a very close call for a
submain.

It's not the classic solution, but I don't feel it's weird beyond it's
merits.

I might use DP RCBOs for the outside circuits, or at least the sockets,
which are 2 module wide, so at the extreme the above occupies 14 ways
from a 20 way board.

I usually have a policy of making sure that anything outside does not
share a RCD with something inside. Since the outside circuits are a more
likely source of nuisance trips etc.

I should have said - it's an all RCBO solution. That's one of the reasons I
settled on Hager - generic single pole RCBOs are around 20-something + VAT,
30-something for type C which isn't bad for a quality brand.

Here I have installed two split load CUs in the house[1] (preceded by
one main switch in a separate two module enclosure)). Both TT, the
second smaller CU is reserved for outside stuff. The 30mA RCD carries
outside sockets and lights, the 100mA type S RCD directly feeding
things like the garage and my workshop (via BS 88 cartridge fuse
carries). The garage/workshop has another split load CU with inside
lights on the main switch part, and sockets and outside lights on the
30mA RCD section.

Nice

[1] When we got here there was one CU with everything sharing a single
30mA trip RCD. Needless to say a horse farting would plunge the place
into darkness.

Ow!

Cheers

Tim

Tim S wrote:

I should have said - it's an all RCBO solution. That's one of the reasons I
settled on Hager - generic single pole RCBOs are around 20-something + VAT,
30-something for type C which isn't bad for a quality brand.

Ah, sorry had missed that bit about RCBOs. Yes in which case separating
outside circuits is less of an issue anyway. (about the only capability
you lose is the ability to shut off all outside power with the flick of
one switch)

--
Cheers,

John.

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"BigWallop" wrote in message
om...
I've taken the chance to contact EDF Technical Services by e-mail, with a
simple description of the move of supply to a new postion on the same
property. I'm going to wait for their reply and make comment on it at
that
time.

I prefer e-mail in these cases, because you have the chance to paste the
whole reply into the group and let everyone see what's what.

Here's hoping they get back to me. (from their call center in Istanbul)
:-)

(All e-mail addresses munged)

This is the reply I received. To which I have sent another e-mail to Power
Connect for a response.

From: "Home Services"
To: "Ben Nald"
X-OriginalArrivalTime: 13 Nov 2008 15:30:15.0999 (UTC)
FILETIME=[BA570CF0:01C945A4]
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Hi

Sorry we can not help you with your enquiry, but I believe that Power
connect may be able to help you. The telephone number is 0845 2340 040.
--------------------------------------------------------

From: Ben Nald ]=20
Sent: 12 November 2008 17:58
To: Home Services
Subject: Home Services enquiry

Hi Team,

I would like to move our current supply head to a new position in the
house. The new place would be approximately 14 metres from the existing
head and meter position. If I connect with a temporary supply to the
new position for the consumer unit and supply head, just to keep me
working on a reduced supply, would it be possible for you to arrange for
the supply to be re-enforced and connected to the new position?

This move is due to a new extension we are constructing to the rear of
the property, and which gives us clearer space to create a proper
services area for all the supplies to the house. We would like to know
of your procedures in such cases. We believe it was customary to allow
for these changes, if the new final circuit installations are tested
independently by your service engineers, before arrangement is made to
make the new supply head available. Is this still the procedure?

Many thanks, in advance, for any help you can give on this matter.

Sincerely

Ben Nald

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EDF Energy plc
Registered in England and Wales No. 2366852
Registered Office: 40 Grosvenor Place, London SW1X 7EN

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Tim S wrote:
John Rumm coughed up some electrons that declared:
I can't see the need for a RCD at the switchfuse position.

Me neither, but [...]

It would of course be essential if the earthing were TT. It may be
required to get 5 s disconnection time with Ze ~ 0.8 ohm, which doesn't
appear to apply in your case (and hopefully DNO's network alterations
would only reduce Ze, not increase it).

[...]
When (if) I get a reply from EDF Technical, I'll post a summary here.

You can be 100% certain, IMHO, that they will require a fuse (or cct
breaker) and not just a switch-disconnector, regardless of
discrimination (lack of) and however nonsensical it may appear from a
purely technical point of view. Demarcation of design responsibility is
the issue, see my previous comments he
http://groups.google.co.uk/group/uk....n&dmode=source
and
http://groups.google.co.uk/group/uk....n&dmode=source

BTW are you sure the main fuse is 100 A? TN-S suggests an older supply,
likely to have a 60 or 80 A fuse fitted, perhaps.

--
Andy

Hi Andy,

Andy Wade coughed up some electrons that declared:

Tim S wrote:
John Rumm coughed up some electrons that declared:
I can't see the need for a RCD at the switchfuse position.

Me neither, but [...]

It would of course be essential if the earthing were TT. It may be
required to get 5 s disconnection time with Ze ~ 0.8 ohm, which doesn't
appear to apply in your case (and hopefully DNO's network alterations
would only reduce Ze, not increase it).

[...]
When (if) I get a reply from EDF Technical, I'll post a summary here.

You can be 100% certain, IMHO, that they will require a fuse (or cct
breaker) and not just a switch-disconnector, regardless of
discrimination (lack of) and however nonsensical it may appear from a
purely technical point of view. Demarcation of design responsibility is
the issue, see my previous comments he

Demarcation - yes that seems a good point.

http://groups.google.co.uk/group/uk....n&dmode=source
and

http://groups.google.co.uk/group/uk....n&dmode=source

Just read them - most informative, thank you.

BTW are you sure the main fuse is 100 A? TN-S suggests an older supply,
likely to have a 60 or 80 A fuse fitted, perhaps.


No, it's really 100A by all indications. There's a sticker on the cutout
that says BS1361 100A. Haven't looked at the fuse, obviously... An early
conversation with EDF on the phone, they managed to dig up the records. The
supply connection was replaced and a new head and meter fitted within the
last 10 years and they confirmed the supply was good for 100A max.

They also told me the local network was suitable for TN-S and TN-C-S
possibly explains the excellent Ze readings I'm getting?

Cheers

Tim

Tim S wrote:
John Rumm coughed up some electrons that declared:

[1] When we got here there was one CU with everything sharing a single
30mA trip RCD. Needless to say a horse farting would plunge the place
into darkness.

Ow!

FWIW, that's the setup I have, and I can only remember one trip in the
last year (which may have been associated with a blowing bulb). Of
course, I've probably jinxed it now :-)

On the excellent advice from this group, though, I did add my shed
supply as its own submain rather than tempt fate with the single spare
CU way.

Pete

Tim S wrote:

No, it's really 100A by all indications.

Fairynuff.

They also told me the local network was suitable for TN-S and TN-C-S
possibly explains the excellent Ze readings I'm getting?

Your reading - around the 0.2 ohm mark - is pretty typical, IME, for
urban domestic supplies.

--
Andy