Am looking into replacing an antique fuse box with a split load
Contactum consumer unit - in part to provide a modern ring circuit.
But a survey of the current installation indicates potential earthing
problems, so help please!

By way of background the supply is PME - upgraded (new cable, meters
etc etc) around 1995-98 from TT(?) after a fault by the successors(?)
of SEEBOARD.

The incoming supply cable looks quite beefy. It supplies a largish
house plus several small flats via switchgear leading off in parallel
to the main house switchgear (everything goes through the same elec
board meter & cutout). Can only guess as to the incoming supply cable
size, but the cable from the cutout is 16.3mm diameter overall
(including the brown insulation/sheath), which suggests something
around 70mmsq to 100mmsq depending on the sheath/insulation thickness.
Can anyone identify this size more exactly please?

There's quite a lot of switch gear attached, one of which is a 60A
switch fuse leading to the antique fuse box in question. It is hoped
to leave this switch & the cable (imperial?? - around 25mmsq) to the
antique fuse box in place as it looks healthy. However the earthing
is via steel conduit & looks to require upgrading to a proper 'modern'
copper earth cable.

The yellow/green insulated multi-strand earth-to-water bonding cable
appears to be at least 25mmsq. It has crimped lugs so it isn't easy
to measure directly: the overall diameter (including the yellow/green
pvc insulation) is 10.4mm and it is multi-stranded - perhaps 15+
strands. I can't get an exact fix on the size of the strands, but
they are perhaps 2-2.5mm in dia. Please, can anyone identify the
exact size/spec of this cable?

This is way over the 'typical' sizes shown for PME on page 28 of the
IEE On-Site Guide. Then looking at the table 10a on page 161 gives a
wider picture, and it all rather implies the supply must be 70mm^2 (or
larger?).

So what size should the new earth bonding cable from the supply earth
point to the new Consumer Unit be? Any opinions/advice will be most
gratefully received.

If the supply cable is really 70mmsq then am I reading On-Site Guide
table 10a (pg 161) correctly that a 35mmsq earth cable should be
specified for the run earth terminal-to-Consumer Unit? Or should it
be sized according to the 25mmsq cable coming from the switch-fuse
(16mmsq if I read pg 161 aright)?

BTW I enquired about 25mm^2 earth cable at a TLC branch only to be
greeted with astonishment, so if a heavy gauge earth cable turns out
to be needed, where can you buy it?

many thanks

[BTW appologies if this posted twice - I think the first attempt
failed, but you never know)

"jim_in_sussex" wrote in message
om...

[...]
The incoming supply cable looks quite beefy. It supplies a largish
house plus several small flats via switchgear leading off in parallel
to the main house switchgear (everything goes through the same elec
board meter & cutout). Can only guess as to the incoming supply
cable size, but the cable from the cutout is 16.3mm diameter overall
(including the brown insulation/sheath), which suggests something
around 70mmsq to 100mmsq depending on the sheath/insulation
thickness.

When you say "from the cutout" do you mean the individual 'tails' to the
meter, or the incoming multi-core cable from the street? If the wires or
cable are recent theyr'e likely to have the size embossed on the sheath
somewhe look carefully.

Can anyone identify this size more exactly please?

A photograph would be useful/interesting, if you can put one up somewhere.
Is this single phase or three? A single-phase supply of more than 100A
would be very unusual, IME. What's the rating of the meter, and what type
of main fuse(s)?

[...]
The yellow/green insulated multi-strand earth-to-water bonding cable
appears to be at least 25mmsq. It has crimped lugs so it isn't easy
to measure directly: the overall diameter (including the yellow/green
pvc insulation) is 10.4mm and it is multi-stranded - perhaps 15+
strands. I can't get an exact fix on the size of the strands, but
they are perhaps 2-2.5mm in dia. Please, can anyone identify the
exact size/spec of this cable?

Size Stranding
---- ---------
25 7/2.13
35 19/1.53
50 19/1.83
70 19/2.17
95 19/2.52

- does that help?

This is way over the 'typical' sizes shown for PME on page 28 of the
IEE On-Site Guide.

You're beyond the scope of the OSG at 100 A.

So what size should the new earth bonding cable from the supply earth
point to the new Consumer Unit be? Any opinions/advice will be most
gratefully received.

BS 7671 requires main earthing conductors (i.e. from PME terminal to
installation main earth terminal) as follows:

Supply Earth Main
size size bonding
------ ----- -------
25 16 10
35 16 10
50 25 16
70 35 25
95 70 25

NB: the earthing conductor size is based on the phase conductor of the
supply, whereas the bonding size is based on the supply neutral size. Local
supply network conditions may require a larger bonding size.

Bear in mind that oversized earths may have been used.

Also bear in mind that the earth to your new consumer unit (it's actually a
dis-bord in this case) is a CPC and not a main earthing conductor. If the
fuse in the switch-fuse which feeds this board is 100A or less then a
'standard' 16 mm^2 earth will be OK. OTOH the main bonding for the house
will go to the main earth terminal and should follow the table above (but
you're not changing that, are you?).

BTW I enquired about 25mm^2 earth cable at a TLC branch only to be
greeted with astonishment, so if a heavy gauge earth cable turns out
to be needed, where can you buy it?

That doesn't say much for TLC (whom I've never used). Any decent wholesaler
will have it, or be able to get it - try somewhere like Newey and Eyre.
Most wholesalers will sell you cut lengths of the larger sizes - but watch
the price, it's easy to get ripped-off on cable where stupidly high list
prices tend to be quoted. Make sure you get the 95% discount!

[BTW appologies if this posted twice - I think the first attempt
failed, but you never know)

Well I'm following up to the first of two copies of it on Zen's news server.

--
Andy

On 6 Jul 2004 23:16:14 -0700, (jim_in_sussex)
strung together this:

snip lengthy post as I'm not sure which bit to reply to!

Basically, the only bit of info needed was '60A'. If the supply to
your proposed consumer unit is 60A then 10mm^2 cable is all you need.

Everything before it isn't relevant in this instance. If it sounded
ropey I'd ask for more info but it sounds as if there has been a fair
bit of work carried out recently to a decent standard, although
without seeing it you never know.
--

SJW
A.C.S. Ltd

"Andy Wade" wrote in message ...

first many thnks for yuour extremely helpful reply, it's made me feel
a bit saner...

snip

Can only guess as to the incoming supply
cable size, but the cable from the cutout is 16.3mm diameter overall
(including the brown insulation/sheath), which suggests something
around 70mmsq to 100mmsq depending on the sheath/insulation
thickness.

When you say "from the cutout" do you mean the individual 'tails' to the
meter, or the incoming multi-core cable from the street?

err....that should read 'across the fields', the old cable which was
replaced came in from other fields on a 180deg bearing!

it is a 3phase swa supply - maybe overall swa dia c.50mm (from memory)
but only 2 phases are used - 1 for normal elec supply the other for
(ancient) off-peak heaters. The swa disappears into a sealed box of
(presumably) board fuses, out of that emerge the 2 line cables as
described which immediately go through a current transformer. The 2
meters (1 on each phase used) are indirect and actually monitor the
CTs.


If the wires or
cable are recent theyr'e likely to have the size embossed on the sheath
somewhe look carefully.

Can anyone identify this size more exactly please?

A photograph would be useful/interesting, if you can put one up somewhere.
Is this single phase or three? A single-phase supply of more than 100A
would be very unusual, IME. What's the rating of the meter, and what type
of main fuse(s)?

A pic is tricky within the next week as I've come home (the
installation is at my parents). However I think your next comments re
the earth clear up most of the issues - I was only poking about trying
to size the incoming supply because of the top line of table 10A in
OSG & to get as much info together as poss.




[...]
The yellow/green insulated multi-strand earth-to-water bonding cable
appears to be at least 25mmsq.

snip

Please, can anyone identify the
exact size/spec of this cable?

Size Stranding
---- ---------
25 7/2.13
35 19/1.53
50 19/1.83
70 19/2.17
95 19/2.52

- does that help?

immensely. 19/1.83 seems to fit the overall dia best allowing 1.2mm
for the sheath - OTOH 50mmsq is not in the BS7671 table (below), so
maybe 70mmsq?? 35mmsq would make the insulation/sheath much thicker
(1.7mm) than it looks.

for future ref what is the source please?


This is way over the 'typical' sizes shown for PME on page 28 of the
IEE On-Site Guide.

You're beyond the scope of the OSG at 100 A.

Another key point I missed, thanks

So what size should the new earth bonding cable from the supply earth
point to the new Consumer Unit be? Any opinions/advice will be most
gratefully received.

BS 7671 requires main earthing conductors (i.e. from PME terminal to
installation main earth terminal) as follows:

Supply Earth Main
size size bonding
------ ----- -------
25 16 10
35 16 10
50 25 16
70 35 25
95 70 25

NB: the earthing conductor size is based on the phase conductor of the
supply, whereas the bonding size is based on the supply neutral size. Local
supply network conditions may require a larger bonding size.

Bear in mind that oversized earths may have been used.

that must be the key, possibly they only carried 1 earth cable size &
used it everywhere.


Also bear in mind that the earth to your new consumer unit (it's actually a
dis-bord in this case) is a CPC and not a main earthing conductor.

thanks, just wish the IEE & OSG gave clearer definitions (ie decent
illustrative pics/diagrams) of these fundamnental terms.

If the
fuse in the switch-fuse which feeds this board is 100A or less then a
'standard' 16 mm^2 earth will be OK.

that's the answer I want to hear, 16mmsq it is, many thanks.

OTOH the main bonding for the house
will go to the main earth terminal and should follow the table above (but
you're not changing that, are you?

not on your life!!!

So all cleared up enough to do the job, many thanks, but one last
niggling query:

As stated elec board put in this massive bond to the house cold water
supply at the point where the incoming water mains changes to copper.
What they missed is that there is a second (Cu/galv iron(?) water pipe
going underground into the garden to an outside tap. There is a long
(c. 3m) section of plastic pipe between the main house water Cu and
this outgoing pipe.

Presumably it needs earthing/ bonding back to where? - just bridge the
plastic insert, link back to the main bond join to the house water, or
link back to the elec board earth terminal? & what size? No doubt
there'll be some 16mmsq spare, but if not is 10mmsq OK?

snip
[BTW appologies if this posted twice - I think the first attempt
failed, but you never know)

Well I'm following up to the first of two copies of it on Zen's news server.

Thanks, I thought it had posted first time but right at the end my
computer went into a deep contemplation of its navel


cheers

"jim_in_sussex" wrote in message
om...

err....that should read 'across the fields', the old cable which was
replaced came in from other fields on a 180deg bearing!

OK :-)

it is a 3phase swa supply - maybe overall swa dia c.50mm (from
memory) but only 2 phases are used - 1 for normal elec supply the
other for (ancient) off-peak heaters. The swa disappears into a
sealed box of (presumably) board fuses, out of that emerge the 2
line cables as described which immediately go through a current
transformer. The 2 meters (1 on each phase used) are indirect and
actually monitor the CTs.

Ooh, I've never seen CT metering in a house before. (Might "mansion" be a
better description?) It used to be common (standard?) for anything over 100
A, but, round here anyway, whole-current meters are normal up to 200
A/phase.

Size Stranding
---- ---------
25 7/2.13
35 19/1.53
50 19/1.83
70 19/2.17
95 19/2.52

- does that help?

immensely. 19/1.83 seems to fit the overall dia best allowing 1.2mm
for the sheath - OTOH 50mmsq is not in the BS7671 table (below), so
maybe 70mmsq??

Or it might be because I made a mistake - see below.

for future ref what is the source please?

Err, my head, for the no. of strands; the rest is just simple arithmetic.
Actually I've seen 35mm^2 wires in both 7 and 19 strand versions, but I
think the other sizes are always as described. The no. of strands always
follows the sequence 1, 7, 19, 37, ... which follows from the geometry of
the situation, so to speak.

Supply Earth Main
size size bonding
------ ----- -------
25 16 10
35 16 10
50 25 16
70 35 25
Correct so far
95 70 25
^^
Drat, that 70 should be a 50 (& relates to your comment above). Basically
for phase sizes greater then 35mm^2 the earth has to be at least half the
phase size.

thanks, just wish the IEE & OSG gave clearer definitions (ie decent
illustrative pics/diagrams) of these fundamnental terms.

IMO the OSG is a model of how _not_ to produce an accessible reference
manual.

So all cleared up enough to do the job, many thanks, but one last
niggling query:
As stated elec board put in this massive bond to the house cold water
supply at the point where the incoming water mains changes to copper.
What they missed is that there is a second (Cu/galv iron(?) water
pipe going underground into the garden to an outside tap. There is
a long (c. 3m) section of plastic pipe between the main house water
Cu and this outgoing pipe.

Presumably it needs earthing/ bonding back to where? - just bridge
the plastic insert, link back to the main bond join to the house
water, or link back to the elec board earth terminal? & what size?

Is the plastic section entiely within the house? If it is then the galv.
pipe ought to be main-bonded (sizes as above). OTOH an outside tap should
definitely not be earthed to the PME terminal because it's outside the
equipotential zone and could become live under fault conditions. Some more
plastic pipe would be useful here.

No doubt there'll be some 16mmsq spare, but if not is 10mmsq OK?

In the circumstances described (i.e. the pipe isn't going to be metallicaly
connected to the water mains or to any other electrical installation) I'd
say that was entirely adequate.

--
Andy

Ooh, I've never seen CT metering in a house before. (Might "mansion" be a
better description?) It used to be common (standard?) for anything over 100
A, but, round here anyway, whole-current meters are normal up to 200
A/phase.

I`ve never seen a whole current meter rated above 100A before (Manweb
area)

--
Please add "[newsgroup]" in the subject of any personal replies via email
--- My new email address has "ngspamtrap" & @btinternet.com in it ;-) ---

"Colin Wilson" wrote in message
t...

I`ve never seen a whole current meter rated above 100A before (Manweb
area)

Well in premises where I used to work there were two 3x 200 A meters, big
square-ish chunky things fitted with electronic MRUs. 15 revs/kWh on the
disc, IIRC. I can't remember the manufacturer - Siemens, perhaps. This is
the former Eastern Electricity area, BTW.

--
Andy

In article ,
Colin Wilson writes:
Ooh, I've never seen CT metering in a house before. (Might "mansion" be a
better description?) It used to be common (standard?) for anything over 100
A, but, round here anyway, whole-current meters are normal up to 200
A/phase.

I`ve never seen a whole current meter rated above 100A before (Manweb
area)

In my building, we pull well over 1MW.
The meters are driven via current transformers though,
the main feeds don't go directly through the meters.
I think we've now got 3 feeds into the building, as each
time we needed more power, it was done by adding another
feed rather than upgrading the existing ones.

--
Andrew Gabriel

I think we've now got 3 feeds into the building, as each
time we needed more power, it was done by adding another
feed rather than upgrading the existing ones.

I`m guessing you have your own HV ring then, rather than taking it
directly off the local REC`s network...

--
Please add "[newsgroup]" in the subject of any personal replies via email
--- My new email address has "ngspamtrap" & @btinternet.com in it ;-) ---

In article ,
Colin Wilson writes:
I think we've now got 3 feeds into the building, as each
time we needed more power, it was done by adding another
feed rather than upgrading the existing ones.

I`m guessing you have your own HV ring then, rather than taking it
directly off the local REC`s network...

Originally, we were on the 1MVA transformer feeding the local
residential properties. With the first upgrade, a second
transformer was added to the pad. I guess our feeds must now
be split between the two transformers since the total exceeds
the capacity of either one, but I don't know which feed is
from which transformer. We managed to trip our main 630A
incoming 3-phase breaker whilst we were waiting for the new
transformer to be installed for the first upgrade. Funny thing
was a company electrician was on site it the time, and it
tripped just as he fitted a replacement lamp in low voltage
downlighter;-)

--
Andrew Gabriel

"Andrew Gabriel" wrote
| In my building, we pull well over 1MW.

See folks, an instant electric shower /can/ beat a high flow combi :-)

Owain

On 8 Jul 2004 08:38:15 GMT, (Andrew
Gabriel) strung together this:

it
tripped just as he fitted a replacement lamp in low voltage
downlighter;-)

Some of those cheap imported bulbs are useless, try fitting a type C
630A MCB. ;-)
--

SJW
A.C.S. Ltd

Some of those cheap imported bulbs are useless, try fitting a type C
630A MCB. ;-)

Or a thick steel nail. Much the same thing, really!

Christian.

On Thu, 8 Jul 2004 13:04:38 +0100, "Christian McArdle"
strung together this:

Some of those cheap imported bulbs are useless, try fitting a type C
630A MCB. ;-)

Or a thick steel nail. Much the same thing, really!

I think a short length of galv\copper pipe would be better. The nail
will probably melt quicker than the MCB would blow!
--

SJW
A.C.S. Ltd

See folks, an instant electric shower /can/ beat a high flow combi :-)

You can get 24kW instant electric heaters, provided you can spare a 100A
phase for one. I wouldn't attempt to apply diversity given that it will draw
the full 100A when operational. Alternatively, I believe you can get 3 phase
ones that will only draw 33A off each phase.

These could be a good solution for a shower liking person with lots of
electricity and no gas or oil. They would be particularly suitable for a
holiday house that is left unattended, as water tanks can be a worry for
leakage and accidental heating for long periods (such as over winter).

Christian.