Another moving a consumer unit question
 

Hi, I've done a substantial amount of googling on this but I wanted to
get an up-to-date opinion.

I had an old fuse box (with fuses) hidden away under my stairs so I've
moved it into a new coat cupboard nearby. It's a 63A fused supply
which I think is PME or whatever the earthing system is called.

I've put a 100A isolator switch (non-fused) where the old CU was and
added 2 metres of new meter tail type cables from this to the new CU.

My questions a

Does this sound OK? I read conflicting advice as to whether fused or
non-fused switches are needed.

Can I simply extend the 16mm earth cable to the new CU using an earth
block? Can i put the block in the isolator switch box?

Thanks!

Steve

Another moving a consumer unit question
 

wrote in message
oups.com...
Hi, I've done a substantial amount of googling on this but I wanted to
get an up-to-date opinion.

I had an old fuse box (with fuses) hidden away under my stairs so I've
moved it into a new coat cupboard nearby. It's a 63A fused supply
which I think is PME or whatever the earthing system is called.

You had better check it is pme - ie the supply company provide you with an
earthing point and not a TT (with an earth rod).
If you were moving the box I would have been tempted to replace it with a
new one

- so does this mean that at the moment you have unearthed the supply as you
have not connected the CPC?

I've put a 100A isolator switch (non-fused) where the old CU was and
added 2 metres of new meter tail type cables from this to the new CU.

Each supply authority has its own requirements for the maximum length of
meter tails whether joined or not - to do with the tens of thousands of amps
that can flow in a short circuit in the tails (molten metal everywhere!)


My questions a

Does this sound OK? I read conflicting advice as to whether fused or
non-fused switches are needed.

I am a bit rusty on the IEE Regs but I do not think it makes much
difference - both are rated for the maximum load but fuses are better.

Can I simply extend the 16mm earth cable to the new CU using an earth
block? Can i put the block in the isolator switch box?

Yes you can extend it using an earth block - and if you have an unearthed
installation I would do it quickly!

All connections are supposed to be accessable - I am not sure how big the
isolator switch box is but if it can be got to easily I suppose so -
provided it is securely fitted to a surface and not waggling about inside

if in doubt you could always get it checked by an electrician - as the work
you have listed requires certification under part P

Thanks!

Steve

Another moving a consumer unit question
 

MadJen wrote:

You had better check it is pme - ie the supply company provide you with an
earthing point and not a TT (with an earth rod).

It sounds as if the OP has a TN system, i.e. the DNO does provide an
earth connection. This might be either TN-S, where the earth comes via
a separate conductor (the sheath of a lead sheathed cable, or a separate
core in a concentric service cable), or TN-C-S (=PME) where the service
has a combined neutral and earth conductor.

If you were moving the box I would have been tempted to replace it with a
new one

A "new CU" was referred to, so that may have been done.

Each supply authority has its own requirements for the maximum length of
meter tails whether joined or not [...]

The maximum length allowed will be either two or three metres. This is
the total length between the supply terminals (i.e. the meter output
terminals or the output terminals of any isolator provided by the DNO)
and the consumer's main switchgear.

If the total distance between the supply terminals and the input to the
consumer unit exceeds the limit, fusing must be provided on the
consumers side of the supply terminals within the 2 or 3 m length limit.
In other words the OP's unfused isolator is OK if the total distance
to the CU is within the limit, AND all sections of the tails are sized
in relation to the supply fuse (= 25 mm^ for a 100 A main fuse or = 16
mm^2 for a 60 A main fuse). If the total distance exceeds the limit
then the OP will need to replace the isolator with a suitable
switch-fuse unit, fitted with a 60/63 A or 100 A fuse as appropriate to
the assessed max. demand of the installation. Note that the wiring
between the switch-fuse and the CU is not then part of the meter tails,
but is a distribution circuit, which needs to be designed in accordance
with BS 7671.

Yes you can extend it using an earth block - and if you have an unearthed
installation I would do it quickly!

Agreed, although a better alternative might be to replace with a single
length of 16 mm^2 earthing conductor. (NB: if a metal-clad isolator or
switch-fuse is used that will need its case earthed, so the earthing
conductor would then go via the isolator/switch-fuse's earth terminal,
provided that there is sufficient terminal capacity.)

There has been no mention of the main bonding conductors in this
discussion. Where are those connected at the moment?

[...] as the work you have listed requires certification under part P

This is true, but only in England & Wales.

--
Andy

Another moving a consumer unit question
 

Andy Wade wrote:
MadJen wrote:

You had better check it is pme - ie the supply company provide you with an
earthing point and not a TT (with an earth rod).

It sounds as if the OP has a TN system, i.e. the DNO does provide an
earth connection. This might be either TN-S, where the earth comes via
a separate conductor (the sheath of a lead sheathed cable, or a separate
core in a concentric service cable), or TN-C-S (=PME) where the service
has a combined neutral and earth conductor.

It's PME - a single round cable connects to the main fuse unit & the
earth wire comes out of there. I would have put new wires in but it
means more opening up of the company's boxes.

The combined length of the wires to the CU from the meter is 2m. All
wiring is with meter tail type cable.

Currently the system is earthed using 16mm cable connected with an
earth block.


There has been no mention of the main bonding conductors in this
discussion. Where are those connected at the moment?

Bonding to pipework etc? I beleive that some 10mm earth wire comes out
of the CU and runs up into the ceiling/wall and that would be the
bonding.