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 are:
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?
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?
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!)
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.
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
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.
A "new CU" was referred to, so that may have been done.
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.
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?
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.
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.
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