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