I have an installation where the main switchboard is approximately 150m away from the 8 distributions boards it is supplying. The supply authority are giving us a 250amp TP&N supply and have suggested they will supply an earth connection but this will be maximum 0.35ohms (as stated in the regs)
If you know the supply can deliver 250A within a reasonable voltage tolerance you can realistically predict Ze Particularly on a TN-C-S.
we are having problems with earth loop impedance as to get 250/315amps fuses at the source to trip the Zs is required to be around 0.16ohms.
Aplogies if I misundertsood this but as your total supply capacity is only 250A anyway where did a requirement for 315A devices come from
Equally if you are feeding 8 Dist Panels at the load ends from this 250A supply then clearly there should be no requirement for each device to be anywhere near 250A if the downstream installation is reasonably designed
Can anyone advise on how/what the power authorities are likely to install at the source with regards to fuse sizes, realistic Ze value, disconnection times expected/required for their equipment etc
Probably a 400A cut out assembly with 315A BS 88 fuses on board. I wouldn't worry about disconnection times on thier kit - they have substantially a much more pragmatic approach to cable temperature rise under fault on thier network. Again, for your 250A fuses in the main switch, then why bother - an on load isolator is all you need.
Let the wind blow you, across a big floor.