David Hansen wrote:

How would this help? The customer's fuse would need to be able to
discriminate with the "electricity board" fuse to be of much use.
However, it would also need to be able to cope with the maximum
demand. That would be fine if the "electricity board" fitted say
150A plus fuses and customers fitted say 80 or 100A fuses.

This is as much about demarcation of design responsibility as anything
else. The distribution network operator's (DNO's) fuse is provided to
protect _their_ equipment, not the consumer's installation. It is
accepted that the DNO's fuse may be relied upon to provide overload and
s/c fault protection to _short_ consumer's meter tails of up to 2 or 3
metres length, as in a normal domestic consumer unit type of installation.

Where a consumer unit is to be located at a distance from the supply
intake the consumer is expected to provide 'main switchgear' (i.e. a
switch-fuse, etc.) to protect the distribution circuit(s) between the
meter and the consumer unit(s) and to satisfy the isolation requirements
of BS 7671. It's the installation designer's responsibility to ensure
that such distribution circuits/sub-mains are properly protected,
regardless of the DNO's fuse. As others have pointed out it's not
necessarily practical to obtain discrimination between the fuses in this
situation.

--
Andy