"Andrew Gabriel" wrote in message
.. .

So you are always going to get some length of 2.5mm^2 T&E which
is inadiquately protected if the PSC at supply end is greater
than 2936A. In the case of your 6kA PSC supply and 2.5mm^2 T&E,
that's going to be the first 2.3 metres of cable (if I did the
calculation right;-)

OK, we're sort of agreeing now.

Given that all circuits of = 2.3 metres are susceptible to faults WITHIN
the first 2.3 metres, this situation must pertain for almost every final
circuit in real life.

I can only assume that one must check I2t let-through figures for the
protection device, rather than disconnection time graphs.

What I don't understand is why some of the examples I've seen seem to
include the final circuit's own loop impedance in this calculation - that
seems to be a mistake.

Cheers,

Will