On Fri, 05 Aug 2005 18:06:10 +0100, Will Dean wrote:

"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.

In real life PSC of 6kA are uncommon although theoretically possible.
A plausible scenario would be a cleaners' socket in a meter cupboard on
the ground floor of a block of flats containing the sub-station in the
basement.

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.


I thought the resistance of the MCB might come to our rescue but I've just
measured one and it's DC resistance is less than 0.01 ohms as measured by
good test gear. Likewise the meter might help out a bit? However the AC
impedance might be quite a bit more as MCBs do get a little warm in
operation.

On such short circuits what fault could you have that both gave
rise to a full short circuit and which did not implicate the cable.




--
Ed Sirett - Property maintainer and registered gas fitter.
The FAQ for uk.diy is at http://www.diyfaq.org.uk
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