Sorry but brain not working this evening and can't get an adiabatic equation to give me a reasonable answer
PFC=12.2Ka
OCPD BS88-2 E 100A might be 80A but disconnection should be 0.1 at those fault currents
k=115 ie copper sleeved conductor
I'm getting over 30mmCSA main earth which doesn't seem correct
Thanks
That is because disconnection of a BS88 100A fuse at 12kA will be a heck of a lot faster than 5 cycles of mains.
Try page 22 of this PDF for the values directly as a 'bullrush' plot, or squinting at a curve like one of these will give you an idea of how fast things happen - once you get to a PSSC about 10-20 times the nominal never blow rating you are looking at current being interrupted in less than one half cycle of mains (0.01 secs).
And it is better than that - the current may not cut off for so many milliseconds, but for the last few while molten metal is moving and an arc is forming, there are already some extra milli-ohms in the path, and so the fault current is rather below the full PSSC.
Once you allow for less than 1/10th the energy you will probably land between 6mm2 and 10mm2, which may be closer to your expected value.
Regards Mike
That is because disconnection of a BS88 100A fuse at 12kA will be a heck of a lot faster than 5 cycles of mains.
Try page 22 of this PDF for the values directly as a 'bullrush' plot, or squinting at a curve like one of these will give you an idea of how fast things happen - once you get to a PSSC about 10-20 times the nominal never blow rating you are looking at current being interrupted in less than one half cycle of mains (0.01 secs).
And it is better than that - the current may not cut off for so many milliseconds, but for the last few while molten metal is moving and an arc is forming, there are already some extra milli-ohms in the path, and so the fault current is rather below the full PSSC.
Once you allow for less than 1/10th the energy you will probably land between 6mm2 and 10mm2, which may be closer to your expected value.
Regards Mike
