Hi Guys
Seeing as I am pretty old I thought I would jump in on this one.
Fusing factor is a historical throwback which is not explained in the book. Note that fusing factor is not in the definitions either.
Fusing factor was last used in the 14th edition, for real. It is a guarantee of popping a fuse or breaker with enough amps....
I shall give one example to clarify.
If you look a the time current curves for rewireable fuses on page 297 green book you will see that you need around double the current to blow a fuse. So that makes it 60A for a 30A and so on.
In the 14th, to guarantee a 'blow' the rewireable fuse was given a fusing factor of 3 so you needed 90A for a 30A unit.
Even more interesting, if you go back to the time/current curve on p297 and read off 90 A on a 30 A unit, it takes around 5 seconds for a disconnection so that is the origin of the 5 second discomnnection times that linger till today. :|
Now it is a long time ago but AFAICR the factors were....
Rewireable fuse 3
Cartridge fuse 2.4
Breaker 1.5 (no C and D in them far off halcyon days)
Finally, to close the lesson, looking at the above the 14th called the rewire-able 'coarse protection' and the rest 'close protection'
Regulation 533.1.1.2 basically states that you must not replace a cartridge unit ( close) with a rewire-able ( coarse) of the same rating as it will risk loop impedance compliance ....
I feel much better now for getting that off my chest
ut the kettle on
Even though I have recently quit teaching, i cannot help myself:innocent
