Hi John:
> Because continuity at low current does not ensure that the protective
> circuit will carry a large fault current - it might be 'hanging on by
> one strand'.
Yes, for one strand. No, for five strands.
Some years ago, I did some experiments on what
problems the 25-amp test would detect. I
simulated broken strands by cutting them one
at a time. With five strands intact, the circuit
passed the 25-amp, 2-minute test. It failed at
4 strands and 1 minute.
(The tested wire was 18 AWG comprised of 36
strands of 34 AWG.)
The ability of a few strands to carry the 25-amp
current depends on the free length of the
those few strands, which in turn determines the
heat-sinking provided to those strands. The
free length was on the order of 3 mm.
My experiment assumed the problem was caused by
an incorrectly set wire stripper, that cut a
number of strands. So there was a very small
free length of strands.
I published this study in the Product Safety
Newsletter, Vol. 10, No.1, January-March, 1997.
Best regards,
Rich
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