But, Hedley, did you also test them in a 24v circuit?
It's helpful to look at a manufacturer's catalogue to see for the expected
trip characteristics of a particular circuit breaker should be. These will
depend on whether its operation is magnetic or thermal, or combined
magnetic/thermal. While you are correct in thinking that the trip current
should be independent of the circuit voltage, how quickly it trips will
depend on the rate at which the current rises and the current level reached.
For example, a typical thermal circuit breaker, rated at up to 240 V 50 Hz
AC/28 V DC, is quoted as having typical trip times in seconds at 23 degrees
C. of:
100% -- no trip
200% -- 10-60
300% -- 3.5-14
400% -- 2-7
500% -- 1.5-5
600% -- 1-3.5
This means that you would see your test wire glowing at 200% rated current
for up to a minute before the breaker trips. As your test wire heats up, its
resistance would increase and reduce the current unless you slide the
tapping to maintain a constant current. If the current dips below 200%, the
trip time would even greater.
Hope this helps
Mike
Europa Club Safety Officer
-----Original Message-----
From: hedley brown
Subject: Re: amps
I have been deluded by my simple mind into thinking that a 5-amp contact
breaker for dc circuits would break at 5 amps regardless of whether it
was rated at 12 or 24 amps. Not so. Having installed 11 of them from a motor
trader's catalogue, with the great advantage that they were exchangeable
with spade-type motor fuses, I was warned that because they were deemed
suitable also for 24-volt circuits they might not break at the right
amperage. I was incredulous - amps are amps.... relative to wire
thickness that's what makes the heat that breaks the fuse or clicks the
breaker, surely. No; they broke contact at twice the rated current, by which
time my trial variable-length of hotwire was glowing brightly. Had to resort
to good 'ole fuses and write off 110.00. Still don't understand the
physics.
Ah.,
well....
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