hedley brown wrote:
>
> 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....
Hedley, did you give 'em enough time to pop at rated current? Seems I
remember among Bob Nuckoll's many fine epistles some data on this, and
it's measured in many seconds. He's anti-breaker anyway, as the
advantage of being able to easily reset them is actually the problem.
I agree especially for modern avionics. The scenario is: a 50-cent
diode fails and creates havoc in the supply voltage to the integrated
circuits. The breaker pops before anything else is damaged. Reset
the breaker, and a $300 IC cooks. Reset it again (duh!!) and....
Regards,
Fred F., A063
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