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All of you using circuit breakers and fuses should be
aware of the operating characteristics of these devices. With the exception of
magnetic circuit breakers, fuse's and thermal circuit breaker's characteristics
are affected by ambient temperature. I'm not quite sure where you would use a
fuse in an aircraft application except for some ancillary device that has no
direct effect on the aircraft operation; I use one in my air-horn circuit.
Typically, a circuit protection device is to protect the wiring. not the load,
but in some applications that may not hold true.The Littlefuse data sheet says
that you should use a fuse at 75% of its 25C rated value in order to
prevent nuisance blowing. At an ambient temperature of 80C, 176F, the fuse
should be de-rated to 95% of its 25C rating. If you are using one of the
re-settable PTC devices, it is de-rated linearly to 40% at 90C ambient.
W23/W31 series thermal cuircuit breakers de-rate linearly to 70% at an ambient
temperature of 60C, 140F, of their nominal 25C rating. Note that these
breakers will not trip at their rated value at 100% of their de-rated rating,
and will trip in one hour at 135% of rating. Depending on the breaker size 5-50A
breakers trip at 200% in 6-22 seconds, whereas 0.5-4A breakers trip in
11-30 seconds at 200%. The reason I am bringing this up is that if you are using
one of these devices in a hot area, such as within a closed box with other
heat-producing devices, or at the top of the firewall in the engine compartment,
you may not get the operation you think is taking
place!
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