RE: [VAL] 40 Amp Fuse

[root <geraldj@xxxxxxxxxxx>]

OK, its Buss that says the AGS is marginal above 30 amps. They repeat it
for every variation on that size in my oldest Buss catalog. They list
these variations in my newest catalog (at least a decade old) only in
the index with a * and a footnote saying "Older style fuse or device and
not recommended for new design."

Littlefuse says, "Fuses of 35 amperes and over are not recommended for
continuous duty at 100% of their rating due to heat dissipation problems
in this size of fuse and holder."

Fuses that are usable in AGS holders include:
AGS
4AG
ABS
4AB
MDM
MDC

The first two are fast blow with glass tubes. The middle two are fast
blow with ceramic tubes. The last two are slow blow (time delay) in
glass or ceramic tubes. The old Buss catalog recommends using the time
delay fuses for incandescent lamps and motor loads where there is an
increased current at starting. 4AG and 5AG are both ancient military
nomenclature and Littlefuse family  I recall seeing the 5AG size fuse in
WW2 vintage surplus transmitters sometimes. As 9/32" is not far from 7
mm, I wonder if that size was chosen to match European fuse sources for
military equipment.

Fuses with ceramic tubes are considered more rugged though they make
visual inspection of the fuse wire impossible. A fuse can fail open
without blowing so that a visual inspection of the glass and wire
doesn't reveal the failure. Its possible for the wire to develop a
hairline crack from slight overload, many repeats of on/off cycling, or
vibration so its often necessary to check a fuse with some electrical
test to be sure its good.

Slow blow or time delay fuses reduce the incidents of nuisance blowing
from short term overloads like lamp or motor starting.

Some DC motors can draw several times their running current until they
get up to speed. High speed DC motors don't have such a starting surge.

Incandescent lamps draw 15 to 16 times their operating current when
first connected to the power. That's because the cold resistance of the
filament is that much lower than its resistance when white hot.

Gerald J.