Fuse the Wire or Fuse the Device?

Thanks for everyone's comments and responses. Generally it seems that
people like;

1) One large fuse at the battery rated for the maximum load and not
rated for the fusing potential of the wire. See comment #1 below

2) Several small fuses, one for each device, rated for that specific
device. See comment #2 below.

Thanks, John

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Comments

#1 - Yes, it is to protect the wire as overheating from excessive load
can melt the insulation before the wire fuses (melts) and can send
caustic smoking into the cockpit. Moral: don't use any old wire, use
Tefzel wire!! I suspect that the fuse needs to be rated 20% or more
over the maximum load to deal with inrush current surges when you first
hit the master switch. That being said some older devices have
significant inrush currents but most new avionics were designed to have
low or no significant surge currents. Only one way to tell, measure
it. You do have a peak reading current meter handy don't you? See
slo-blo fuse comment #3 below. Note the one comment about measuring
all the MAXIMUM currents involved is done with vario(s) on and audio
blaring, PDA charging with backlight on, computer at max gas and your
transceiver transmitting. Is all that likely to happen at the same
time? No, but we have to think worse case current draw.

#2 - Calling the manufacturer or looking in the manual to find the
proper fuse size and type seems a good idea.

#3 - slo-blo fuse use - While a slo-blo fuse seems useful for devices
with large inrush currents, there are two downsides to their use.
First, they are slow...hence the name...like breakers. Thus the device
you are trying to protect may not get protected in time. Second all
fuses and breakers suffer a voltage drop across the fuse caused by the
resistance of the fuse element. Volage drop means that some of the
power of your battery is being wasted by heating the fuse and thus less
energy (and voltage) is reaching your device. Wire suffers from this
also, so generally keeping wire lengths shorter and gauges larger is
better. However with slow-blow fuses and low amperage breakers the
voltage drop is significantly higher than regular fuses. In an
airplane with a generator, this may not be much of an issue but with
the limited battery power we have in gliders I suggest sticking with
regular fuses.

#4 - Transponders - As this device transmitts more often than you are
likely to do with your transceiver (and maybe continuously in high
traffic areas), this will be a significant power drain. Because I live
near a mode-C vail, I might have to add altitude encoding on top of
that. Therefore I hope that we don't have to have them. Have there
been accidents that would have been prevented if the glider had had a
transponder?

ContestID67 wrote:

#4 - Transponders - As this device transmitts more often than you are
likely to do with your transceiver (and maybe continuously in high
traffic areas), this will be a significant power drain.

For the typical Microair or Becker installation, the best choices for
gliders in the US, it's about 400 ma at idle. That rises to about 500 ma
in areas of VERY heavy radar coverage, and another 100 ma in the winter
to heat the encoder. The peak current demand is much smaller than a
communication radio, but it's higher on average.

Because I live
near a mode-C vail, I might have to add altitude encoding on top of
that. Therefore I hope that we don't have to have them. Have there
been accidents that would have been prevented if the glider had had a
transponder?

No disasters like an airliner hitting a glider (at least in the US), but
several collisions with general aviation aircraft and at least one
fighter aircraft might have been avoided if the glider had used a
transponder and the airplane pilot was in contact with ATC (IFR flight
plan or using Flight Following). Or, if either one was using a
transponder detector, the collisions might have been avoided.

"A lot" of glider pilots in high traffic areas like Minden/Reno,
Southern California, and elsewhere have installed transponders, so one
could easily imagine some collisions have been avoided by doing so. It
would be very hard to estimate how many.
--
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Eric Greenwell
Washington State
USA