Fuse the Wire or Fuse the Device?

I am about to open a debate on fusing in Gliders. Which of the
following fusing methods do you believe should be used and why?

1) Place one fuse at the main battery sized to protect the wire. That
is if the wire "fuses" (vaporizes) at 20A then the fuse should be 20A.
Therefore it takes a 20A device fault to blow the fuse.

2) Place one fuse at the main battery sized to the devices being
sourced. That is if all the devices being powered have a maximum draw
of 5A, then the fuse should be 5A or slightly larger. Therefore it
takes a 5A fault to blow the fuse.

3) Place one fuse at the main battery sized to protect the wire (see
#1). Place one fuse at each device sized to the devices being sourced.


4) Place one fuse at the main battery sized to the devices being
sourced (see #2). Place one fuse at each device sized to the devices
being sourced.

5) fill in the blank

Thanks, John

I use 5A fuze at the battery and a 5A C/B for everything in the panel.
Don't ever remember blowing a fuse. I have had wires break, fuse
holders not keep good contact, plugs fail to do their thing, etc. My
latest problem was a "clunking" noise, turned out to be the plug at the
end of a 6 inch battery lead, clunking against the deck. After about 10
flights of clunking, the wire broke at the end of the solder connection
to the battery. Now have the plug epoxied to the battery. BTW, we need
2 batteries and 2 varios and 2 GPS's and 2 data loggers, etc to make
everything 100 % reliable. When my wire broke I was 40 miles from home,
switched the Borgelt B-40 to "internal" power and had a vario and audio
to get old JJ back home.

wrote:

BTW, we need
2 batteries and 2 varios and 2 GPS's and 2 data loggers, etc to make
everything 100 % reliable. When my wire broke I was 40 miles from home,
switched the Borgelt B-40 to "internal" power and had a vario and audio
to get old JJ back home.

I had a battery die in flight once. I was abouit 100nm from home. No
audio, no GPS. Got back home anyway.

Stefan

wrote:
When my wire broke I was 40 miles from home,
switched the Borgelt B-40 to "internal" power and had a vario and
audio
to get old JJ back home.

Off topic, but: On Monday I flew a racing task here without any
electronics for the first time in years. Logger in "dumb" mode (ie. no
interface)due to disconnected cables and L-NAV back at the shop for
repairs. It was one of the nicest flights ever, just looking out the
window and glancing at the map now and then. Funny how we seem to have
forgotten the old fashioned way of doing things! I still managed to
limp around at 110km/h using "just" a Scheuman vario and seat of the
pants.

Disclaimer: Yes, I know not having an audio vario is dangerous and yes
I know that moving maps are a wonderful thing. Just waxing nostalgic
for a moment.

P3

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

=========================
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.
--
Change "netto" to "net" to email me directly

Eric Greenwell
Washington State
USA

ContestID67 wrote:

I am about to open a debate on fusing in Gliders. Which of the
following fusing methods do you believe should be used and why?

4) Place one fuse at the main battery sized to the devices being
sourced (see #2). Place one fuse at each device sized to the devices
being sourced.
I use a slow fuse somewhat bigger than the total my devices will draw on the
battery. This prevents damage to the battery and the main wires to the
instrument pannel. Note that it's hard to say at wich current your cable
will melt. This is not only determined by the wire as it is when you just
installed it, but also by the (soldering) connections, againg plastics,
damaged isolation, the time a short circuit continues, etc.

For each instrument on it's own, I have a faster fuse. 1 Amp for most of
them. Now, if a device causes a problem, only that device will be out of
order. The rest will continue to function just fine. You can use automatic
fuses for these if you want.

André

I really like scenario # 3. Most of the time you'd be OK with just the
devices fused, but would you want to have an electrical fire in the
glider at altitude, with oxygen floating around in the cockpit? I have
mine fused as close to the battery as possible.

I'm a telecom manager for a class 1 railroad, and we do all our
installations with this fusing method. We have a lot of specialized
electronics in locomotives and vehicles. We don't want electrical fires
in any of those, either.

Be safe up there...

Jack Womack

The fuse is there to protect the wiring not your devices.

The best bet is to do the following:

1 Large fuse at the battery just bigger than all of your devices, take
this to a fuse box and fuse individual devices at their correct current
rating. ie if the device is rated at 1 amp fuse it at 1 amp.

Do NOT fuse two devices from one larger fuse.

Use the correct size wire for the current to be drawn, many nasty
things can happen if you get to larger voltage drop along the wire and
you start driving a 12 volt device with a lower voltage.

Be aware that the fumes released by some wire is very toxic and you may
find you need to leave an other wise perfectly serviceable aircraft for
no other reason that you did not fuse your wires correctly.

One other point, if the device blows a fuse the chances are that there
is something wrong with it. Do not start putting larger fuses in the
circuit get the item looked at.

With speacial thanks to Steve K, who recently lectured on this subject
on a Form 2 course I attended, I hope I did you justice

Sean


--
jorgie
------------------------------------------------------------------------
Posted via OziPilots Online [ http://www.OziPilotsOnline.com.au ]
- A website for Australian Pilots regardless of when, why, or what they fly -

I use scenario #3. In the UK it is mandatory to have
a fuse 'as close to the battery as possible' to protect
the whole system and I use a 3 amp fuse. There is also
a seperate fuse, rated to the individual instruments
mainly to try and prevent one single instrument taking
the whole system down. That does not always work as
the only problem I have had was with a fault in the
distribution box/power supply for the PDA/Logger/GPS.
All the output leads from the box are fused but a fault
in the box blew the main battery fuse.
The power requirements of all my instruments are well
below 3 amps. A standard battery is 7ah a current draw
of 3 amps would flatten the battery in 2.5 hours (max)
and this is one of the reasons why we are so set against
adding further avionics such as transponders over this
side of the pond.




At 03:00 20 April 2005, Jack wrote:
I really like scenario # 3. Most of the time you'd
be OK with just the
devices fused, but would you want to have an electrical
fire in the
glider at altitude, with oxygen floating around in
the cockpit? I have
mine fused as close to the battery as possible.

I'm a telecom manager for a class 1 railroad, and we
do all our
installations with this fusing method. We have a lot
of specialized
electronics in locomotives and vehicles. We don't want
electrical fires
in any of those, either.

Be safe up there...

Jack Womack