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.

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

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

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
------------------------------------------------------------------------
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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

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

Don Johnstone wrote:

The power requirements of all my instruments are well
below 3 amps.

Have you checked the amperage while the radio is transmitting? A glider
with a GPS (.2 -.3 amps), PDA (.4 to .8 amps), vario, and a good radio
transmitting (Becker 4201 2+ amps) can draw 3 amps, so you might be
closer to the fuse rating than you think. And if you turn on the T&B
(.7-.9 amps), you will almost surely exceed 3 amps during transmission.

I suggest you go to at least 5 amps, since you use smaller fuses to
protect the individual devices. This will also reduce the voltage drop
across the fuse, a good thing.
--
Change "netto" to "net" to email me directly

Eric Greenwell
Washington State
USA

"ContestID67" writes:

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.

The idea is right, but you should use a fuse well under the rupture
current of the wire. You want the fuse to blow, not both!

Also check the inrush current of your panel, you may get a supprise at
how high it is when the master switch is flicked on with all of your
stuff already switched on. Also check the battery rating, the fise
should be below that. Dumb to protect the wire and boil the battery.

5) fill in the blank

Ask the insrument maker what fuse they sepecify. It could be none, unit
has internal protection, or a type of xA. Unless you use a fast blow,
most electronics get no protection from a fuse. The electronics are dead
before the fuse heats up enough to blow.

Carry spares of ALL sizes you use.

Don't even think about doing the trick of wrapping foil around a blown fuse
to fix it; the blow current of cigarette packet fiol over an AG3 fuse is
3000-10000A! Yes, THOUSANDS of amps.

The type could be a standard(ish) fuse, a slo-blo or a fast blo, or an
specific part. The correct rating for fuses is not Amps, but I^2T,
that is, the higher the current, the less time it lasts, up to a limit
where it lasts `forever'.

--
Paul Repacholi 1 Crescent Rd.,
+61 (08) 9257-1001 Kalamunda.
West Australia 6076
comp.os.vms,- The Older, Grumpier Slashdot
Raw, Cooked or Well-done, it's all half baked.
EPIC, The Architecture of the future, always has been, always will be.

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