Fuse the Wire or Fuse the Device?

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.
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Eric Greenwell
Washington State
USA