Fuse the Wire or Fuse the Device?

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
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