Fuses versus Breakers

John Giddy wrote:
On Fri, 11 Mar 2005 14:57:37 +0100, Bert Willing wrote:


Yes and no. Of course one needs a big fuse (mine is 7 Amps) on the battery
to protect the cable running to the instrument panel.
Individual fuses on simple instruments like a T/S do protect the main fuse
to blow off. On more sophisticated intrumentation like a radio or a flight
computer, the protect the instrument in the case you connect the battery
with the instrument switched on. That usually produces a spike which some of
the instruments (the LX160s is a good example) absolutely don't like.
Fuses which protect an instrument need to be fast fuses, fuses protecting
the main fuse or cables are slow fuses.


You would need very fast fuse to stop the spike you describe. Maybe a
surge supressor on the line near the sensitive instruments would do a
better job ? e.g. a 5W or so zener diode with a breakdown voltage of
14v. (or 16v if you are one of those who advocate 14v batteries) This
will also blow the battery fuse if you connect the battery in reverse
polarity by mistake, and prevent large negative voltages being applied
to insufficiently protected instruments.
It also saves you having to carry a supply of instrument fuses so you
can replace the fuse of the instrument which was ON when you connected
the battery.
Cheers, John G.

Uh, there is no voltage spike produced from doing that. That's an old
myth that comes from power planes, where the starter motor can induce
big spikes in the system during starting. You don't want sensitive
instruments powered on at that time.