Valin Thorne wrote:
I’m looking for opinions on the use of circuit breakers and/or
fuses in our Legacy under construction. …
Valin,
I have three circuit breakers in my iV-P, one each for the primary
and backup alternator field circuits, and one for the main hydraulic pump. Consider
wiring the hyd pressure switch “downstream” of this large CB so you will disconnect
relay power as well as pump power when it’s pulled. Otherwise, the intermittent-rated
relay remains energized if you pull the pump CB with pressure below the pre-set
turn-on. The alternator field CB’s allow each to be shut down independently of
the other, and of the rest of the electrical system, should you have a
run-away.
Everything else is on fuses. I have four fuse panels from
B&C in the airplane, a home-made one on the side of the battery box in the
engine compartment, plus a few in-line fuses where it made better sense to do
this—e.g. the hydraulic pump “on” sensing circuit. This uses a 24 v signal from
the pump side of the relay to illuminate a warning light on the panel. This 22
AWG wire is toast if any part of the circuit is shorted with the pump energized,
so I put an in-line fuse back with the pump relay. Rational is that the small
fuse (1 A) protects the wiring, and the system works just fine without the
warning light, so the the fuse can be changed if/when needed.
I have a 10- and 20-position B&C fuse panel hinged to the
forward bottom of the instrument panel just over the co-pilot’s knees. It latches
behind the panel with the fuses pointing downward above the passenger knees,
and unfolds “face up” to rest on the passenger legs for checking/changing
fuses. These panels power all the instrument systems (to include the fuel probe
oscillators out in each wing tip), and are continuously fed by both electrical systems
through high power/high quality diodes.
The other main fuse location is on the right passenger side of the
fuselage. Most aircraft systems beyond the instrument panel are fused through
this pair of panels. They hang vertically and are hinged at the top to lift up
for access/visibility, and are completely concealed by a Velcro’d cover built
into the fuselage side panel.
Although I plan to carry spares for each type/size of fuse I
used, my general plan is to carry on with whatever remains after a “pop” and
sort it out on the ground. In more than 40 years of flying, including 25+ in AF
fighters, I don’t recall a situation that resetting a popped CB restored the
function—and I had two occasions where attempting caused much more serious
problems. I know that resetting occasionally works—such as with a lightning
strike—but this never worked for me either.
So, IAW Bob Nuckols’ recommendation, I used CB’s only for
equipment I thought I might need to shut down in flight (e.g the hydraulic
pump) and that doesn’t have it’s own switch. I used fuses for all else, and don’t
plan to routinely change them out in flight…
The cost of fuses is about 10% of good CB’s, they are lighter, faster
and easier to install, and take up less space.
Let me know what you decide, or if you want additional
information.
BOB