I think the key
point in this discussion is not so much fuses vs circuit breakers; but that we
follow the philosophy of not having single point failures in the flight
critical systems. The rotary engine has dual plugs/rotor, most induction
systems have two injectors/rotor; Tracy’s ECU has dual
units built in (although unfortunately, common input power point); and most of
us are using dual fuel pumps. Separate and independent feeds to each of
these can eliminate single point failures.
The choice of fuse
or breaker is as much opinion as it is technical fact, and a winning argument
for either in our airplanes isn’t going to happen. There are advantages
and disadvantages to both.
I have separate and
independent power feeds to each set of coils, to each set of injectors, to
each pump, and to the ECU. I have a mix of breakers and fuses. I
happened to choose fuses (blade type) for the injectors, coils, and pumps
somewhat along the lines of Jim’s argument – I can protect those wires with 15
amp fuses. The ECU has one circuit through a breaker, and one
fused. These circuits are as simple and direct as possible.
Battery - to fuse - to
disable switch – to component. No intermediate contactors or connectors
to fail. Of course I also have two batteries.
Fuses, of course,
cost nothing, and can be changed out on a whim; so there is no reason for them
ever to be old and fatigued. CB’s are a thermal switch; a bit more complex,
expensive and generally difficult to change, so they are likely to get
old. That will be the only point I will make either
way.
You choose; CB or
fuse. I think that choice is less important than how you design and
execute your circuit.
FWIW,
Al
-----Original
Message-----
From: Rotary
motors in aircraft [mailto:flyrotary@lancaironline.net] On Behalf Of Russell Duffy
Sent: Monday, May 02,
2005 8:21
AM
To: Rotary motors in aircraft
Subject: [FlyRotary] Re: finally changed
to fuses vs circuit breakers
You've been working field service long enough to
know
that fuses go bad, too. They de-solder; they break due
to
vibration; the ends corrode and lose connectivity.
I've seen
this happen on cheesy fuse holders. I have never seen, or heard of
it happening on an automotive fuse. Now, don't start telling me
stories of some crappy old car that had rats in the fuse holder, because that
ain't comparable.
I have
items like fuel pumps separately fused, and to the wiring rating, so they
should never blow unless there's a good reason. Items like the EC-2
are directly tied to power, via two attachment points. This is how
Tracy recommends it, because the controller will be it's own
fuse.
If I had
the panel space, and believed CB's offered any significant benefit, I wouldn't
care about the cost and weight.
Gotta go
pick up my new company van, and replace a switch in
Mobile.
Rusty
(discussion for entertainment only)