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)