Fuses

[mhadley1@juno.com]

All,

I have seen, heard, and read about several versions of "self-resetting'
current protection devices over the years. I have bitten my lip while
installing some of these items for customers, at their insistence. For
some of you, this will be rehash. For others, this is my take on circuit
breaker protection in an aircraft.

I would objectively say that for some applications these type devices may
be OK to use in an airplane. Landing, Nav, and Strobe light circuits have
on / off switches that give the operator some external manual control
over that circuit. Fuel pumps and Pitot heat would also fall into this
category.

Avionics are a bit different. In most current generation avionics and
digital equipment (engine monitors, etc.), the on / off switch on the
panel does not directly switch the incoming power. It switches a
transistor on and off (usually located in the internal power supply)
which allows the power into the radio. I have personally on several
occasions replaced these transistors because they had shorted 'closed'.
This meant the radio panel on / off switch was rendered useless.

Typically, turn coordinators, clocks, hobbs meters, and the like have no
user on / off switching. In the event of a hard short, the circuit
protection device is the only thing able to remove power without shutting
down the entire aircraft electrical system.

To me, that is the heart of the question. Any electrical short in an
aircraft can be handled by shutting down the entire electrical system.  I
consider this to not be a reasonable approach. The question is "How much
control do I (we) want to relinquish to an automatic device?" I
personally do not like the non-pullable circuit breakers for that very
reason. "Automatic" devices fail. While it is easy to assume and
understand that a solid state radio might fail, would it not be just as
easy to assume at some point in time one of these solid state "fuses"
might fail to perform its designed function? 

>From both an on ground troubleshooting / maintenance standpoint and an
in flight emergency standpoint, I want to be able to selectively and
manually disable ANY circuit, at ANY time, at the power distribution
source. My circuit short just might be between the panel switch and the
circuit breaker. While I would concede most shorts take place at the
'end' of the electrical circuit, I have repaired many  wire bundles that
screws, metal shavings, safety wire, etc. have damaged and shorted wires.
Toggling the switch on and off isn't going to remove some of these
shorts.

Granted, all of this is for a "what if" scenario. What if the tire falls
off? What if a winglet breaks off? What if?...

What if there is an electrical short in the cabin? I've been in that
position before and waited (and I mean waited!) for an automatic device
to do its job (a non-pullable circuit breaker in that case). It's kind of
like when your wife asks you to take the trash out...it'll happen when it
happens, right?. It may not happen as soon as it should have or could
have though.

When a short happens in the car, or a piece of electronics in the home,
even on the boat, I have alternatives that can put me out of harms way.
When it happens in the air, my life may balance on my trust and
willingness to wait for the automatic device to do its thing, or so
equipped, reach over to that $17.25 circuit breaker and make it happen.

I just finished a panel with 28 of the best pullable circuit breakers
that I know of. $483.00 worth. Let's say for argument sake I could save
$250.00 or more by going with these automatic devices and the LEDS, etc..
We could even save about $440.00 if we went to a big fuse block with
glass fuses. Seems kind of a false savings when what I am (we are) trying
to do is protect a $70,000+ investment. After all, it only takes once,
doesn't it?

We all have to make choices based on what we believe to be and / or know
to be true. I made what I consider to be an informed decision about
circuit breakers in 1979 at 2500 feet AGL in a Cessna 1975 model 172
while my turn coordinator was smoking. Like 'Loreal, good, pullable
circuit breakers may not be cheap, but they are darn well worth it!

Guess it's safe to say you won't find any solid state or automatic
resetting circuit breakers in my airplane. (unless they were installed by
the manufacturer in some of the equipment!)

Martin

P.S. Progress is what a glacier makes, right? I was a holdout on gas
discharge and LED displayed radios for several years, too. Before the
displays became as reliable as they are today, it seemed ridiculous to me
to have a perfectly good working radio that you had no idea what
frequency you were transmitting on or navigating on! Now I am a big fan
of Gas Discharge and LED Displays! Maybe in twenty years I'll change my
mind about auto reset and solid state CB's.  

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