Re: Fuses

["James F. Agnew" <RRT0162@IBM.NET>]

mhadley1@juno.com wrote:
> 
> 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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Martin, let's all get on the same page.  The PTCs are not
"self-resetting", they are "RESETTABLE" and require the power to be
removed to reset.  Therefore, you must have a switch in series with the
PTC to reset it so there is no functional difference between the
"pullable circuit breaker".  The PTC also has an interesting design that
after overload ramps down the current flow like a current limiter so a
very quick overload like a motor starting does not trip them.  The reset
time is not instant, depending on the overload it can take several
seconds after power is removed.  After tripping, the current drops to a
tiny holding current.  I have also waited for a mechanical CB tripped
and I can assure you that the PTCs react quicker and reaction speed is
directly related to the ammount of overload.  

Try dead shorting a P&B W28 low amperage CB and you may find that you
now have a blown fuse simulator that will not reset, the PTC will take
this in stride.

Jim    


-- 
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// James F. Agnew
// Tampa, FL
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