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Date: Sat, 21 Oct 2006 21:56:31 -0400
From: Ernest Christley <echristley@nc.rr.com>
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To: Rotary motors in aircraft <flyrotary@lancaironline.net>
Subject: Re: [FlyRotary] Re: Fuse Ratings for Wiring??
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Ed Anderson wrote:
> Yes, Bill, that is what I understand as well.  BUT, if you are fused 
> for the equipment load, the wiring is automatically protected (in a 
> proper design). In fact, by using a smaller rated fuse (designed just 
> to protect the equipment rather than the wiring) you are even better 
> ensuring that the wiring won't carry a dangerous load and catch fire.  
> So again, I can see no rational reason for fusing to protect the wire 
> rather than a lower rated fuse/CB to protect the equipment (regardless 
> of whether it actually protects the equipment).  But, then I've never 
> been accused of being the brightest turnip on the tomato truck.
>
> Ed

The equipment needs at least X current.  The wire can only have Y 
current.  The fuse's rated value can lie anywhere between X and Y.
By moving the value closer to X, you add more protection to the 
equipment...but subject yourself to nuisance trips.  Better to go with a 
circuit breaker.
Push the value toward Y, and you move out of the nuisance trip zone.  A 
fuse would do just a well then, because if it ever tripped because of 
bad equipment, that piece is already toasted...and good.
I expect that airliners and the military like circuit breakers because 
of institutional momentum, and the fact that they actually have multiple 
people on the airplane to do something about a popped breaker besides 
just pushing it back in.