Previously I said:
Quote:
Originally Posted by DanH
Shorting a 6AWG to ground is "...far short of getting it to smoke and/or open a fuse breaker."
Interesting. I like the logic, but this may call for an experiment 
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I have huge respect for Bob's work, and like many of you, consider his AEC manual an essential bookshelf reference. However, in retrospect I do find some fault in his logic regarding circuit protection on a main bus feed.
The cautionary tale involved a 40A feeder which came into shorted contact with an elevator cable. Eventually the cable parted, without blowing the fuse, the supposed lesson being whatever component a robust cable shorts against is likely to burn up (burn an open) before the fuse can blow. That is undoubtedly true when the victim is
not robust. However, does it hold true if the victim is a motor mount, or the stainless firewall, or some other component with significant mass?
Now consider time vs fuse opening ("before it can blow"). Alex posted a link to some sample fuse specifications, where it states the expected open time is 1 second at 200% of rated load. We're talking about a battery-fed short, which can dump very large currents, so 200% or more can be expected. There is more than enough amps available to pop the fuse.
Assume we do insert circuit protection at the main bus feeder source, properly sized for the wire...say 60A max for a #6 wire. Now let's short that wire somewhere in the cockpit. if the short burns an immediate open in less than one second, great. We'll get a flash and a few sparks and maybe some dirty drawers, but smoke should be limited and it probably won't release enough energy to serve as an ignition source for fibers and plastics. This is what Bob speaks of when he describes why the certified airplanes don't have circuit protection on bus feeds.
Should the short
not burn open, given a typical Nuckolls wire plan the pilot is expected (1) overcome his surprise, (2) close the essential bus switch, and (3) open the master switch. No argument; it should work. However, how
fast does it work? Is it likely to be less than one second? That's all the time the above passive circuit protection would have required, with no pilot action at all.
(BTW, remember Bob's cautionary tale leading to his introduction of the essential bus concept? The big cockpit short
didn't burn open.)
Good design is the art of compromise. The key compromise to be weighed here is the addition of another set of connections (and a few ounces), vs the probability of any practical value. I would caution that blanket judgments may not be appropriate. Allow an example.
Assume a VFR RV-8 with a rear battery. We all seem to agree about the value of circuit protection on the alternator B-lead. There isn't any reason why the main bus feed can't share the same fuse...no additional connections or components.
