Bob Nuckols on protecting "fat" wires
I can think of both pros and cons, but tell us why Bob doesn't like it.
In working on my electrical system design I reference Aeroelectric Connection and Aeroelectric List. I assembled some notes on Bob Nuckols' opinion regarding circuit protection of "fat" wires, from Aeroelectric List, below.
Note there is protection in the cockpit in terms of being able to shut off the master relay and being able to remove alternator field current. And of course every connection is a potential failure.
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Bob quotes the FARs:
Sec. 23.1357 Circuit protective devices.
(a) Protective devices, such as fuses or circuit breakers, must be installed in all electrical circuits other than--
(1) Main circuits of starter motors used during starting only; and
(2) Circuits in which no hazard is presented by their omission.
(b) A protective device for a circuit essential to flight safety may not be used to protect any other circuit.
(c) Each resettable circuit protective device ("trip free" device in which the tripping mechanism cannot be overridden by the operating control) must be designed so that--
(1) A manual operation is required to restore service after tripping; and
(2) If an overload or circuit fault exists, the device will open the circuit regardless of the position of the operating control.
(d) If the ability to reset a circuit breaker or replace a fuse is essential to safety in flight, that circuit breaker or fuse must be so located and identified that it can be readily reset or replaced in flight.
(e) For fuses identified as replaceable in flight--
(1) There must be one spare of each rating or 50 percent spare fuses of each rating, whichever is greater; and
(2) The spare fuse(s) must be readily accessible to any required pilot.
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Some quotes from Bob:
“I'll direct your attention to paragraph (a)(2) which speaks to battery feeders and feeders to main power distribution busses that are typically the largest wires in the airplane and are not subject to being "smoked" by downstream faults. You can comfortably leave out the fuse you've cited.
A 12AWG wire is probably too small to be a bus feeder. When wiring with truly "fat" wires (6AWG or larger) they're not at high risk for burning due to shorts or overloads. Take a look at the wiring diagrams for any single engine TC aircraft and you'll find that few if any will incorporate fuses or current limiters in these pathways. This philosophy is echoed in the FARS…
Faulted robust wires generally arc to ground and burn their faults clear. Further, they're easily installed with attention to mechanical details such that faults to ground are as probable as losing one's propeller due to bolt failure.
The Z-figures are crafted with this philosophy in mind supported by a confidence in nearly 100 years of field history. I'll suggest that none of your fat wires should be smaller than 6AWG and that protection beyond what is illustrated in the Z-figures is no-value-added weight, cost and complexity.”
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