---------------------------------------------------------- AeroElectric-List Digest Archive --- Total Messages Posted Sun 11/27/22: 1 ---------------------------------------------------------- Today's Message Index: ---------------------- 1. 09:59 AM - Re: Re: Fuse or CB for largish wires into cabin? (Robert L. Nuckolls, III) ________________________________ Message 1 _____________________________________ Time: 09:59:14 AM PST US From: "Robert L. Nuckolls, III" Subject: Re: AeroElectric-List: Re: Fuse or CB for largish wires into cabin? > >Don't put a fuse in series with the starter motor circuit. >A fuse has a certain amount of resistance which will reduce engine >cranking speed. >As for the main power bus feeder, it is normally not fused, but some >builder do install a fuse. The FAT wire feeders in light aircraft are typically never actively protected i.e. breakers or fuses. In both Part 23 and Part 25 of the FARS, starter power pathways are specifically exempt from such protection. They are demonstrably immune to hard faults that might be expected to trip active protection. A robust wire's inadvertent connection with ground is more likely to be a soft fault that burns structure rather than the wire itself. I've cited an example incident waaayyy back when where a Beech C90 on short final to New Mexico airport suddenly found the elevator mechanically disconnected from the control column. They managed to land the aircraft using pitch trim. Subsequent investigation found the elevator cables completely slack. It seems that during maintenance activity some time ago, some wire bundles under the floorboard were mis-positioned for access but not properly returned to as-manufactured. One of the wires was feeder to the copilot's windshield electric de-ice inverter . . . a relatively fat wire protected at 75A or so. This wire was being rubbed against the elevator control cables until insulation was penetrated. The intermittent arcing was so subtle that the breaker never opened . . . and nothing untoward was noticed by crew. Over a period of time the steel control cable was eroded to failure while copper wire strands were barely marked. A similar thing would happen if a rear mounted battery feeder were to become inappropriately involved with the edge of a bulkhead lightening hole under the floorboard . . . A little insulation might smell bad but subsequent electrical effects are more likely to burn away the aluminum as opposed to over-stressing the wire. This is why most FAT wires are passively protected by virtue of judicious installation and maintenance practices as opposed to breakers and/or fuses. Bob . . . Un impeachable logic: George Carlin asked, "If black boxes survive crashes, why don't they make the whole airplane out of that stuff?" ------------------------------------------------------------------------------------- Other Matronics Email List Services ------------------------------------------------------------------------------------- Post A New Message aeroelectric-list@matronics.com UN/SUBSCRIBE http://www.matronics.com/subscription List FAQ http://www.matronics.com/FAQ/AeroElectric-List.htm Web Forum Interface To Lists http://forums.matronics.com Matronics List Wiki http://wiki.matronics.com Full Archive Search Engine http://www.matronics.com/search 7-Day List Browse http://www.matronics.com/browse/aeroelectric-list Browse Digests http://www.matronics.com/digest/aeroelectric-list Browse Other Lists http://www.matronics.com/browse Live Online Chat! http://www.matronics.com/chat Archive Downloading http://www.matronics.com/archives Photo Share http://www.matronics.com/photoshare Other Email Lists http://www.matronics.com/emaillists Contributions http://www.matronics.com/contribution ------------------------------------------------------------------------------------- These Email List Services are sponsored solely by Matronics and through the generous Contributions of its members.