Quote:
Originally Posted by rv7charlie
Just to be clear, Nuckolls' writings/drawings include a protection device at the battery end of the alternator B-lead.
The 'fat wires' (post 18) mentioned are the very heavy gauge lines from the battery to the main contactor, and to the starter.
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My understanding is:
The lead from the battery contactor to the main bus is a long "fat" wire.
The starter contactor is there to disconnect in case of a problem downstream from it.
One of Bob Nuckols' stories:
"Bottom line is that you're many, many times more likely to have a bad day in the cockpit for reasons far removed from a hard ground fault on your 6AWG bus feeder wire. . . and THAT because you didn't conduct due diligence in its installation.
In the case of the bus feeder, the risks are not so much to the wire as to the thing the wire touches.
Case in point: C90 twin turboprop on short final experiences disconnect of elevator cables. Pilot uses trim commands and power to execute go-around, assesses the condition and successfully lands the airplane with rudder, trim and power.
Pulling up floorboards in the cockpit revealed a 40A protected feeder to the windshield de-ice inverter had been mis-positioned against the elevator control cable during a maintenance operation. Over what had to be many hours of operation, motion of the cable wore through the insulation bringing the cable into contact with the hot wire. The arcing and sparking was of insufficient intensity to come to attention of crew in spite of the fact that it was going on virtually under their feet.
The copper wire was barely damaged. The breaker never popped while the elevator cable eventually eroded through and parted. Compare thermal properties of copper versus steel . . . this explains why the best steel safes have intermediate layers of copper in their construction. It's EASY to burn through steel . . . next to impossible on copper.
This narrative explains the high order probability that even if you DID get your 6AWG feeder faulted to ground, it's most likely to be a soft fault that burns a hole in your airplane while doing little damage to the wire . . . and certainly far short of getting it to smoke and/or open a fuse/breaker.
Adding 'protection' to this pathway doubles the number of joints in the pathway and adds nothing demonstrable in terms of fault response . . . which is why the spam-can builders don't do it either.
... Bob Nuckolls ..."