Calling all Nuckoll-heads

[PCHunt]

I am rebuilding the "Fat Wire" portion of my purchased RV-6A, moving the battery and contactors to the engine side of the firewall.
Bob Nuckolls fig Z-11 does not show any protection on the 6AWG wire going from the Battery Contactor to supply the Main Power Distribution Bus. In my case, I am using a 12AWG wire, which passes through the firewall. Seems to me this wire needs protection of some sort, such as a fusible link, or an in-line fuse on the engine side of the firewall.

Looking for suggestions/recommendations, thanks!

[amilder]

I think the reasoning is that it is short, and the circuit protection adds a failure point that increases risk beyond the safety advantage of protecting that wire.

I am a little uncomfortable with that myself and my plan was to add more anti-chafe protection and to cover the connections at both ends, since the main threat is a short to ground.

[erich weaver]

I believe Andrew is correct, but hey, why not ask the Nuckoll-head himself on the Aeroelectric List?

erich

[PCHunt]

D'OH ! Good Idea. Glad I thought of it!

[frazitl]

Industry practice doesn't use circuit protection here, unless you think of the battery contactor as circuit protection. IIRC, the logic is that the really fat wires are short, and if they do arc to ground at the firewall, they will typically burn the hole bigger in short order thus stopping the short. This bothered me too. I used no extra protection on the fat wire from the contactor to the main bus, but DID include a 40 amp MAXI fuse on the fat wire to the always hot battery bus.

[DGlaeser]

The only plane I know that provided protection for the fat wires unfortunately crashed (both pilot and pax survived with relatively minor injuries). The salvage crew did not disconnect the batteries before they attempted to move the plane, and electrical sparks started a fire which destroyed the plane (the tanks were filled just before the flight, and ruptured during the crash).
That tells me that the fuses didn't provide the protection intended, and therefore are of dubious value. If the fuses are big enough to handle the starting current, they will definitely allow the wire to burn holes in thin sheet metal without blowing.

[az_gila]

Originally Posted by frazitl

Industry practice doesn't use circuit protection here, unless you think of the battery contactor as circuit protection. IIRC, the logic is that the really fat wires are short, and if they do arc to ground at the firewall, they will typically burn the hole bigger in short order thus stopping the short. This bothered me too. I used no extra protection on the fat wire from the contactor to the main bus, but DID include a 40 amp MAXI fuse on the fat wire to the always hot battery bus.

...physical protection would be a good idea for the short section that actually penetrates the firewall.

[Bushcaddy]

and he replied as previuosly stated in this thread that due to the short run it was considered unnecessary. He did suggest additional protection on the wire.

[PCHunt]

The wire I am talking about is about 3 feet long, and goes from the master contactor on the engine side of the firewall, through the firewall, and then up to the panel row of CB's, and supplies the main power distribution bus. The wire, and the bus behind the panel, are "hot" whenever the Master Switch is on.

Is 3' considered to be a "short" wire?

And if I did want to put a fuse or a fuselink in the system, what would be your recommendations??

Thanks again!

[apatti]

All,
What about putting another ANL-60 in-line with this wire before penetrating the firewall? We do this for the alternator B-lead; why not for this wire?