Bus feed protection??

springcanyon · #1 02-07-2007, 11:52 AM

Hi all,

I am following the AeroElectric Connection Z-14 schematic to wire my RV-7. Z-14 does not show wire protection (i.e. fuse-link, current limiter, etc.) for the wires that feed the distribution buses. I am told that some certified aircraft do not protect these wires at all. I am just wondering what others have done and opinions on this.

Thanks in advance,
Don Owens

rv6ejguy · #2 02-07-2007, 12:19 PM

A short in the main power wires feeding the bus would cause a serious problem given the amperage involved. I'm protecting my circuits with automotive flat fuses. Available in 75-250 amp models.

Yukon · #3 02-07-2007, 12:33 PM

I have been wrestling with that issue myself. Not a comforting feeling to know that 8 gage wire is in such close proximity to a direct, un-fused fault with the firewall. Sort of a standard in the industry, though. Fuses/breakers out of reach of the pilot are few and far between on certificated aircraft.

I think a bullet-proof firewall penetration along with consciencious wire protection and support are your best protection (but I am still considering a 150 amp current limiter on the firewall.)

f1rocket · #4 02-07-2007, 03:43 PM

With the wire sized properly (2AWG or 4AWG), protection is not necessary and thousands of spam cans are wired this way. First, the runs from the battery should be short. Second, even if it did gound, it would just drain the battery.

FredMagare · #5 02-07-2007, 04:19 PM

Any fault in the wires that lead from the battery contactor to the bus is not an issue as once the wire burns through no more current flows. (I suppose it kind of rectifies itself, although it presents a whole new realm of challenges to deal with.) Ensure the feed wire(s) is(are) sized for the capacity necessary and carry handheld back-ups for communication and navigation as required.

Fred

rv6ejguy · #6 02-07-2007, 06:08 PM

If the main + lead from the battery touches ground, you will have several hundred amps flowing and a big fire quickly. If the short is after the contactor and the contactor opens as a result, no real problem other than losing bus power.

Is the fuse more or less of a liability than having the cable touch ground? Hard to say.

f1rocket · #7 02-08-2007, 08:03 AM

You know, I don't know the answer to this question but I should. I wonder what would exactly happen if I grounded the positive lead of my Panasonic battery with a 2AWG wire. Somehow I doubt a big fire would erupt, but to be honest, I don't really know. That battery is pretty small and you'd have to really heat up a Tefzel covered 2AWG wire to burn it up.

Anyone tried this?

jcoloccia · #8 02-08-2007, 08:09 AM

If it ever shorted out, as far as I can tell the 2AWG wire simply turns into a 2 AWG fusable link. I don't think there's a whole lot in the engine compartment that's prone to bursting into flames (unless you have a leak).

Remember, if you fuse off the battery, it needs to be able to handle cranking currents (hundreds of amps). The best thing to do is probably to support the wire so that if something does break off it still can't short against anything.