From: TheWireGuy@aol.com
Message-ID: <0.acf9172.25434424@aol.com>
Date: Sat, 23 Oct 1999 13:02:28 EDT
Subject: Re: Wire insulation
To: lancair.list@olsusa.com
Mime-Version: 1.0

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In a message dated 10/23/99 9:36:00 AM Eastern Daylight Time, dfs@gateway.net 
writes:

<< As for running the wiring in a PVC conduit, probably not extremely swift,
 however, if you have properly protected ALL wire runs in your airplane with
 breakers or fuses (remember, the breakers and fuses are there ONLY to
 protect the wiring - not the equipment being connected) it should not be a
 problem. >>

You are correct, of course, regarding wire protection. You should be safe 
under normal circumstances, assuming proper breaker sizing and all. However, 
there a scenario that still haunts me. On a L-IV you have heavy gage power 
and ground wires running back to your hydraulic pump. Visualize what happens 
if, due to some sort of damage to the bundle, you get a short between the 
pump power wire and a 22 gage wire connected to ground. You coule easily have 
a red-hot 22 gage wire without blowing the large hydraulic pump breaker. If 
that red-hot wire is inside that PVC tube, well, you get the idea. Now figure 
one end of the conduit is inside the pressurized cabin, and the other end is 
in the un-pressurized baggage compartment. Now you have a lot of air flow to 
fan the flames. Did you use flammable upholstry? Now imagine you ran your 
(plastic?) oxygen system hose along side the conduit...
Such nightmare scenarios, though unlikely, warrent consideration. In my 10 
years or so working on airplanes, I've seen 3 or 4 instances of just such a 
scenario. In each case the damage was pretty much limited to the wires 
themselves because the bundle was either properly supported in an open run, 
or, as in one case was in metal conduit. In one case an upholstry screw was 
driven through a bundle and shorted some small wires to a large 
power-carrying wire, causing a couple small ones to smoke and damage other 
adjacent ones. In another case, a mechanic, while changing a pump, 
re-connected the wires wrong and smoked a wire. Fortunately, both of these 
happend on the ground. Another time, long-term exposure to hydraulic fluid 
broke-down a bunch of wires in a bundle and caused arching and blew breakers. 
There was no fire in that instance, but some systems were damaged by 110v 
power getting into low voltage circuits.
I guess I'm trying to illustrate the point that it is important to use proper 
materials and methods. It's impossible to visualize every potential failure 
scenario, but many of them have already been played out, and that's how a lot 
of standards were developed. Read AC 43.13 at least. There's some good 
information in there. Mil-5088 is another good source.
Ok, ok, I'll get off my soap box now.
Cheers,
Jim McMillan
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