A reader on one of the lists writes:
>The concept is that a 4 inch length of wire, at least 4 wire sizes smaller
>than the wire being protected, will work as a fusible link by heating and
>opening before the protected wire is damaged. Damage is limited to melting of
>the insulation on the fusible link plus heating of anything touching it, so
>try to isolate the link and/or insulate it with a fiberglass sleeve. Fusible
>links may be purchased at auto parts stores or made from pieces of tefzel
wire
>using crimp terminals. Fusible links are apparently used in cars to protect
>certain areas of wiring from screwdrivers dropped into the works.
I recommend that fusible links for aircraft be limited to the
24AWG and 22 AWG sizes for supplying some of the always-hot
feeds to thing like the essential bus alternate feed, electronic
ignitions in any form, electronic controlled fuel injection, and
ammeter shunts. These feeds are generally limited to 5 amps or
so max continuous demand.
I just got home from a fire-fighting trip to CA for Raytheon
and I found a good buy on silicone impregnated fiberglas sleeving
in a size 12 . . . it fits inside the insulation support barrel
on a 22-18 AWG (red) PIDG terminal. We'll be offering 24 and 22
AWG fuselinks from our website catalog as soon as I can get the
.html files revised. They will be p/n S905-xx-y where xx is
either 22 or 24 for the wire size and y is 6 for #6 screw stud,
8 for #8, 10 for #10, 25 for 1/4", 31 for 5/16", 38 for 3/8" or
50 for 1/2" ring terminal. The other end is fitted with a PIDG
butt splice for continuing on with the wire that's getting
protected.
>In my installation I wanted the breakers in the panel but this left the wires
>between the master relay and the breakers (including the panel connectors)
>unprotected. I added a fusible link to protect these wires, unlikely as a
>failure here is. Of course, since the link is about 25A and the panel
>connectors are rated at 15A per pin, a connector meltdown is still
possible if
>a short occurs in the panel....
Fuses, breakers, and/or fusible links are not needed in the location
cited . . . I have had a lot of builders suggest "master breakers"
to protect against bus shorts . . . check your installation and see
what might CAUSE the short you are guarding against. Once you find
that failure, eliminate it, don't protect against it. The trip I just
got home from was to help debug a system that was so worry-driven about
finding and annunciating every conceivable fault that system
complexity grew by 20-50 times what it should have been for this
very critical application. The end result was that the system was
over budget by 10X and unusable . . . Take care lest unjustified
worries drive complexity up and reliablity down.
>Another fusible link was added between the alternator's regulator and the
>capacitor. These links replace the high current fuses called for in Europa's
>diagram. I added fusible links on the wires controlling the master and
>starter relays for the same reason. All fusible links are on the engine side
>of the firewall (to avoid smoke in the cockpit) and are positioned to be
>easily accessed.
I wouldn't suggest that you take these out . . they don't hurt
anything but I wouldn't recommend that other folks put them in either.
Some are not needed due to absence fault that would cause them to
operate, others are not needed because your system should be designed
to eliminate the probability of faulting.
Take a look at the wiring diagrams which are downloadable at
http://www.aeroelectric.com/errata/z8.pdf If you don't find
a fusible link on those diagrams where you think one might be
useful or necessary, let's talk about it.
Bob . . .
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