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Ed Anderson wrote:
I am in full agreement with you. If you design your power circuit properly (wire sized to carry the required load plus a margin) then fusing for the equipment will ALWAYs protect the wire. Fusing for the wire will almost never save the equipment. I have seen more equipment catch on fire than I have wiring bundles. I recently had a small display (cell phone size) catch fire being powered with 9 volts - I was amazed at how well the plastic used to contain the circuitry caught fire and how well it burned.
I have long been puzzled by the save-the-wire sizing of fuses. If a smaller fuse is used to save the equipment then it will surely save the wire.
I believe the reason that practice came about is that when you install wiring in an aircraft (or anything) that is offered for sell to a customer, you don't necessarily know or have control over what future equipment(s) might be hooked up to it or what loads they might require. Therefore, if you fuse to the size of the wire, then damage is prevented to the wiring from any overload. So undoubtedly not a bad practice from that viewpoint.
Actually, there's another crucial factor. The truth is, fuses (or CBs) will often not even save the equipment anyway. If there's something inside the equipment that's drawing so much current it blows a fuse, that equipment probably IS the problem, and resetting it won't help (unless the fuse/CB was undersized for a nuisance trip - but that's not what's going on if we're talking about "saving" the equipment). So, except for motor loads where you EXPECT a problem (nose gear retract, for instance), as long as you don't have nuisance trips that piece of equipment is almost certainly gone, and at the VERY least is not something you're going to fix in the air, especially by power cycling.
No, instead, what's likely going in is a short somewhere, and let's face it, the MOST likely cause of a short in the subsystem feeding an EFIS is the wiring WE install, not the surface-mount technology in the EFIS assembled under controlled factory conditions. We've got a stray wire whisker, a loose connection that's just touching another, or something similar. In this case, the equipment generally isn't even at risk, except in rare cases (such as in radio transceivers that can't protect themselves from antenna/speaker wiring shorts). And you most CERTAINLY want to protect the wire, because your risk here is fire.
If you want proof, go research failure rates for EFIS devices where the failure was the device blowing a (properly sized) fuse. It's basically unheard of in well designed products.
Regards,
Chad
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