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Date: Tue, 20 Feb 2007 09:14:20 -0500
From: Chad Robinson <crj@lucubration.com>
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To: Rotary motors in aircraft <flyrotary@lancaironline.net>
Subject: Re: [FlyRotary] Re: CBs and fuses
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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