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Date: Mon, 16 Oct 2006 12:48:22 -0400
From: Ernest Christley <echristley@nc.rr.com>
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To: Rotary motors in aircraft <flyrotary@lancaironline.net>
Subject: Re: [FlyRotary] Re: What size CB or fuses.. EC 2 items..
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Tracy Crook wrote:

>  
> To reiterate more explicitly,  Do NOT size the circuit breakers 
> according to the amp load of the devices hooked to them.
>  

Tracy,

While this is correct, it is confusing for the unitiated when we're told 
to fix a problem with a device by using a larger fuse.  The problem is 
that we're given the current ratings for the devices, and we don't know 
how to fit those into the equation.  I believe I'm nearly through the 
initiation, so if I may offer some pointers that will make it go down a 
little easier (at least it made it easier for me).

As background, a fuse is just a wire that is carefully calibrated to 
burn through with a specific amount of POWER is applied.  Yes, they are 
sold with current ratings, but that is because that is what everyone 
cares about for the most part.  A 10A fuse can take a 100A spike, if the 
duration of the spike is short enough (1).  When you dig deeper, you 
find that there are different types of fuses that are rated according to 
how fast they burn up.  This is a nod to the fact that they are power 
limiters and not just current limiters.  A wire rated for 10A will not 
melt into a liquid and flow out the bottom of the airplane the 
millisecond 11A is applied.  The more likely scenario is that it will 
slowly heat up till the insulation melts.  Then it will short out to a 
metal ground, suck down 100A, melt to liquid in a few milliseconds and 
flow out the bottom of the airplane.  I jest, but the key point is that 
a wire is also a POWER limiter just like the fuse, except that the wire 
isn't contained in a nice protective package when it melts.

Now we get to that expensive piece of avionics that we want to protect.  
Since the fuse is a protection device, we just know that it needs to be 
rated to protect that big investment.  The idea that is missing is that 
we pay up to $10,000 for some pieces of avionics.  If the designer 
didn't spend $5 to protect it against the reasonable mistake of someone 
sticking in the wrong size fuse, someone deserves a refund.  So we don't 
really need to 'protect' the device;  just keep the power wire going to 
it from smoking if the device does go bad.

Now, the ultimate protection for a 10A wire would be a 1A fuse.   But 
our device needs 5A, so a 1A fuse would burn out as soon as the device 
is turned on (2).  If the device draws a nice quiet 5A, with no spikes, 
anything between a 5 and 10A fuse would work.  Very few devices are that 
quiet.  Most will suck down more electrons when first turned on, or when 
some system event occurs, such as a radio when keying the mic.  Motors 
are more prone to these surges than solid state devices, but it happens 
everywhere.  If you went with a 5A fuse for a 5A device, it would blow 
every time you turn the device on.  If you went with a 10A fuse for a 
10A wire, there would be a race condition as to which would burn up 
first.  For the most part, this is not the sort of game we want to be 
playing.  A much safer choice for a 5A load on a 10A wire would be a 
7.5A fuse (3).

My 'engineering' process, when there is no manufacturer recommendation, 
is to determine the current requirements of the device, and whether it 
has significant surge requirements.  Use that information to choose a 
reasonable wire size that provides significant headroom.  I could use 
double-aught battery cable to wire a 5A PM motor just to be safe, but 
18AWG might save a pound or so and will do for 20A (4).  Since it is one 
of those "noisy" motors, and I don't want to be replacing a fuse every 
time it strains a little (5), I'd go for a 10A fuse.  15A would work 
just as well.  In fact, I might choose 15A if the motor were a hydraulic 
drive for gear retraction.  I would expect such a motor to experience 
more surges and strain much more than something like an electric fuel or 
water pump.  If I went with 15A for both types of motors, I could expect 
a lot of happy flying without problems.  It's just a matter of finding a 
comfortable spot somewhere in the middle of the headroom.  Keeping it 
closer to the wires limit will mean that the device has more room to 
play in.  I might also use 18AWG to wire something with a very small and 
quiet current requirement like the EC2.  While not technically correct 
thing to do, it wouldn't make baby Jesus cry either (6).  It follows the 
honorable engineering imperitive "that's what I had on the rack".  In 
this case, I might only need a 1A fuse, but I could still use a 15A if 
that is all that I had on hand.  As above, everything would work, and 
I'd never expect to have a problem from it.

To reiterate more explicitly,  Do NOT size the circuit breakers 
according to the amp load of the devices hooked to them.  You're only 
protecting the wire, while giving the device hooked to it room to play 
in.  The more room you give the device, the less fuse panel headaches 
you'll encounter.


(1) for some undefined value of short
(2) actually, a couple hundred milliseconds AFTER it is turned on, but 
who's counting
(3) I think the standard sizes are 1A, 3A, 5A, 7.5A, 10A, 15A, and 20A, 
though nearly any size can be had
(4) I think that is the right number, but I always check my wire chart 
before settling on a specific size.  I could memorize the chart, but why 
bother.  I have it taped to my wire rack.
(5) PM (permanent magnet) motors suck more current to maintain their 
speed...to the point of happily burning themselves up
(6) I'm sorry.  I watched that stupid Will Farrell NASCAR movie, and 
I've been infected.


-- 
         ,|"|"|,              Ernest Christley       |
----===<{{(oQo)}}>===----    Dyke Delta Builder      |
        o|  d  |o        http://ernest.isa-geek.org  |