Newbie question

I have a simple question about electric flight: where is the fuse (or fuse-equivalent)?
Long, long ago I learned that the purpose of a fuse (the thing that
melts when too much current goes through it, not slang for "fuselage") was to protect the wiring in a circuit, not the device at the end of the wire. That makes sense.
Now let's look at a typical setup:
Bat --- ESC --- Motor where of course there's another branch, too:
ESC --- Receiver --- Servos
Suppose that something goes wrong at the motor -- your anti-noise capacitor gets shorted, for instance. Suddenly you've got a large potential current draw at the motor, and it seems like a good chance to melt the motor wires, the Bat <-> ESC wires, or both. If the ESC has some sort of over-current protection, you're probably OK...but if not, you're headed for a fire.
This isn't entirely hypothetical, of course -- while flying with my kids, with a 700mAh Lipo powering a J250 motor, this is just what happened to me. The time between "hunh...something's wrong here... the prop isn't turning when we goose the throttle" to "there's a smell of melting/burning insulation" was about 6 or 7 seconds, tops. By the time I grabbed things and ripped them apart, I had a couple of pieces of burning insulation and an ESC that looked like a small roman candle. (Ammazingly enough, the plane -- fanfold foam -- was undamaged. Damn. Why couldn't the $.50 plane get wrecked and the $15 ESC be saved?)
So...should I henceforth solder in a fuse in the battery-to-ESC wire? Something like 7A fast-blow should do the job, since that's the top discharge rate for my 10C lipo, right? At the cost of a few grams, I'll feel a lot safer...
--John
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| I have a simple question about electric flight: where is the fuse (or | fuse-equivalent)? ... | So...should I henceforth solder in a fuse in the battery-to-ESC | wire? Something like 7A fast-blow should do the job, since that's | the top discharge rate for my 10C lipo, right? At the cost of a few | grams, I'll feel a lot safer...
Yes, adding a fuse isn't a bad idea. You'll add a bit of resistance and weight to the plane, but not much.
If you have a brushed motor, the best place for a fuse is between the ESC and motor -- that way, the BEC still provides power if the fuse blows. There is a danger that the BEC is what's shorted, and in this case this won't help, but it's rare.
If you have a brushless motor, you'll need either three fuses on the motor (one on each wire) or just between the battery and ESC. Though in this case, if the fuse blows, the plane loses all power and crashes.
Where the fuse goes really depends on what you're worried about -- will the plane survive a crash? Do you want to try to save the ESC and lose the plane, or lose the ESC and save the plane?
In my experience, there's two things that usually are what cause you to blow a fuse -- either the ESC shorts out somehow, or the motor is stuck. The latter can happen when a gear breaks, or the plane lands and the prop is on the ground and can't move, but it's still getting power.
I've had the former happen a few times. I don't know why. I also don't know if a fuse between the motor and ESC would have helped.
--
Doug McLaren, snipped-for-privacy@frenzy.com
I'm not worried about the bullet with my name on it... just the
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Not that I know much, but isn't it bad sense to put a fuse where it will disconnect the radio (thru the BEC) (and lose total control of the plane) in the event of a motor/ESC/wiring fault? A fuse between the ECS and motor is fine, but elsewhere???
I would have though protecting the radio and servos access to the battery for safety reasons would be the prime approach - ie better to lose an ESC than risk hitting someone with a totallly out of control plane
David
Doug McLaren wrote:

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| Not that I know much, but isn't it bad sense to put a fuse where it will | disconnect the radio (thru the BEC) (and lose total control of the plane) | in the event of a motor/ESC/wiring fault? A fuse between the ECS and | motor is fine, but elsewhere???
It's a tradeoff.
Between ESC and battery is easy, works everywhere, and protects everything.
Between ESC and motor only protects the actual ESC part, and not the BEC part, and if you have a brushless motor, you need three fuses, not just one (though two ought to work too.)
| I would have though protecting the radio and servos access to the battery | for safety reasons would be the prime approach - ie better to lose an ESC | than risk hitting someone with a totallly out of control plane
It depends. If it's a small foamy plane, especially if it's got a brushless motor, between the battery and ESC is probably best.
If it's a large plane, you may be better off not relying on an ESC at all, in which case it doesn't matter.
From what I've seen, most small electric planes don't even have fuses ...
--
Doug McLaren, snipped-for-privacy@frenzy.com
Vegetarians eat vegetables. Beware the humanitarians.
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John F. Hughes wrote:

I'd make it a 6A fuse, or maybe even 5A just to be sure. It needs to go between the batt and ESC as that ensures you're protecting all the expensive gear (ESC, RX and servos) in your plane from electrical failure.
Although, some might consider an uncontrolled plane plummeting towards the ground a less-than-ideal alternative.
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Thanks for the thoughts.

Well, I see your point...but if the other alternative is a *FLAMING" uncontrolled plane headed towards the ground, which would you choose? :-) In my case, the problem arose at the ground before launch, so it was clear that a fuse would've been a Good Thing.
--John
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