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prkaye

Well Known Member
My planepower alternator is currently wired thruogh a 5A fuse. I want to change this to run through a 5A panel breaker instead. By far the most convenient way for me to implement this change will be to keep the existing wiring through the fuse block and just add the breaker in series with the alt switch. But I need a fuse in the fuseblock for continuity. My idea is to plug a 10A (or larger) fuse in (just as a "blank", to provide an electrical connection in the fuse block). The circuit protection will be provided by the breaker, but I want to be sure that teh breaker will always trip before the fuse (so that I never have to replace that fuse).
Can I be sure that the 5A breaker will open before the 10A fuse will blow? This would depend on how fast the current might rise during a fault, and also on the relative speeds at which breakers open and fuses blow. Will the 5A breaker in series with a 10A fuse prevent that fuse from ever blowing?
 
Ever

prkaye said:
Will the 5A breaker in series with a 10A fuse prevent that fuse from ever blowing?
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Ever is a long time. If you are adamant about doing it this way, put a very large slo-blo fuse in the slot. But I think the better thing to do would be to disconnect the wires from the fuse block and tie them together (solder, good crimp, etc.) Not quite as easy, but from a troubleshooting point of view, I envision someone in the future beating their brains out trying to figure out this issue. If you should go the large fuse route, update the documentation.

I really have no idea what AC 43.13 says about this, if anything.

Don
 
Phil,

I don't see a problem with your plan. That 10amp fuse will hold. I am sure there are lots of "well, what if....". I'd probably do the same thing.
 
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Will the wire from your fuse block to the CB be protected appropriately ?
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Same as it would if the wire were going directly to the switch. As I said, the CB is providing the protection... the larger fuse is basically serving the purpose of a wire splice.

Another failure point in the future when you least expect it.
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A fuse or CB is an intentional point of failure. If a high current was going to cause that fuse to break, I would certainly WANT either it or the CB to open up. Preferable if just the breaker trips, so that it can be easily reset at the panel. If both the CB and the fuse open, it's no worse than if i just had a single 5A fuse in there... just means I'd have to replace the fuse.

Unless you're suggesting that a large fuse might fail spontaneously, for a reason other than a high electrical current? But in that case, it's no worse than just having the 5A fuse I have in there now (with no CB). Lots of people use fuses for circuit protection, and so I would suspect that spontaneous breakage of an installed fuse (at normal current levels) is pretty rare.
 
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Really splitting hairs but if the wire shorted between the fuse and the CB the upsized fuse may not blow at the appropriate load . You may have to upsize that wire to be suitable for the 10 A fuse.

Marc
 
Marc makes a good point here, and I dont think its splitting hairs at all. Fuse blocks serve as a bus and also provide protection for the wires between the bus and the various devices. Changing the location of the built in weak link will change what is being protected - you are defeating the purpose that the fuse is there for.

erich
 
I am not suggesting that the larger fuse would blow. But a fuse in a fuse holder is not as good as a properly made splice. Fuses have been known to fail with time and corrosion in the connections to the fuse. Why install multiple failure points when doing it properly should be the prime objective?
 
if the wire shorted between the fuse and the CB the upsized fuse may not blow at the appropriate load . You may have to upsize that wire to be suitable for the 10 A fuse.
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Ahhh... now i see. So suppose I bypass the fuse entirely, and go straight from the battery to the CB... then I would still have to use big wire for this (battery to CB), in case this wire from the battery shorts before the CB. Is it normal to have the alternator field on an "always-hot" circuit? What is the typical way of wiring this (alternator field through a breaker)?
 
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prkaye said:
Is it normal to have the alternator field on an "always-hot" circuit? What is the typical way of wiring this (alternator field through a breaker)?
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No, it is not normal.

You should have the main contactor in there.
 
Oh yes, of course I do have it through the main alt switch, but to my bat bus, not my main bus. I guess "always hot" was a poor choice of words there... I just meant that it's on a bus that is always hot, but yes of course through the alt master switch.
Since I use my fuse blocks as busses, if I want to use a fuse instead for my alt field, what are my options? Run another big wire right off the battery to the fuse? I kind of like my plan of running the fuse in series with a 10Amp fuse to avoid having to make another fat-wire connection at the battery. Of course a good point was made earlier that I'll have to size the wire from the 10Amp fuse to the breaker according to 10 amps.
 
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A 5A breaker in series with a 10A fuse is not a good idea

Always obey the K.I.S.S. principal. Use one or the other, but not both. I used a 5A combo switch/breaker. There are instances when it is necessary to cycle the alternator on and off.

Here is my reasoning, a 5A breaker does not immediately open at the rated current. Usually it takes a few extra seconds and a few more amps for the resistance heater inside the breaker to heat up enough to trip the breaker. Also a 10A fuse does not always open at exactly 10A. So, if you have a current surge above 5 amps, the fuse, or the breaker, or both could open. Now you have just lost your alternator current and are left without a simple or easy way to turn it back on.
 
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I used a 5A combo switch/breaker.
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Ok, but as was pointed out before, if you want to use a CB you have to run really fat wire (16 or even 12 gauge??) from the battery all the way back to the CB on the panel, in case this wire shorts to ground between the battery and the CB (in which case it would carry all the current the battery can dish out). I guess this is one of the advantages of using fuseblocks as power distribution busses. I only need to have a short fat wire from the battery to the battery bus fuse block, and the rest of the wires from there can be just sized according to the corresponding fuse. Wherever you use a breaker on the panel instead of a fuse, it seems (in light of Marc's observation above) that you'd have to run all fat wires between the power bus and the fuse (in case a wire shorts to ground between the power source and a CB). For a panel that uses many CBs instead of fuses, this would be a lot of fat wires!

How did you wire your 5A switch/breaker?
 
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Power to circuit breakers

I used a copper buss bar to supply current to all the circuit breakers. The main buss bar is attached to the master contactor by an 8 ga large wire. I used this same setup for the avionics, and acc buss. All circuit breakers are lined up and have one post connected directly to the copper bar and the other post to smaller output wires.

If you are supplying current to an individual circuit breaker, the input wire does not have to be any larger than the alt field output wire.
 
That copper bus bar setup sounds nice.

If you are supplying current to an individual circuit breaker, the input wire does not have to be any larger than the alt field output wire.
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If a wire on the battery side of the fuse (i.e. between the battery or bus bar and the fuse) shorts to ground, you get a direct path from battery to ground. I think this would probably melt an 18AWG wire, so it has to be considerably bigger (the alt field output wire is 18AWG).
 
Nuckols recommends a fusible link in all of his Aeroelectric Connection schematics for this purpose (short length of wire, 4 AWG wire sizes smaller, with a fiberglass sheath over it in case there is a circuit fault and the fusible link does burn up).

Having said this, I have seen this question come up many times. Phil, why don't you rig up a test with a fuse and CB in line and manually short it out 20 or 30 times and report the results? Simple test, conclusive results, no? That will settle this question once and for all. Using a fuse in your fuseblock is easier than fabricating a fusible link and if the CB always pops instead of the fuse, then I would say do it your way rather than Nuckols way or having (yikes) no protection up to the CB.
 
Aeroelectric Connection

prkaye said:
Ok, but as was pointed out before, if you want to use a CB you have to run really fat wire (16 or even 12 gauge??) from the battery all the way back to the CB on the panel, in case this wire shorts to ground between the battery and the CB (in which case it would carry all the current the battery can dish out).

How did you wire your 5A switch/breaker?
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Why don't you follow the Aeroelectric Connection recommendation for the alternator field circuit? Use an 18 gauge wire w/22 gauge fusible link connected by ring terminal to the input stud of your Main Buss fuse block to feed the field breaker?

Skylor
RV-8
Flying
 
fusible link connected by ring terminal to the input stud of your Main Buss fuse block to feed the field breaker
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Ah, very interesting... hadn't considered a fuseable link for that (been a few years since I read Nuckoll's). I guess doing what I originally proposed (with the 10Amp fuse followed by wire (fat enough for 10amps) to the breaker) is basically equivalent. Failure of the 10Amp fuse would be analogous to failure of the fuseable link... neither are resettable in-flight. Of course there will always be some mode of failure that will not be fixable in-flight. For a VFR RV, a local repeat offender (a former AME who has built several RVs) often says "remember this is not a space shuttle you're building".
 
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I just haven't been a fan of fusable links. In the interest of simplicity, my 5 amp switch/breaker does connect to a 5 amp fuse on the block/buss,
It's worked fine for it's two years in service. If it blows, yes I might have to replace the fuse & go without an alternator for a short period of time. All wiring is #18.

L.Adamson --- RV6A
 
prkaye said:
...Failure of the 10Amp fuse would be analogous to failure of the fuseable link... neither are resettable in-flight. Of course there will always be some mode of failure that will not be fixable in-flight.
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Just my opinion, but I wouldn't reset any breakers in flight.
 
Field Breaker Reset

apkp777 said:
Just my opinion, but I wouldn't reset any breakers in flight.
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The alternator field breaker for a Plane Power alternator (or any alternator/voltage regulator with crowbar OV protection) would be an exception to this rule, since it uses crow-bar over voltage protection which is designed to trip the breaker if there is a voltage spike.

Skylor
RV-8
 
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