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electrical system circuit protection
I'm getting near the point of adding the electrical system to the 601XL I'm building. I would be interested to hear some commentary on the various forms of circuit protection which can be used. I've identified three candidates (but there may be others): fuses, circuit breakers, and the so-called self-resetting fuses (see page 71 in the October Plane and Pilot for a description).
My own thinking is that the self-resetting fuses are cheap enough by themselves, but by the time I put them in some sort of protective housing, and add associated circuit elements, the price approaches that of circuit breakers. Plus, it requires extra fabrication and work. They also don't have a lengthy track record in aircraft applications.
Circuit breakers have a higher initial cost, but don't require spares (compared to fuses). They also weigh more than fuses.
Fuses appear to be the cheapest and lightest approach, but they require carrying spares. From a resale point of view, they also don't have the "pizzaz" of the other two approaches.
I suspect this exchange will be equally applicable to the 701, 750, 801, etc etc. I'll ignite the potential for controiversy by saying that I'm leaning slightly in the direction of fuses, although that has changed almost on a daily basis.
My own thinking is that the self-resetting fuses are cheap enough by themselves, but by the time I put them in some sort of protective housing, and add associated circuit elements, the price approaches that of circuit breakers. Plus, it requires extra fabrication and work. They also don't have a lengthy track record in aircraft applications.
Circuit breakers have a higher initial cost, but don't require spares (compared to fuses). They also weigh more than fuses.
Fuses appear to be the cheapest and lightest approach, but they require carrying spares. From a resale point of view, they also don't have the "pizzaz" of the other two approaches.
I suspect this exchange will be equally applicable to the 701, 750, 801, etc etc. I'll ignite the potential for controiversy by saying that I'm leaning slightly in the direction of fuses, although that has changed almost on a daily basis.
Replies to This Discussion
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Permalink Reply by Bob Pustell on
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I am partial to fuses, myself. If a circuit protection device opens up in flight I have no intention of making it active again by resetting a C/B or replacing the fuse, or whatever. You can do without that device until you land. Many is the airplane that has burned up in the sky because the crew reset a circuit protection device. It tripped for a reason, wait until you are on the ground to play with it.
Because I have no intention of repowering a tripped circuit in flight, I plan to use the simple, inexpensive fuse. As far as the hassle of having spare fuses, not a big deal. Build a little storage compartment or just install a few fuse holders that have no wires attached to them and use those to hold the spare fuses. Just be sure to lable them clearly as spares. You do not need to have a spare fuse for every fused circuit, only a spare fuse for each amperage of fuse in your system. In other words, you may have five 15 amp fused circuits, but you only need one spare 15 amp fuse. You can carry more if you feel like it.
If you must go with Circuit Breakers, I would suggest the "switch ciruit breaker" which combines the toggle swirtch and the C/B in one unit. They cost a little bit more but they save you the cost and the weight of a switch.
I would stay away from the self resetting devices. They defeat the purpose of having circuit protection, in my opinion. As I said at the start of this post, if a circuit protection device shuts down a circuit in flight I want it to stay shut down.
I will be interested to see some other inputs on this topic.
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Permalink Reply by Jim Belcher on
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I largely agree with your comments about fuses, although I'm waiting until I either get a bunch more ideas from fellow-travelers, or it becomes apparent the ideas are exhausted.
The one comment I have concern about is the number of spare fuses required. FAR91 discusses fuses in section 91.205. Unlike FAR23, which applies only to certificated aircraft, FAR91 applies to all aircraft. It's a set of operating requirements, not a specification for the design/construction of aircraft.
Perhaps interestingly, it separates fuse requirements into VFR day (91.205(b)), and VFR night (91.205(c)). If an aircraft is to be used solely for VFR day, part 91 has no requirement for spare fuses.
From this, although I don't think having no spare fuses would be wise, since LSAs are day VFR only, there is no requirement for spare fuses in an LSA aircraft. The exception would be a factory built or ELSA, where the manufacturer has already decided about fuse sparing.
I have installed landing lights and nighttime illumination. While I plan to fly the aircraft as an LSA, I don't want to rule out night flight for myself or some future owner.
FAR 91.205(c)(6) states: “One spare set of fuses, or three spare fuses of each kind required, that are accessible to the pilot in flight.”
In other words, FAR91 requires, for night flight, that there be a full set of spares, or, if the same kind of fuse is used more than three times, it is only necessary to have three fuses of that kind. This is very similar to the sparing requirements the government imposes on other things, like some commercial radio equipment.
Unless there is some FAR of which I'm unaware that obviates this requirement, the number of spare fuses can be significant. I think it's overkill, but I'm just a taxpayer. :-)
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Permalink Reply by Phill Barnes on
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Hi Jim
I used the thermal breakers from Jabiru which ranged from $6 - $8 AUD each direct from jabiru. I used them because they were economical to buy, simple to install and most importantly, easy to notice if they have triped from a red tag and easy to reset. Another good point is that Jabiru have been using them for years and continue to do so. I hope that I have been of some help.
Regards
Phill Barnes
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Permalink Reply by Stephen R. Smith on
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Its useful to note that electrical gear has been changing. Current draw for most modern avionics is way down from years ago. The remaining power hogs are the lights. If you use LEDs in place of incandescent bulbs even these power hogs can be tamed. Lower power, lower current, safer circuits.
I agree resetting breakers and replacing fuses without finding the cause is an act of ignorance or desperation especially in the air in a light-sport experimental plane.
I Chose the EXP BUS because it got the job done with the least amount of effort. It has worked perfectly and I would not hesitate to use it again.
To protect against fire one should concentrate on high-current circuits. The battery is by far your most dangerous source of current followed by the alternator. Put an appropriately small (low current) fuseable link at the battery and put a fuse on the alternator's output.
Use fuse holders of good quality. I had the fuse holder on my alternator circuit melt down in flight. The fuse did not blow - the holder melted into a puddle of plastic.
Steve
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Permalink Reply by Tom Blejwas on
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I use Bob Nuckolls' "The AeroElectric Connection" as my bible for anything electrical. He recommends the fuses, which I am buying from B&C Specialty Products. I'll also use a couple of Tyco W31 switch circuit breakers. Like Bob Pustell, I do not plan to try to change fuses in the air. By the way, I have e-mailed B&C and Bob Nuckolls and received very helpful replies.
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A point of clarification: If I correctly understand the FAA, circuit protectors are there to protect the wiring, not the boxes. Or at least, that's what I was told in A&P class about 20 years ago.
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I haven't gotten any additional comments for a couple of weeks, so I think decision time is upon me. For my purposes, I'll go with fuses in good quality holders. I don't think I'm going to do any better for size, weight, and reliability. There's a pretty good write-up on electrical systems in some of the late Tony Bingelis' books. I'll probably clone one of those approaches.
One thing I note when looking at this books is that we need a lot more circuits now, but they mostly each use less current.
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Permalink Reply by David Gallagher on
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Jim,
I also have followed the Nuckoll's approach. I used fuses on everything except for wound motors, which could pop a breaker/fuse due to an overload condition not related to a product failure or short circuit. On my 601XL, this meant breakers for the starter, trim, flaps and alternator. The alternator is the only one that has ever popped. For some reason, it will pop on maybe 1-in-20 engine starts. I simply reset it and all is well.
I used the B&C fuse holders on a plate hinged below the passenger side of the instrument panel. Zero problems so far with my electrical system.
Good luck,
Dave G.
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You have a good point. Motors have a very heavy "draw" current initially - driven by whatever the resistance of the winding is. Once they're up and running, they usually put out enough reverse emf that the current draw drops substantially. Unless, of course, the motor is "hanging," in which case we really want the circuit to open.
Maybe I need to rethink this, and put the heavy draw things like motors on breakers, since they're far more likely to "pop." I've lost count of how many starter fuses I've replaced or breakers I've reset over the years. I don't recall ever replacing a fuse or having a breaker reset on a radio. The internal fuse usually goes first.
This is the kind of commentary I was looking for. Thanks!
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Permalink Reply by Patrick M. Stafford on
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Hey Jim -
You might want to consider the Vertical Power system. I'm a beta tester for the VP-200 system and it works great. They have less pricey systems as well.
Pat Stafford
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I'm not familiar with the vertical power system. What is it? How new is the technology or implementation of technology?
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After my earlier reply, I did a search on Vertical Power system. Assuming the correct URL is http://www.verticalpower.com, this is too much of a high end system for my simple aircraft. It's probably a better match for a more complex aircraft.
My concerns are as follows, based on the bottom-of-the-line unit:
1) This is a two box implementation. I'm not sure the control box would gain me any panel space, and I'd need to find a home for the second box.
2) The cost seems high (~$1,500) to replace a handfull of fuses and 3-4 circuit breakers.
3) If this thing fails, it sounds as though I might not lose just a single electrical circuit, but all the electrical circuits.
4) There does not appear to be a significant field history, which means there may be (and probably are) problems yet to be uncovered. My experience has been that it is best not to install new generation equipment unless it solves a real problem not otherwise solvable. Or, since it is an experimental aircraft, if one really wants to experiment with the technology.
5) Although I scanned the installation manual, I was unable to learn the weight impact of the unit. I don't know if I gained or lost installed weight over fuses and circuit breakers. My guess is that it is heavier.
My conclusion is that, if I knew nothing about electrical wiring, this might be a way to get electrical protection without that knowledge. But fuses and circuit breakers are not complex items to wire, and take minimal technical knowledge to size. I find myself wondering what problem this is intended to solve.
My comments should not be taken as too much of a diss of the system. I simply don't see how it meets my needs. Thanks for the reference; I'll keep the device in mind. One never knows when there may be a need for something in the future on a different aircraft.
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