Fuses or Circuit Breakers?

(printed in Sport Aviation, March, 1993)

Fuses and circuit breakers are equivalent devices with respect to function; not so with respect to cost. Miniature, high quality circuit breakers start at $17 each; switch/breaker combinations will cost twice that much. The lowly fuse costs about twenty-five cents; its holder sells for under two dollars. All things considered, the fuse represents an excellent cost/performance ratio alternative to circuit breakers.

Fuses were widely used in general aviation aircraft up through about 1965. They began to take a bad rap when they tended to be flaky after a few years in service. Actually, fuse holders were most of the problem. Fuse holders then used large area, low pressure contacts to both hold the fuse and carry current. This type of contact is subject to precipitous corrosion exacerbated by moisture and current induced heating. Fusible elements of modern, low voltage fuses are integral with the contact blades mated in turn to high pressure, gas-tight contacts. They promise low cost, reliable alternatives to circuit breakers.

A number of vendors have introduced cute, very small, low cost circuit breakers for commercial applications. At this time, I don't think I can recommend them for use in airplanes. The reason is simple: a breaker is a fairly complex, electro-mechanical device. Airplane interiors experience punishing cycles of humidity, temperature and vibration which degrade breaker performance over time. The stone-simple fuse is much more impervious to such environmental abuse. If you do use breakers, use good ones.

I've observed much concern during discussions with my readers (also during forums at Oshkosh) about being able to quickly amd conveniently reset a tripped circuit breaker in flight. After all, at one time or another, we've all experienced a "lights out" event which was rectified by resetting the appropriate breaker. A major difference exists between homes and airplanes. Except where a breaker feeds a single device such as a stove or air conditioner, branch circuits in your home's breaker box are subject to widely variable loading. I've surveyed many a breaker box load distribution. It is not uncommon for most breakers to be very lightly loaded and one or two loaded to within milliamps of tripping. Branch circuits in your automobile and airplane are load specific; each branch is designed to carry a predictable load. One may be confident that the breaker (or fuse) will remain closed except for unusual events.

Two circuit failure modes predominate in power distribution systems. (1) The circuit becomes open and the system simply fails to operate and (2), some failure causes excessive current perhaps exceeding the capability of wires. With the former, current drain drops to zero; failure may have occurred inside a device being powered (motor brushes worn out, lamp filament burned through) or some part of the wiring opens (broken terminal or disconnected plug). With the latter again, fault may lie within the device being powered or perhaps compromised insulation on a poorly supported wire has created a short to ground. The potential for high current faults dictate use of a circuit breaker or fuse to prevent damage from precipitating further.

Now, let us suppose some failure has caused a circuit breaker to open. What is the likelihood of getting that system back on line by resetting a breaker? In airplanes I'd say it was pretty close to zero. Whoa! Some of my forum attendees have experienced situations where resetting a popped breaker did indeed get the system back. Some have reported it happening several times. The astounding observation here is that the reason for the first malfunction wasn't researched and fixed! It is certain that the circuit which fed the system is marginally sized or the breaker is faulty.

Remember, circuit protection is there to protect wires. If you made a super deal on 10 amp breakers, it would be perfectly acceptable to wire nearly EVERY branch in your airplane with 18 AWG wire and protect each with a 10amp breaker. The only requirement is that the breaker and wire be big enough; there's no prohibition for being two or three times too big.

This begs the question . . . Why use up all the precious panel space with rows of circuit breakers (or fuses)? Why, indeed. The only answer I can think of for that is, "Because that's the way we've been doing it . . . for a long time." I can tell you that a goodly number of breakers in the Piagio P-180 are in the tail of the airplane, well out of reach!

Future articles will address failure mode effects analysis techniques for helping you make decisions about whether a fuse (or circuit breaker) really needs to be on the panel. In the meantime, give serious consideration to the use of fuses as primary branch circuit protection in your next application. I perceive a certain elegance in their simplicity and modern variations have made them quite serviceable. Further, mounting a fuse block in a relatively handy place OFF of the panel is an option to consider.