Do Aircraft Electrical Systems Use FUSIBLE Links???

In car world, the design of some electrical wires utilize a FUSIBLE Link of wire that is designed to conduct a great deal of electrical current up to a certain point and then after a certain threshold this short piece of link will burn through and protect the rest of the electrical system should there be a direct short. It works kind of like a fuse however instead of a removable fuse that can cause connection problems, this link is grafted into the wire and is less prone to corrosion and the associated connectivity issues.

I ask because I am buying a pusher airplane where the battery is in the nose and the alternator and engine are in the tail and heavy gauge wire connects the two units and there is no protection should there be a direct short in between.

If airplanes do not have fusible links then what kind of protection is available for these large current providing items?

THANKS IN ADVANCE!!!

 

Circuit breakers?

 

Resettable breakers are the norm on the plans and designs I have seen.

 

It would make sense to have one close to the alternator.

 

I see no reason why one would choose that approach over a circuit breaker or fuse. Proper design, installation and maintenance- which one should be doing anyhow- would likely seriously reduce any of the benefits you list for a fusible link.

 

See notes 4 and 5 here: http://www.aviator.cc/bonanza/mx/avionics/www.aeroelectric.com/articles/Rev10/z10.pdf

If you don't know who Bob is, and you are designing a HBA, you should learn, and consider all of his recommendations.


BJC

 

Yes, some people are fitting fusible links at the battery in case of a crash where battery movement could short to the airframe.

 

I use fusible links right at the alternator lugs. They are light and reliable. I also have a direct connection to the battery for my "emergency" mode on my ignition. This wire is hot all the time so it too has a fusable link right at the battery.

CB's work in many situations, but the purpose of circuit protection is to save the wiring, not the component in the circuit. You cant save the wiring coming from the alternator or battery unless the fuse/CB is located AT the source of power. And if that happens to be at the extreme end of the airplane, then you cant reset in flight anyway. It might as well be a fusible link.

 

Yes, proper design, installation and maintenance relegates the need to a "Hail Mary" event, but a fusable link is very light and very reliable. As nothing more than a few inches of wire 4AWG smaller than the main wire size, the fusible link virtually eliminates the possibility of a main buss electrical fire for essentially zero penalty in cost, weight or complexity.

 

There are usually fusable links on the firewall on most certified planes built 70's and up.

 

Really? Never seen one. Maybe you're seeing the ammeter shunt used in some airplanes; it looks like a fuse strip but it's not.

Standard aircraft practice is a master contactor (heavy relay, what we call a "solenoid") right at the battery so that the short section of the positive lead between the battery and contactor is as short as possible. The pilot can turn the master off if an electrical glitch develops. There's a heavy breaker between the alternator's output and the bus that will pop if there's a massive short anywhere, a sign that perhaps one should leave it popped and get on the ground and be ready to turn the master off at any time, too.

That contactor and its control circuitry are designed to enable the pilot to completely isolate the battery from the rest of the airplane's systems in case of electrical fire or a forced landing where metal might get torn (and torn metal rips wires) or G forces will be large. The last thing you want when fuel tanks are being ruptured are some big fat sparks or glowing wires.

 

Below is the schematic for the electrical system in Bob Barrows prototype Bearhawk and Bearhawk Patrol and the Bearhawk LSA.












If its not there, it weighs nothing, cost nothing and is 100% reliably.
Dan

 

Should look like this:



x---------------------------------------x **



**(non-specific length of 18ga wire from cockpit switch to p-lead) lol

 

But how do you protect the big fat wire between the alternator output (on the engine) and the CB (in the panel). CB's by default become "the source" and only protect stuff downstream. If you have a 60+ amp alternator feeding the CB, you have essentially 5 feet of unprotected arc welder cable running next to sharp metal, severe heat and flammable liquids. Might be fine in the days of 20 amp generators, but with the increasing electrical demands of current aircraft, the alternator itself is becoming a significant hazard source.

 

Instead of a smaller piece of wire (which actually burns away with out a protective casing), use one of these (they come in different amps) as used in several vehicles and boats I have owned:





NAPA, O'Reillys, Marinas, ect all have these with the fuse holders.

 

You can't use a fusible link at the battery because it will reduce the ampacity of the large wire to the starter, alternatively a high starter draw could blow the link. In a pusher, these large wires are not usually protected. Aside from the starter, there is nothing else in the airplane that would overheat an AWG 2 or 4 wire other than a dead short. If you suspected a short, you'd turn off the master relay which is usually close to the battery in the nose.

 

There are two protections there: (a) regular inspections of the alternator and its wiring to catch any looseness or chafing, and (b) the diodes in the alternator will blow if the loads get too high and the alternator will fail. And the wires coming off the stator to feed those diodes aren't very big and they'll let go the same as any fuse.

 

That's what I used in my Fly Baby during my electrical rebuild a couple of years back.

Previously, the airplane had the classic "in-line" fuse holders that typically come with mobile electronics. However, I've had trouble with these over the years. As the plastic ages, it gets brittle. Eventually one of the "ears" breaks off and the fuse holders come apart. Not only does this kill power to the device, it leaves a hot lead dangling behind the panel. Had that happen with both airplanes and cars.

They make single-unit blade fuse holders that can be direct replacement to in-line holders, and since the fuse isn't spring-loaded, it doesn't have the same problem. I put a piece of Velcro on the fuse holder and stick it to the back of the panel near the bottom.

I'm looking into buying or making a small fuse bank using these blade fuses.

Ron Wanttaja

 

I was thinking more of this kind of heavy duty fuse holder for the batt/starter circuit..






Sorry for the potato quality, couldnt find better..

 

Take a look at the link below.

The ANL type fuse listed is a "slow blow" current limiter that is commonly used for high-output generators as found on turboprop powered and other aircraft with high current generating devices.

DC Fuses for many applications, Marine, Automotive, Solar, and Alternative Energy.

Terry