Do Aircraft Electrical Systems Use FUSIBLE Links???

Discussion in 'Instruments / Avionics / Electrical System' started by HomeBuilt101, Feb 16, 2016.

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  1. Feb 16, 2016 #21

    Toobuilder

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    And compared to a fusible link, you have much more weight, footprint, expense, and because there are multiple connection points, you have multiple failure modes.

    Remember, we are not talking about replacing fuses or CB's where appropriate. We are talking about protection against wiring that has historically gone unprotected. And this protection comes at no cost, weight or size and is extremely reliable. It's as close to a free lunch as you can get in aviation.
     
  2. Feb 16, 2016 #22

    Matt G.

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    Use caution with these if the fuse is exposed in a holder. Sometimes the metal prongs protrude slightly from the top of the fuse (the part where the amperage rating is labeled) and can be shorted out if the fuse contacts a conductive object. I don't have a link handy, but there was a glider in-flight fire a few years ago caused by this happening. I have one of these directly on my glider battery, but it is inside of a rubber enclosure to prevent this issue.
     
  3. Feb 16, 2016 #23

    Pops

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    Your are correct, forgot about that. :) Thanks

    Dan
     
  4. Feb 16, 2016 #24

    Dan Thomas

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    Right. Too many connections lead to unnecessary inflight shutdowns, which is why certified airplanes don't use them to fuse alternator outputs, for example. Cessna uses an overvolt sensor to kill a runaway alternator, but that won't protect against a short between the alternator output cable and ground. Only good maintenance can do that.

    Small aircraft aren't required to have such protection, but transport-category aircraft have it. They'll have automatic feeder-fault devices at the generator and bus to compare amperage flows at each end and if there's a difference, that generator is shut down. If the cable has shorted to ground, bus current will reverse flow and the generator breaker will trip.
     
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  5. Feb 16, 2016 #25

    Aviator168

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    This is like the master circuit breaker. If one instrument/bad breaker strips it, it will take down the rest of the plane. It might even take down the ECU of the engine.
     
  6. Feb 16, 2016 #26

    rv6ejguy

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    You want a backup power source for an ECU in an airplane, not only fed off the main buss.
     
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  7. Feb 16, 2016 #27

    bmcj

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    If I am correct, a fusible link is slower than a breaker. If an errant component has a runaway draw, the breaker will trip and remove power from the device before the link reacts. The point of the link is to protect ALL wiring in the event of a short... it burns the link before any wiring has a chance to ignite; the fault need not be at the component level, it might also be in the wiring itself before it ever gets to a breaker panel. Chances are, if you have a failure bad enough to burn the link, you have a situation that cannot be remedied in the air, so it is better to have lost all electrical power than to contend with an inflight fire to which there may not be a manual shutoff option (magneto ignition is on it's own circuit, so the engine will still run).
     
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  8. Feb 17, 2016 #28

    SVSUSteve

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    I stand corrected. Plus with Ross' suggestion, I am going to have to look into that. Thanks guys. I always learn new stuff from you guys.
     
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  9. Feb 17, 2016 #29

    Midniteoyl

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    http://www.homebuiltairplanes.com/f...cal-systems-use-fusible-links.html#post311411
     
  10. Feb 17, 2016 #30

    Midniteoyl

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    Do you not still have multiple connections with a fusible link? ;)
     
  11. Feb 17, 2016 #31

    Toobuilder

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    You will add one permanent connection between the link and the main wire. This will be properly crimped and environmentally sealed to aviation standards. It will be as reliable as the wire itself.
     
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  12. Feb 17, 2016 #32

    Toobuilder

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    Exactly. In my case with a single battery, I still have redundancy in my ignition because the "normal" source of power is off the main buss, provided by the alternator. If all hell brakes loose and I need to shut the whole system down from the main contactor forward, I still have the always hot single wire connected directly to the battery, switched by my "Emergency" ignition power switch which sends power directly to the ignition, bypassing even the CBs. It is a selective "hotwire" connection in case I need to bypass every possible source of equipment malfunction. It wont fix a dead short, but then again, that can take down any airplane.
     
  13. Feb 17, 2016 #33

    cluttonfred

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  14. Feb 17, 2016 #34

    AdrianS

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    I have seen a competition car char a heavy wiring run because something inside the alternator went dead short to the case, and there was no fusible link between alt. and battery.
    Fire avoided because the driver smelt "electrical smoke" and pulled the main isolator.
     
  15. Feb 17, 2016 #35

    Mad MAC

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    If a 2 AWG wire grounds on the edge of sheet metal say at the fire wall penetration, it is likely that it will not blow the fuse and will instead spark erode the the offending metal. So it also pays to take great care with routing and edge protection.
     
  16. Feb 17, 2016 #36

    Dan Thomas

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    In an airplane, the alternator output breaker would have killed it for him. The alternator feeds the bus via the breaker, and the battery is also connected to the bus via the master contactor. A dead short inside the alternator sends battery current backwards through the breaker and pops it.
     
  17. Feb 18, 2016 #37

    HomeBuilt101

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    I m glad I asked!!!

    I now know who "Bob" is and I am now officially a Bob Stalker!!!

    REALLY GOOD information on Bob's site. I am currently plowing through and devouring all he has on his site...Fusible links included!!!

    THANKS everyone for your help...This is a steep learning curve for me going from CarLand to HomebuiltLand...very educational and fun in the process!!!
     

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