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Alternator Fuse/CB Location?

in 912 / 914 Technical Questions 6 years ago
Hi,

I'm building a Sling 4 with the 914 and I'm trying to figure out where people (or what Rotax recommends) for the location of the fuses (or Circuit Breakers - what I'm using) for protecting the alternators. I understand that the kit company puts them in the cockpit, but that means dragging high current wires into the cockpit and that doesn't make sense to me.

I've bought 25A and 50A circuit breakers for this purpose and would rather not have to cut into my custom pre-made panel if I don't have to.

Thanks!
Craig

  • Re: Alternator Fuse/CB Location?

    by » 6 years ago

    You are correct that you do not want to run unfused wires long distances.
    An InLine, Weatherproof, ATO Blade Style Fuse Holder can be installed within 6 inches of the Regulator.
    Remember that the Capacitor needs to be installed on the Regulator (Un-Fused) side of this Fuse.

    Circuit breakers are a convenience because they can often serve double duty as a switch.
    Breakers do not trip without a reason.
    Do not reset a breaker without first determining the reason it tripped.
    This can rarely be accomplished in flight.

    Bill Hertzel
    Rotax 912is
    North Ridgeville, OH, USA
    Clicking the "Thank You" is Always Appreciated by Everyone.

    Thank you said by: Craig E Maiman
      
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  • Re: Alternator Fuse/CB Location?

    by » 6 years ago

    OK. But I could use a circuit breaker (already bought), as long as it's near the regulator as you described, right?
      
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  • Re: Alternator Fuse/CB Location?

    by » 6 years ago

    It might be difficult to properly install a Breaker in the hostile environment of an engine compartment.
    Consider the day you find yourself unintentionally flying through a rain shower.
    Is everything in the engine compartment Water resistant?

    I understand your unwillingness to retire a brand new Breaker.
    Consider this option...

    A Breaker/Fuse is used to protect the WIRING downstream from the fuse.
    Assuming you have 12 AWG wire from the battery to the panel,
    A 30A fuse at the regulator would protect the Wire running to the panel in the event it ever chaffed through to the frame.
    An additional 25A Breaker at the panel would trip before the 30A fuse in the event of a slight overcurrent condition in the panel.
    This would allow you to isolate the problem device from the circuit and reset the breaker in flight without the need to land to replace the Fuse at the regulator.

    A "Belt and Suspenders" construction.

    Bill Hertzel
    Rotax 912is
    North Ridgeville, OH, USA
    Clicking the "Thank You" is Always Appreciated by Everyone.

      
    Report

  • Re: Alternator Fuse/CB Location?

    by » 6 years ago

    Just be aware that a circuit breaker and a fuse are quite different.
    For example a 30 amp rated fuse will blow almost immediately (within a second) if 30 amps are drawn through it. And of course there are slow blow fuses.
    A 30 amp circuit breaker on the other hand may require 50 amps for it to trip immediately or be able to carry 30 amps for say 3 minutes before it trips.
    Check your individual circuit breaker specifications very carefully.
    What this means in practical terms is that you could end up with a fully established fire in your wiring before the breaker trips.

    Regards............Chook.
      
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  • Re: Alternator Fuse/CB Location?

    by » 6 years ago

    A 30 amp Fuse or Breaker carrying 30A will take from 100 hours to the 12th of Never before they open. Neither are precision devices!

    Look at the Time-Current Curves on the Data Sheets for the devices.
    https://www.littelfuse.com/~/media/automotive/datasheets/fuses/passenger-car-and-commercial-vehicle/blade-fuses/littelfuse_atof_datasheet.pdf

    An ATO Fuse carrying 135% (40A) of its rating is spec'd to take 1 second to 5 minutes to open.
    You will need to draw 300% of rating (90A) to guarantee an opening in under a second.

    The response times are highly dependent on the ambient temperature.
    A fuse located in the engine compartment will open long before an identical fuse located with cool air blowing over it.

    Bill Hertzel
    Rotax 912is
    North Ridgeville, OH, USA
    Clicking the "Thank You" is Always Appreciated by Everyone.

      
    Report
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