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Substitute Midi Fuse instead of ANL Fuse

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user9253

Joined: 28 Mar 2008
Posts: 1921
Location: Riley TWP Michigan

Posted: Mon Jan 07, 2013 2:12 pm Post subject: Substitute Midi Fuse instead of ANL Fuse

A friend of mine, Larry, wants to use a Midi Fuse instead of an ANL to protect the battery and wires from a possible shorted alternator.  His Viking engine comes with a 40 amp alternator.  The Midi Fuse is physically smaller than the ANL and will take up less room on his crowded firewall.

Question 1: Is it OK to use a Midi Fuse for this application?

Question 2: Should a 40 or a 50 amp fuse be used?

Midi fuse data sheet: http://tinyurl.com/axeza2t

Thanks, Joe

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Joe Gores

nuckolls.bob(at)aeroelect
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Posted: Tue Jan 08, 2013 11:35 am Post subject: Substitute Midi Fuse instead of ANL Fuse

At 04:12 PM 1/7/2013, you wrote:




A friend of mine, Larry, wants to use a Midi Fuse instead of an ANL 

to protect the battery and wires from a possible shorted 

alternator.  His Viking engine comes with a 40 amp alternator.  The 

Midi Fuse is physically smaller than the ANL and will take up less 

room on his crowded firewall.

Question 1: Is it OK to use a Midi Fuse for this application?



   Absolutely. There are several variations on the

   ANL theme found in all manner of DC power systems

   that range from the 'fusible link' up to and including

   the legacy ANL/ANN style current limiters. This

   would included devices like the Littlefuse MIDI



   The data sheet you linked was for the BF1 which

   is closer to a fuse than a current limiter.

   Compare the specs found here and you'll see

   that operating times versus current for the

   BF1 fuse is asymptotic to a value close to

   its rating while the MIDI is more like a

   current limiter where the asymptote is much

   greater than the device rating.



http://tinyurl.com/by5qnop



   None the less all variations on this them are suitable

   to your task.



Question 2: Should a 40 or a 50 amp fuse be used?

   Keep in mind that the rating of an alternator

   is for worst case conditions. A cold alternator

   is capable of significantly more than rated output

   until it warms up. Hence the condition known as

   the "B-lead breaker designed to nuisance trip"

   which describes the use of 60A breakers on

   60A alternators on thousands of TC aircraft.



   You'll notice that the fusing specs for these

   these Littlefuse BF1 are not as robust as the

   legacy current limiter. I note that Littlefuse

   calls these devices 'fuses' as opposed to

   'current limiters'.  You will note too that

   the BF1 has a few parts tagged with "Note 3"

   suggesting that these parts should be used for

   short circuit protection only . . . more

   like their MIDI cousins.



   The short answer is that one would do well to

   use a substantially larger BF1 FUSE on the 40A

   machine . . . 70A would not be out of line.

   A MIDI-50 (almost a current limiter) would

   be okay too.

   Bob . . .

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user9253

Joined: 28 Mar 2008
Posts: 1921
Location: Riley TWP Michigan

Posted: Tue Jan 08, 2013 1:47 pm Post subject: Re: Substitute Midi Fuse instead of ANL Fuse

Thanks for the info, Bob.  I will pass it on to Larry.

Joe

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Joe Gores

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