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Main bus wire feed

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stevesrv7(at)gmail.com
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Posted: Sun Jul 16, 2006 7:53 pm Post subject: Main bus wire feed

I was reviewing the Z diagrams in Bob's book and am curious about the circuit protection coming into the cockpit from the battery contactor. There is an ANL fuse shown between the alternator and the batt contactor, but there is no fuse on the (typically) 6 or 8 ga wire coming from the batt contactor into the cockpit. What is the reason to not protect such a large wire as it goes through the firewall? I'm sure Bob has thought this through, I just don't know the reasoning behind it and am hoping someone here might know.    Thanks. Steve

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nuckollsr(at)cox.net
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Posted: Mon Jul 17, 2006 5:11 am Post subject: Main bus wire feed

At 09:51 PM 7/16/2006 -0600, you wrote:



 	  Quote: 	
		  I was reviewing the Z diagrams in Bob's book and am curious about the 

circuit protection coming into the cockpit from the battery contactor. 

There is an ANL fuse shown between the alternator and the batt contactor, 

but there is no fuse on the (typically) 6 or 8 ga wire coming from the 

batt contactor into the cockpit. What is the reason to not protect such a 

large wire as it goes through the firewall? I'm sure Bob has thought this 

through, I just don't know the reasoning behind it and am hoping someone 

here might know.    Thanks. Steve


	


   What risk is to be mitigated by adding such

   protection?



   There are numerous instances of "exposed" or seemingly

   risky situations in configuring hardware. For example,

   there's an invisible, fast moving, deadly body basher

   on the nose of your airplane that will turn you into

   hamburger in a heartbeat. Why don't we put a wire cage

   around it to protect against such events?



   The cost-benefit ratio study says that operationally

   we can conduct due-diligence in making sure fragile

   things don't get into the prop and therefore, the

   weight and performance losses for protecting it are

   of no practical value.



   Okay, apply the same thinking to the piece of "un

   protected" wire you've identified. Get out your hammer,

   crowbar or any other favored instrument of force and

   then look over the wire and it's terminating bus installation.

   See if you can identify any piece of the airplane that

   even MIGHT come against it such that there is risk

   of a fault.



   I believe you will find (as have many who have studied

   this problem before us) that there are no risks associated

   with leaving this wire unprotected beyond observing

   due diligence in supporting the pieces so that vibration

   or other motion does not compromise insulation.



   Study the wiring diagrams for any production aircraft

   and see if builders of those airplanes deduced any

   risks that warranted the protection you've cited.

   It's a good question. The answer is, "Risks associated

   with this practice have been deduced by analysis and

   proven by field experience to be insignificant."





        Bob . . .

      ---------------------------------------------------------

      < What is so wonderful about scientific truth...is that >

      < the authority which determines whether there can be   >

      < debate or not does not reside in some fraternity of   >

      < scientists; nor is it divine. The authority rests     >

      < with experiment.                                      >

      <                            --Lawrence M. Krauss       >

      ---------------------------------------------------------

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