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Wire Protection Question

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petehunt(at)earthlink.net
Guest

Posted: Thu Jun 18, 2009 11:41 pm Post subject: Wire Protection Question

I am rebuilding the "Fat Wire" portion of my purchased RV-6A, moving the 

battery and contactors to the engine side of the firewall.

Fig Z-11 does not show any protection on the 6AWG wire going from the 

Battery Contactor to supply the Main Power Distribution Bus. In my case, 

I am using a 12AWG wire, which passes through the firewall. Seems to me 

this wire needs protection of some sort, such as a fusible link, or an 

in-line fuse on the engine side of the firewall.



Looking for suggestions/recommendations, thanks!



Pete Hunt

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nuckolls.bob(at)aeroelect
Guest

Posted: Fri Jun 19, 2009 5:53 am Post subject: Wire Protection Question

At 02:31 AM 6/19/2009, you wrote:

  	  Quote: 	
		  --> AeroElectric-List message posted by: Pete Hunt <petehunt(at)earthlink.net>



 I am rebuilding the "Fat Wire" portion of my purchased RV-6A, moving the battery and contactors to the engine side of the firewall.

 Fig Z-11 does not show any protection on the 6AWG wire going from the Battery Contactor to supply the Main Power Distribution Bus. In my case, I am using a 12AWG wire, which passes through the firewall. Seems to me this wire needs protection of some sort, such as a fusible link, or an in-line fuse on the engine side of the firewall.



 Looking for suggestions/recommendations, thanks! 	


    A 12AWG wire is probably too small to be

    a bus feeder. When wiring with truly "fat"

    wires (6AWG or larger) they're not at high

    risk for burning due to shorts or overloads.

    Take a look at the wiring diagrams for any

    single engine TC aircraft and you'll find

    that few if any will incorporate fuses or

    current limiters in these pathways. This

    philosophy is echoed in the FARS . . .



 

 Sec. 23.1357   Circuit protective devices.



 (a) Protective devices, such as fuses or circuit breakers, must be

 installed in all electrical circuits other than--

 (1) Main circuits of starter motors used during starting only; and

 (2) Circuits in which no hazard is presented by their omission.

 (b) A protective device for a circuit essential to flight safety may not be

 used to protect any other circuit.

 (c) Each resettable circuit protective device ("trip free" device in which

 the tripping mechanism cannot be overridden by the operating control) must be

 designed so that--

 (1) A manual operation is required to restore service after tripping; and

 (2) If an overload or circuit fault exists, the device will open the

 circuit regardless of the position of the operating control.

 (d) If the ability to reset a circuit breaker or replace a fuse is

 essential to safety in flight, that circuit breaker or fuse must be so

 located and identified that it can be readily reset or replaced in flight.

 (e) For fuses identified as replaceable in flight--

 (1) There must be one spare of each rating or 50 percent spare fuses of

 each rating, whichever is greater; and

 (2) The spare fuse(s) must be readily accessible to any required pilot.



    In particular, paragraph 12.1357(a)(2) applies here.

    Faulted robust wires generally arc to ground and

    burn their faults clear. Further, they're easily

    installed with attention to mechanical details such

    that faults to ground are as probable as loosing

    one's propeller due to bolt failure.



    The Z-figures are crafted with this philosophy

    in mind supported by a confidence in nearly 100

    years of field history. I'll suggest that none

    of your fat wires should be smaller than 6AWG

    and that protection beyond what is illustrated

    in the Z-figures is no-value-added weight, cost

    and complexity.



    Bob . . .

     [quote][b]

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rjquillin

Joined: 13 May 2007
Posts: 123
Location: KSEE

Posted: Fri Jun 19, 2009 12:55 pm Post subject: Wire Protection Question

At 06:52 6/19/2009, you wrote:

  	  Quote: 	
		  To: aeroelectric-list(at)matronics.com

 Subject: Re: Wire Protection Question

 At 02:31 AM 6/19/2009, you wrote:

  	  Quote: 	
		  --> AeroElectric-List message posted by: Pete Hunt <petehunt(at)earthlink.net>

 <snip> 	

    A 12AWG wire is probably too small to be

    a bus feeder. When wiring with truly "fat"

    wires (6AWG or larger) they're not at high

    risk for burning due to shorts or overloads.

    Take a look at the wiring diagrams for any

    single engine TC aircraft and you'll find

    that few if any will incorporate fuses or

    current limiters in these pathways. This

    philosophy is echoed in the FARS . . .

 Sec. 23.1357   Circuit protective devices.

 (a) Protective devices, such as fuses or circuit breakers, must be

 installed in all electrical circuits other than--

 (1) Main circuits of starter motors used during starting only; and

 (2) Circuits in which no hazard is presented by their omission. 	

 Bob,

 Trying to turn this into a learning/understanding moment, the above leaves me a bit puzzled as to exactly when the omission of a protective device would be considered to pose no hazard.  Say one has an AWG-6 feeder connected to the switched side of the master relay and the conductor or a terminal faults to airframe.  Alternator current would be limited by the alternator breaker, good; but battery current would only be limited by conductor, termination, device and internal battery resistances.  At first estimate this would seem to be capable of generating a current well above the safe capacity of the AWG-6.

 While my scope of knowledge is limited to but a small spectrum of TC's aircraft, all have had some form of protection on large gauge feeders.  Specifically, for a PA-46-350, the only unprotected unswitched circuit is termed the battery bus and supplies lights, clock and ground clearance com.

 This topic is of particular interest to me as we are currently working on electrical system modifications for a Bellanca BL-17 and found the local, SAN, FSDO and inspectors have conflicting thoughts with what you have stated.

 Appreciate any additional insight you may be able to provide.

 Thanks

 Ron Q.

     [quote][b]

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bobmeyers(at)meyersfamily
Guest

Posted: Fri Jun 19, 2009 3:18 pm Post subject: Wire Protection Question

Did you miss this paragraph in Bob's message.



 	  Quote: 	
		     In particular, paragraph 12.1357(a)(2) applies here.

   Faulted robust wires generally arc to ground and
	


   burn their faults clear. Further, they're easily

   installed with attention to mechanical details such

   that faults to ground are as probable as loosing

   one's propeller due to bolt failure.

 	  Quote: 	
		  


	




Robust mounting keeps that arcing end from moving so it burns the fault clear.  You just end up with a hole were the fault was.



 Bob Meyers



Building Sonex 982SX Web Site Index http://meyersfamily.org/Sonex982.html

 



On Jun 19, 2009, at 3:53 PM, Ron Quillin wrote:

 	  Quote: 	
		   At 06:52 6/19/2009, you wrote:

  	  Quote: 	
		  To: aeroelectric-list(at)matronics.com (aeroelectric-list(at)matronics.com)

 Subject: Re: Wire Protection Question



 At 02:31 AM 6/19/2009, you wrote:

  	  Quote: 	
		  --> AeroElectric-List message posted by: Pete Hunt <petehunt(at)earthlink.net (petehunt(at)earthlink.net)>

 <snip> 	


    A 12AWG wire is probably too small to be

    a bus feeder. When wiring with truly "fat"

    wires (6AWG or larger) they're not at high

    risk for burning due to shorts or overloads.

    Take a look at the wiring diagrams for any

    single engine TC aircraft and you'll find

    that few if any will incorporate fuses or

    current limiters in these pathways. This

    philosophy is echoed in the FARS . . .



 

 Sec. 23.1357   Circuit protective devices.



 (a) Protective devices, such as fuses or circuit breakers, must be

 installed in all electrical circuits other than--

 (1) Main circuits of starter motors used during starting only; and

 (2) Circuits in which no hazard is presented by their omission. 	


 Bob,

 Trying to turn this into a learning/understanding moment, the above leaves me a bit puzzled as to exactly when the omission of a protective device would be considered to pose no hazard.  Say one has an AWG-6 feeder connected to the switched side of the master relay and the conductor or a terminal faults to airframe.  Alternator current would be limited by the alternator breaker, good; but battery current would only be limited by conductor, termination, device and internal battery resistances.  At first estimate this would seem to be capable of generating a current well above the safe capacity of the AWG-6.



 While my scope of knowledge is limited to but a small spectrum of TC's aircraft, all have had some form of protection on large gauge feeders.  Specifically, for a PA-46-350, the only unprotected unswitched circuit is termed the battery bus and supplies lights, clock and ground clearance com.



 This topic is of particular interest to me as we are currently working on electrical system modifications for a Bellanca BL-17 and found the local, SAN, FSDO and inspectors have conflicting thoughts with what you have stated.



 Appreciate any additional insight you may be able to provide.



 Thanks

 Ron Q.



  	  Quote: 	
		  



href="http://www.matronics.com/Navigator?AeroElectric-List">http://www.matronics.com/Navigator?AeroElectric-List

href="http://forums.matronics.com">http://forums.matronics.com

href="http://www.matronics.com/contribution">http://www.matronics.com/contribution



	


	


 [quote][b]

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nuckolls.bob(at)aeroelect
Guest

Posted: Fri Jun 19, 2009 10:12 pm Post subject: Wire Protection Question

 	  Quote: 	
		  Bob,

Trying to turn this into a learning/understanding moment, the above 

leaves me a bit puzzled as to exactly when the omission of a 

protective device would be considered to pose no hazard.  Say one 

has an AWG-6 feeder connected to the switched side of the master 

relay and the conductor or a terminal faults to 

airframe.  Alternator current would be limited by the alternator 

breaker, good; but battery current would only be limited by 

conductor, termination, device and internal battery resistances.  At 

first estimate this would seem to be capable of generating a current 

well above the safe capacity of the AWG-6.



While my scope of knowledge is limited to but a small spectrum of 

TC's aircraft, all have had some form of protection on large gauge 

feeders.  Specifically, for a PA-46-350, the only unprotected 

unswitched circuit is termed the battery bus and supplies lights, 

clock and ground clearance com.



This topic is of particular interest to me as we are currently 

working on electrical system modifications for a Bellanca BL-17 and 

found the local, SAN, FSDO and inspectors have conflicting thoughts 

with what you have stated.



Appreciate any additional insight you may be able to provide.


	


     The style of "protection" one might add to

     fat wires in an array of battery/bus/cranking

     feeders are like the ANL "current limiters"

     found in the power distribution systems of

     many larger aircraft.



http://aeroelectric.com/Pictures/Fuses/Fuses/ANL-ANN_Current_Limiter.jpg



     Fusing characteristics for these puppies are

     shown here . . .



http://aeroelectric.com/Mfgr_Data/Fuses_and_Current_Limiters/Bussman/ANL_Specs.pdf



     Suppose you decided to put an ANL200 in the

     battery feeder for your project. Note in

     the fusing plots above the ANL200 will carry

     300A without breathing hard.



     Now, what kind of fault can you imagine

     that ties your feeders down so firmly to

     the airframe that you'd expect battery fault

     currents of 1000+ amps to flow? You're

     going to rub off some insulation and have

     some low pressure, flying fault that

     arcs a lot and intermittently draws hundreds

     of amps . . . but is unlikely to open the

     ANL limiter. In facdt, you'll find that

     the copper is barely damaged compared to

     adjacent aluminum that simply burns clear

     without even warming up your feed wire.



     Have your skeptics research the wiring

     diagrams for the host of S.E. aircraft

     produced in the hundreds of thousands

     for nearly 100 years and they'll note

     a not so curious absence of fuses/breakers

     in the fat wire feeders . . .



     These wires seldom get faulted and when

     they do, the event is so benign that

     the current protection doesn't operate

     before arcing burns the fault clear.



     It's a lot of careful design and hard work

     to bring two conductors together to be

     an efficient conductor of hundreds of amps.

     It just doesn't happen accidently.

        Bob . . .



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

        ( . . .  a long habit of not thinking   )

        ( a thing wrong, gives it a superficial )

        ( appearance of being right . . .       )

        (                                       )

        (                  -Thomas Paine 1776-  )

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

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nuckolls.bob(at)aeroelect
Guest

Posted: Sat Jun 20, 2009 10:09 am Post subject: Wire Protection Question

 	  Quote: 	
		   First, thank you for your response to my original question.  If I am following your logic:  A fat wire, if shorted to airframe ground,  will burn away the nearby aluminum ground fault very quickly, and thereby eliminate the immediate problem.



 If that is so, then the next question would seem to be:  Why protect the relatively short B-Lead that goes to-from the alternator, as it can be easily physically protected in a similar manner to the bus feed wire. 	


   Because the fault risk there is not to the

   wire itself but for the potential of diodes

   shorting in the alternator. This failure

   would probably burn wires in the alternator.

   That's probably the least "useful" of the fat-wire

   feeder fuses. Probability of diodes shorting

   in modern alternators is exceedingly low.

   That fuse or breaker can probably be eliminated

   with little risk. Cars have never used

   this fuse. Early cars burned some wires . . .

   later ones added fusible links in the system

   to limit the amount of wire that needed to

   be replaced after a "burn". 



 

  	  Quote: 	
		   And a second question:  Why use an AWG-6 wire to supply a main bus that only has a 20 amp max load? 	




   Because to qualify as a fat-feeder with

   little use for protection, the wire needs to

   be more robust than the fault site conductors that

   would put it at risk. We protect the little

   guys because flying faults will often burn

   them . . . a distribution fat-wire unworthy of

   protection needs to be in the robust class.

   I've never seen any rules of thumb but my

   sense is that wires in the 10 to 6AWG range

   are in a grey area and I'd probably fuse them.

   A short 6AWG or larger bus feeder is probably

   good to go without specific protection other than

   extra attention to support and isolation from

   potentially hazardous mechanical damage.



 

  	  Quote: 	
		   Struggling to understand in a few weeks what you have learned in an entire career!!!!............ 	


   No hurry my friend. We've got all the time

   you need.



 

        Bob . . .



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

        ( . . .  a long habit of not thinking   )

        ( a thing wrong, gives it a superficial )

        ( appearance of being right . . .       )

        (                                       )

        (                  -Thomas Paine 1776-  )

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



     [quote][b]

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