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Always hot wire to panel

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gilles.thesee(at)free.fr
Guest

Posted: Thu Jan 19, 2017 3:12 am Post subject: Always hot wire to panel

Hi all,



Could anyone point me to a text or reference document mentioning the 

rule about limiting the rating of always hot battery wires such as the 

wire from battery contactor to Master Switch ?



Thanks in advance,



-- 

Best regards,

Gilles

http://contrails.free.fr

http://lapierre.skunkworks.free.fr

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gilles.thesee(at)free.fr
Guest

Posted: Thu Jan 19, 2017 6:13 am Post subject: Always hot wire to panel

 	  Quote: 	
		  --> AeroElectric-List message           posted by: GTH <gilles.thesee(at)free.fr> (gilles.thesee(at)free.fr)           

           

           

           Could anyone point me to a text or reference document           mentioning the rule about limiting the rating of always hot           battery wires such as the wire from battery contactor to           Master Switch ? 

     	
     

     Hi again,

     I believe I got them :

     FAR Part 23-1361 (b)(1)

     CS 23.1361 (b)(2)

     -- 

       Best regards,

       Gilles

       http://contrails.free.fr

       http://lapierre.skunkworks.free.fr

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user9253

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

Posted: Thu Jan 19, 2017 6:52 am Post subject: Re: Always hot wire to panel

Usually the negative side of the master contactor coil is switched.  So there is no always hot wire running into the cockpit from the master contactor.

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_________________
Joe Gores

gilles.thesee(at)free.fr
Guest

Posted: Thu Jan 19, 2017 7:21 am Post subject: Always hot wire to panel

Le 19/01/2017 à 15:52, user9253 a écrit :

 	  Quote: 	
		   



 Usually the negative side of the master contactor coil is switched.  So there is no always hot wire running into the cockpit from the master contactor.



Joe,
	




Thank you for your response.

Just to be sure, isn't this wire from the contactor coil to the panel 

switch permanently "hot", albeit through the contactor coil ?

-- 

Best regards,

Gilles

http://contrails.free.fr

http://lapierre.skunkworks.free.fr

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nuckolls.bob(at)aeroelect
Guest

Posted: Thu Jan 19, 2017 8:42 am Post subject: Always hot wire to panel

At 08:10 AM 1/19/2017, you wrote:

  	  Quote: 	
		    	  Quote: 	
		  --> AeroElectric-List message posted by: GTH <gilles.thesee(at)free.fr> (gilles.thesee(at)free.fr) 



 

 Could anyone point me to a text or reference document mentioning the rule about limiting the rating of always hot battery wires such as the wire from battery contactor to Master Switch ? 	
	


    The 'rules' pertain only to those wires which

    elevate risks during MAJOR deformation of the

    airframe.  The contractor-to-master switch

    wire is not a 'feeder' in any sense . . . rather

    a control wire. Further, current in that wire

    is limited by the resistance in the battery

    contactor coil.



    In this case, an elevation of risk would be

    driven by the probability of damage that

    takes the wire to ground . . . of course,

    this would RE-CLOSE a contactor that the

    pilot purposefully OPENED in preparation

    for the extra-ordinarily hard landing.



    Probability of this being a factor in post

    crash scenarios is exceedingly small . . .

    if things are so badly munched that the wire

    gets grounded, then consequences for having

    re-closed the contactor are irrelevant.



    The rules are not limited to feeders 'to the

    panel' . . . but to anywhere in the airframe.

    The 5A limit is more a 'rule of thumb' having

    history back as far as I can remember . . . heard

    it first at Cessna in '64.



    As a matter of demonstrable physics, an artfully

    crafted rule would limit ENERGY dissipated in the

    fault . . . as opposed to simply limiting current.

    I'd feel more comfortable about a 10A fuse off

    the battery bus than a 5A breaker. Given that

    energy rates are based on Voltage(squared), the

    hazards presented by a 5A breaker off a 24 volt

    battery are 4x those presented by a 12 volts,



    There is a HUGE range of reaction times for electronic

    breakers, magnetic breakers, thermal breakers,

    fuses, current limiters, etc. etc. Hence, the 5A rule

    is a VERY broad brush. This topic is a VERY

    tiny concern in the grand scheme of things.



    I recall a comment by a greybeard in the accident

    investigation world at Beech who once shared the

    observation, "If the airplane burns, the battery

    is likely to still be in the airplane. If the airplane

    did not burn, go look for where the battery landed

    after it was ejected."



    An interesting anecdote for which I have no

    personal knowledge. But it does re-enforce a notion

    that risks to airframe and passengers are orders

    of magnitude greater for reasons not even

    remotely associated with choices of circuit

    protection.



   



 

   Bob . . .

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