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GPU - 24V System - Crowbar Needed

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ChangDriver

Joined: 15 Sep 2007
Posts: 266

Posted: Sun May 10, 2009 5:16 pm Post subject: GPU - 24V System - Crowbar Needed

Bob:



The B&C OVM-14 is a 14 volt corwbar isn't it?  For a 24 VDC system, what crowbar would I use for a GPU?  Also, wouldn't an inline fuse be helpful as the crowbar basically provides a low resistance path to ground to bypass the contactor.  Don't you need something to a fuse or breaker to protect the crowbar from excessive current?



Craig



PS - I'm working on a GPU for a 24V aircraft application.

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nuckolls.bob(at)aeroelect
Guest

Posted: Sun May 10, 2009 9:12 pm Post subject: GPU - 24V System - Crowbar Needed

At 08:16 PM 5/10/2009, you wrote:

 	  Quote: 	
		  

<capav8r(at)gmail.com>



Bob:



The B&C OVM-14 is a 14 volt corwbar isn't it?  For a 24 VDC system, 

what crowbar would I use for a GPU?  Also, wouldn't an inline fuse 

be helpful as the crowbar basically provides a low resistance path 

to ground to bypass the contactor.  Don't you need something to a 

fuse or breaker to protect the crowbar from excessive current?


	


   Yes. See



http://www.aeroelectric.com/PPS/DCPwr/Ground_Power/Ground_Power_1.pdf



   and . . .



http://www.aeroelectric.com/PPS/DCPwr/Ground_Power/Ground_Power_D.pdf



   The OVM-14 and our CbOVM-14 can be supplied in a

   OVM-28 and/or CbOVM-28 versions.

        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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woxofswa

Joined: 12 Aug 2008
Posts: 349
Location: AZ

Posted: Mon May 11, 2009 10:38 am Post subject: Re: GPU - 24V System - Crowbar Needed

Can I ask a neophyte question in the quest for further light and knowledge?



Why is a contactor, and/or switch needed for ground power?



It seems to me that the very act of plugging in the plug could serve as "activating" the power, and pulling the plug out would deactivate it.



What am I missing?

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_________________
Myron Nelson
Mesa, AZ
Flew May 10 2014

Neal.George(at)hurlburt.a
Guest

Posted: Mon May 11, 2009 11:23 am Post subject: GPU - 24V System - Crowbar Needed

Myron - 



Switching the ground power connection allows the pilot full control of the

circuit.  Over-voltage warning enhancements provide the opportunity to

monitor and verify that the lineboy has indeed plugged in 12v instead of 24v

- before you light up the internal circuits. 



Neal

===================

Can I ask a neophyte question in the quest for further light and knowledge?



Why is a contactor, and/or switch needed for ground power?



It seems to me that the very act of plugging in the plug could serve as

"activating" the power, and pulling the plug out would deactivate it.



What am I missing?



--------

Myron Nelson

Mesa, AZ

Emp completed, legacy build fuse in progress

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nuckolls.bob(at)aeroelect
Guest

Posted: Mon May 11, 2009 1:22 pm Post subject: GPU - 24V System - Crowbar Needed

At 01:38 PM 5/11/2009, you wrote:

 	  Quote: 	
		  



Can I ask a neophyte question in the quest for further light and knowledge?



Why is a contactor, and/or switch needed for ground power?


	


   The implementation of ground power connectors

   published on my website and discussed on the forums

   grew out of the following design goals:



   (1) protection from 28 volt ground power being applied

       to a 14v airplane.



   (2) pilot control of ground power from the left seat.



   (3) reverse polarity protection.



   (4) convenience of an industry-common connection to

       the aircraft relatively free of hazard to ground

       crew from spinning props.



   Twice in my experience, I've had 28v connected to my

   14v airplane. In one case, the line-boy caught the error

   and flipped the switch before I could even react. In the

   other case he couldn't hear me over the roar of the

   ground power cart engine and he had his back to me

   watching the gages on the cart. I had to unbuckle,

   slide over, open r.h. cabin door and yell at him while

   stuff in the airplane suffered.

 	  Quote: 	
		  It seems to me that the very act of plugging in the plug could serve 

as "activating" the power, and pulling the plug out would deactivate it.


	


    Making and breaking a high current load during

    mate/de-mate operations is hard on the connector.



    But if you plan to exercise absolute and personal

    control over how and under what conditions external

    sources of power get hooked to your airplane, then

    it might be as simple a task as hooking up the

    jumper cables.



    But standing in a stream of wet, wind-driven snow

    this winter, I got a pair of jumper cables reversed

    to my son's van . . . didn't hurt anything but it

    sure COULD have. The degree of risk you're willing

    to shoulder is up to you.



        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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bicyclop(at)pacbell.net
Guest

Posted: Mon May 11, 2009 4:11 pm Post subject: GPU - 24V System - Crowbar Needed

Besides keeping the wrong voltage from hitting your battery and buss, a 

ground power contactor keeps the fat wire running to the receptacle from 

being hot all the time and being a potential shorting hazard.



Pax,



Ed Holyoke



woxofswa wrote:

 	  Quote: 	
		   



 Can I ask a neophyte question in the quest for further light and knowledge?



 Why is a contactor, and/or switch needed for ground power?



 It seems to me that the very act of plugging in the plug could serve as "activating" the power, and pulling the plug out would deactivate it.



 What am I missing?



 --------

 Myron Nelson

 Mesa, AZ

 Emp completed, legacy build fuse in progress





 Read this topic online here:



 http://forums.matronics.com/viewtopic.php?p 43572#243572

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woxofswa

Joined: 12 Aug 2008
Posts: 349
Location: AZ

Posted: Tue May 12, 2009 12:00 am Post subject: Re: GPU - 24V System - Crowbar Needed

Thanks for the info guys. 



I am going to do it the proper way and have already ordered the parts. I just like understanding the science as well

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_________________
Myron Nelson
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Flew May 10 2014

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