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Potential for Overvoltage Issue

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mike_tailwind

Joined: 06 Feb 2017
Posts: 12

Posted: Fri Sep 22, 2023 12:40 pm Post subject: Potential for Overvoltage Issue

I've been luring on this list for years and this is my first post.

I've recently completed my plane (Tailwind) and have if flying.

I used the previous version of the OVM crowbar circuit and I have modified an automotive alternator to use an external Ford-type voltage regulator.

All is working well.  But, in going through a FMEA in my mind I have identified a potential issue:  I modified the alternator using the method in Kitplanes, November 2019. 

In this method one of the brush terminals is isolated with insulating washers and an insulating sleeve.  In my case I have grounded the other brush to case ground.  The issue is that if ever the insulating washer or sleeve were breached the B--lead would directly feed the field through the brush and an overvoltage would result that the crowbar could not shut down. Turning off the master switch would disconnect the battery but the run away alternator would still be feeding the bus.

I post this in case anyone else has this setup.  



Solutions are:

1. Add a Alternator disconnect contactor connected to the crowbar (in which case, why use an external regulator?)

2. Externally ground the B-lead side brush with a fusible link and feed the field current from the VR into the other (currently grounded) brush.

Any other thoughts?

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nuckolls.bob(at)aeroelect
Guest

Posted: Sat Sep 23, 2023 2:15 pm Post subject: Potential for Overvoltage Issue

At 03:39 PM 9/22/2023, you wrote:

  	  Quote: 	
		  I've been luring on this list for years and this is my first post.

 I've recently completed my plane (Tailwind) and have if flying.

 I used the previous version of the OVM crowbar circuit and I have modified an automotive alternator to use an external Ford-type voltage regulator.

 All is working well.  But, in going through a FMEA in my mind I have identified a potential issue:  I modified the alternator using the method in Kitplanes, November 2019. 

 In this method one of the brush terminals is isolated with insulating washers and an insulating sleeve.  In my case I have grounded the other brush to case ground.  The issue is that if ever the insulating washer or sleeve were breached the B--lead would directly feed the field through the brush and an overvoltage would result that the crowbar could not shut down. Turning off the master switch would disconnect the battery but the run away alternator would still be feeding the bus.

 I post this in case anyone else has this setup. 	


 Yes they do . . . along with about every single

 engine airplane since 1968 or so.



 FMEA also takes into account probability of

 any particular event. Many potentially catastrophic

 events in aviation are statistically or demonstrably

 so small as to be ignored in process of crafting

 a plan-b, or redesigning to remove potential for

 the fault entirely.



 Number one failure of a wiring pathway is

 disconnect. Wire breaks, stud nut is works loose,

 wire pulls out of terminal crimp, etc. Another

 root cause for failure is short to ground due

 to compromised wire insulation (chaffing or

 mechanical damage). The probability of an

 wire becoming disconnected followed by a

 low-impedance, re-positioning of that wire

 to a robust power source is really TWO

 failures happening simultaneously.

 Original structural failure followed

 by misadventurous 'wandering' of that

 same wire to a hazardous source of

 power.



 This combination of events is so remote

 that it has not been treated as potentially

 hazardous in hundreds of thousands of airplanes

 from Super Cubs to Gulfstream V's



 You are to be commended for considering

 the scenario and asking the question.

 Be assured that the question has been

 considered and discounted countless times

 before.



 



 



 

 

   Bob . . .



                    //// 

                   (o o) 

    ===========o00o=(_)=o00o========= 

    < Go ahead, make my day . . .   > 

    < show me where I'm wrong.      > 

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

  

    In the interest of creative evolution

    of the-best-we-know-how-to-do based

    on physics and good practice.

- The Matronics AeroElectric-List Email Forum -

Use the List Feature Navigator to browse the many List utilities available such as the Email Subscriptions page, Archive Search & Download, 7-Day Browse, Chat, FAQ, Photoshare, and much more:

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mike_tailwind

Joined: 06 Feb 2017
Posts: 12

Posted: Wed Sep 27, 2023 7:02 am Post subject: Potential for Overvoltage Issue

Hi Bob,

Thanks for your response.  But, remember in this case we are dealing with a user modified alternator, not factory.  The two failure events you list are linked in a chain so their probabilities can't be considered independently.  In this case, if the screw with it's insulating sleeve were to back out exposing it's threads the ring terminal or brush lug would only have to move 0.020" to make contact and cause a uncontrolled overvoltage event.  The probability of that much movement in a high vibration environment is almost guaranteed.  

I do agree that making that event extremely improbable so that it doesn't need to be considered is the best approach.  I believe I had put Loctite on that screw.  I think the next time I'm in there I will put a blob of JB-Weld over the screw and terminal, potting the area, as a belt and suspenders to make sure that it never moves.

                  

                                                                    On Saturday, September 23, 2023 at 05:25:33 PM CDT, Robert L. Nuckolls, III <nuckolls.bob(at)aeroelectric.com> wrote:                 

                 



                 



                   At 03:39 PM 9/22/2023, you wrote:

  	  Quote: 	
		  I've been luring on this list for years and this is my first post.

 I've recently completed my plane (Tailwind) and have if flying.

 I used the previous version of the OVM crowbar circuit and I have modified an automotive alternator to use an external Ford-type voltage regulator.

 All is working well.  But, in going through a FMEA in my mind I have identified a potential issue:  I modified the alternator using the method in Kitplanes, November 2019. 

 In this method one of the brush terminals is isolated with insulating washers and an insulating sleeve.  In my case I have grounded the other brush to case ground.  The issue is that if ever the insulating washer or sleeve were breached the B--lead would directly feed the field through the brush and an overvoltage would result that the crowbar could not shut down. Turning off the master switch would disconnect the battery but the run away alternator would still be feeding the bus.

 I post this in case anyone else has this setup. 	


 Yes they do . . . along with about every single

 engine airplane since 1968 or so.



 FMEA also takes into account probability of

 any particular event. Many potentially catastrophic

 events in aviation are statistically or demonstrably

 so small as to be ignored in process of crafting

 a plan-b, or redesigning to remove potential for

 the fault entirely.



 Number one failure of a wiring pathway is

 disconnect. Wire breaks, stud nut is works loose,

 wire pulls out of terminal crimp, etc. Another

 root cause for failure is short to ground due

 to compromised wire insulation (chaffing or

 mechanical damage). The probability of an

 wire becoming disconnected followed by a

 low-impedance, re-positioning of that wire

 to a robust power source is really TWO

 failures happening simultaneously.

 Original structural failure followed

 by misadventurous 'wandering' of that

 same wire to a hazardous source of

 power.



 This combination of events is so remote

 that it has not been treated as potentially

 hazardous in hundreds of thousands of airplanes

 from Super Cubs to Gulfstream V's



 You are to be commended for considering

 the scenario and asking the question.

 Be assured that the question has been

 considered and discounted countless times

 before.



 



 



 

 

    Bob . . .



                    //// 

                   (o o) 

    ===========o00o=(_)=o00o========= 

    < Go ahead, make my day . . .   > 

    < show me where I'm wrong.      > 

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

  

    In the interest of creative evolution

    of the-best-we-know-how-to-do based

    on physics and good practice.

- The Matronics AeroElectric-List Email Forum -

Use the List Feature Navigator to browse the many List utilities available such as the Email Subscriptions page, Archive Search & Download, 7-Day Browse, Chat, FAQ, Photoshare, and much more:

http://www.matronics.com/Navigator?AeroElectric-List

nuckolls.bob(at)aeroelect
Guest

Posted: Thu Sep 28, 2023 9:15 am Post subject: Potential for Overvoltage Issue

At 10:01 AM 9/27/2023, you wrote:

  	  Quote: 	
		  Hi Bob,



 Thanks for your response.  But, remember in this case we are dealing with a user modified alternator, not factory. 	


     . . .and your skill set is not equal to or greater

     than 'factory'?



  	  Quote: 	
		  The two failure events you list are linked in a chain so their probabilities can't be considered independently. 	


     You can consider them in any manner that you wish.



  	  Quote: 	
		  In this case, if the screw with it's insulating sleeve were to back out exposing

 it's threads the ring terminal or brush lug would only have to move 0.020"

 to make contact and cause a uncontrolled overvoltage event.  The probability

 of that much movement in a high vibration environment is almost guaranteed. 	


     We've discussed the value of high pressure, gas tight

     connections to insure conductor integrity . . . how likely

     is it that this terminal, having succumbed to a 'high vibration'

     environment and now flailing around in the breeze

     is going to experience the necessary force/area PLUS dwell time to form

     a conduction pathway necessary to produce the condition

     of concern?

     

  	  Quote: 	
		  I do agree that making that event extremely improbable so that it doesn't need to be

 considered is the best approach.  I believe I had put Loctite on that screw. 	


    Locking fasteners are good too . . .



  	  Quote: 	
		  I think the next time I'm in there I will put a blob of JB-Weld over the screw

 and terminal, potting the area, as a belt and suspenders to make sure that it never moves. 	


    Seems you have answered your own question. If

    you have concerns and perceive a mitigating

    action, then by all means. My offering to this

    thread suggests that quality of workmanship

    and robust design are the first lines of defence

    for failures . . . OV management systems are a

    'band aid' that addresses conditions for which

    we have no influence on design/craftsmanship.



 

   Bob . . .



                    //// 

                   (o o) 

    ===========o00o=(_)=o00o========= 

    < Go ahead, make my day . . .   > 

    < show me where I'm wrong.      > 

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

  

    In the interest of creative evolution

    of the-best-we-know-how-to-do based

    on physics and good practice.

- The Matronics AeroElectric-List Email Forum -

Use the List Feature Navigator to browse the many List utilities available such as the Email Subscriptions page, Archive Search & Download, 7-Day Browse, Chat, FAQ, Photoshare, and much more:

http://www.matronics.com/Navigator?AeroElectric-List

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