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Polyfuses for aircraft?

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nuckolls.bob(at)aeroelect
Guest

Posted: Thu Mar 26, 2009 9:38 am Post subject: Polyfuses for aircraft?

I've been working with a variety of polyfuses

on the bench and have completed some tests in the

chamber. I've killed a few of the critters that

suffered heat-sinking effects of vibration

resistant mounting. These guys first fail shorted.

Depending on source impedance, they either pull

the supply voltage down . . . or blow up and go open.



I'll continue to evaluate mounting techniques

but I gotta tell you that I'm not enthusiastic about

these parts. It's a poor design that pivots about

assembly processes.



Decades of aviation experience have demonstrated

numerous recipes for success that have performed well.

It's unfortunate that LSE saw fit to install

the crowbar ov protection. Were it not for this

feature, there would be no good reason why these

systems couldn't be powered through 5A fuses at

the battery bus. But as soon as you add a requirement

for the 5A, crowbar-friendly breaker, robustness of

always-hot feeders from the bus must be greater than

the legacy design philosophies dictate.



If the LSE products had been designed in compliance

with Mil-STD-704/DO-160 guidelines, there would be no need

to incorporate a second layer of OV protection

and we wouldn't be having this conversation.



I'm not finished with the tinkering. However, I am 90%

sure that the parts being tested are not going to

exhibit equal-to-or-better performance than ingredients

and processes which go into demonstrable recipes for success.



LSE's recommended installation philosophies

will FUNCTION as advertised but are contrary

to contemporary aviation design philosophies for

protection of feeders in the power distribution

system. It's unlikely that the 'Connection will

recommend any alternatives for incorporating LSE

products into OBAM aircraft. Installers should

comply with LSE recommendations.

        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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bcondrey

Joined: 03 Apr 2006
Posts: 580

Posted: Thu Mar 26, 2009 12:28 pm Post subject: Re: Polyfuses for aircraft?

Bob,



Earlier you had mentioned that it would take a 30 amp fuse to guarantee that the 5 amp breaker would trip first from a crowbar event.  I'm not enthusiastic about that much current being allowed through - what CB size would be required to guarantee that the 5 amp up front would trip first?  Would a 7.5 in back be OK or would it have to be significantly larger?  Would this be of benefit over a large (30 amp) fuse?



These wouldn't be in flight resettable devices but would reduce the amount of current allowed in the always hot wires.  In theory the only thing that they're protecting against is a dead short in the airframe wiring since anything past the 5 amp panel critters (wiring, maybe a switch and the actual ignition box) would trip them first.



Bob

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nuckolls.bob(at)aeroelect
Guest

Posted: Thu Mar 26, 2009 1:39 pm Post subject: Polyfuses for aircraft?

At 03:28 PM 3/26/2009, you wrote:

 	  Quote: 	
		  

<bob.condrey(at)baesystems.com>



Bob,



Earlier you had mentioned that it would take a 30 amp fuse to 

guarantee that the 5 amp breaker would trip first from a crowbar 

event.  I'm not enthusiastic about that much current being allowed 

through - what CB size would be required to guarantee that the 5 amp 

up front would trip first?  Would a 7.5 in back be OK or would it 

have to be significantly larger?  Would this be of benefit over a 

large (30 amp) fuse?


	


   Do your own tests. Put in any size fuse you like

   and deliberately crowbar the downstream breaker.

   Once you find a fuse that stays put after a half

   dozen trips, go up one more size. Since this

   is hard-fault protection only, you don't need to

   upsize the wire. 20AWG is still good.



 	  Quote: 	
		  These wouldn't be in flight resettable devices but would reduce the 

amount of current allowed in the always hot wires.  In theory the 

only thing that they're protecting against is a dead short in the 

airframe wiring since anything past the 5 amp panel critters 

(wiring, maybe a switch and the actual ignition box) would trip them first.


	


   Now you understand the struggle for

   reconciling LSE recommendations with

   contemporary design goals. I haven't figured

   out a way to do it yet. There is probably some

   form of polyfuse that would meet reliability

   goals . . . but not the ones I have on hand

   now. Assuming the magic device could be identified,

   now we need to insure that folks can get them

   and that other folks don't substitute them.



   I don't like "fine tuning" a design like this.

   I'm working a serious accident right now that

   involved total loss of dual electronic ignition

   where rudimentary design goals were not established,

   understood and met.



   Risk of engine failure due poor hacks is greater

   than risk of fire if you roll it up into a ball.

   Until some better idea emerges, wire it per

   LSE recommendations . . .

        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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longg(at)pjm.com
Guest

Posted: Thu Mar 26, 2009 2:07 pm Post subject: Polyfuses for aircraft?

I have also thought of testing a slow-blo (20) style ATC for this

mission. We'll give it a go. LSE likes to tie the 5 amp breaker right to

the battery post, so a bit more testing each way.



Glenn



--

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