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Fusible Links and Bus Interconnects

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tx_jayhawk

Joined: 10 Jun 2006
Posts: 9

Posted: Thu Mar 26, 2009 12:31 pm Post subject: Fusible Links and Bus Interconnects

I was looking at an old Z drawing (Z-13/20 but that doesn't really matter for the purpose of my question), and it raises some questions that I I am struggling to recall the rationale.



1) What is the theory behind those connections (typically between busses and contactors) they specify minimum length (6" or less).  Examples I am thinking of are between battery contactor and battery bus, battery bus and e-bus contactor, e-bus contactor and standby alt shunt, etc.?  Is it voltage drop along the wire or something else?

2) There is a fusible link shown between the battery bus and e-bus contactor (which ultimately connects to the e-bus when switched on).  Since all of the loads on the busses are separately fused, what exactly is the fusible link protecting against?

3) I've noticed all of the designs show a fusible link in-line with the alt field breaker.  Why are two circuit protection devices in-line with no loads between them?  Seems redundant.

4) I've seen some people put fusible links in-line between the main and e-bus diode feed.  Not sure I appreciate why that is needed?

5) Are there any limits as to how short or how long the fusible links should be?



Thanks,

Scott

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nuckolls.bob(at)aeroelect
Guest

Posted: Thu Mar 26, 2009 1:29 pm Post subject: Fusible Links and Bus Interconnects

At 03:31 PM 3/26/2009, you wrote:

 	  Quote: 	
		  



I was looking at an old Z drawing (Z-13/20 but that doesn't really 

matter for the purpose of my question), and it raises some questions 

that I I am struggling to recall the rationale.



1) What is the theory behind those connections (typically between 

busses and contactors) they specify minimum length (6" or 

less).  Examples I am thinking of are between battery contactor and 

battery bus, battery bus and e-bus contactor, e-bus contactor and 

standby alt shunt, etc.?  Is it voltage drop along the wire or something else?


	


   These notations suggest that connected devices should

   be adjacent to each other. If you gotta go 10" for the

   conductor, then so be it. But close proximity in

   the airplane is a design goal.

 	  Quote: 	
		  2) There is a fusible link shown between the battery bus and e-bus 

contactor (which ultimately connects to the e-bus when switched 

on).  Since all of the loads on the busses are separately fused, 

what exactly is the fusible link protecting against?


	


    Z-13/20 was eliminated from the z-figures. After

    some considerable, afer-the-fact consideration I

    decided I could claim to have been hung over when

    I did it. It's just too clumsy. Z-12 or Z-14 are

    better integrations of an SD-20 with a larger main

    alternator.

 	  Quote: 	
		  3) I've noticed all of the designs show a fusible link in-line with 

the alt field breaker.  Why are two circuit protection devices 

in-line with no loads between them?  Seems redundant.


	


    Long lengths of wire (greater than 6" in the FAA

    world) need some form of fault protection. Unless

    your main bus is located within 6" of the breaker,

    then protection is generally advised. Further, it

    must be MUCH more robust than the 5A breaker which

    is expected to operate FIRST during a crowbar ov event.



 	  Quote: 	
		  4) I've seen some people put fusible links in-line between the main 

and e-bus diode feed.  Not sure I appreciate why that is needed?


	


    If it's not on my drawings, it's not advised by

    me. Fuses, breakers, fusible links, polyfuses, etc

    are NOT interchangeable technologies. I've shown

    fusible links in a very few locations and only

    after consideration for their operating characteristics.



 	  Quote: 	
		  5) Are there any limits as to how short or how long the fusible 

links should be?


	


    Fusible links smoke insulation. They don't want

    to be too short for thermal characteristics. 6"

    is a good number used on many cars. But certainly

    no longer.

        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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tx_jayhawk

Joined: 10 Jun 2006
Posts: 9

Posted: Fri Mar 27, 2009 9:54 am Post subject: Re: Fusible Links and Bus Interconnects

Hi Bob,



Thanks for the feedback...I re-reviewed 43.13 to try and better answer my own questions, but there was still a few things from your reply I was not clear on.



 	  Quote: 	
		  These notations suggest that connected devices should 

be adjacent to each other. If you gotta go 10" for the 

conductor, then so be it. But close proximity in 

the airplane is a design goal. 	




Is this just from a circuit protection standpoint (want it to be like "one" device)?  10" is ok...what about 2'?



 	  Quote: 	
		  Z-13/20 was eliminated from the z-figures. After 

some considerable, afer-the-fact consideration I 

decided I could claim to have been hung over when 

I did it. It's just too clumsy. Z-12 or Z-14 are 

better integrations of an SD-20 with a larger main 

alternator. 	




I'm now sure I understand why this is?  My problem with Z12 is that both alternators are feeding the same point.  If the connection from the main to ess bus fails, you've lost both alternators.  It seems preferable to have the standby alt feed the e-bus more directly.  Z14 has another battery, contactors, and other complexity I chose not to pursue.



 	  Quote: 	
		  Long lengths of wire (greater than 6" in the FAA 

world) need some form of fault protection. Unless 

your main bus is located within 6" of the breaker, 

then protection is generally advised. Further, it 

must be MUCH more robust than the 5A breaker which 

is expected to operate FIRST during a crowbar ov event. 

	




I would be interested in learning the origin of the 6" rule (I could not find in AC 43.13), but it still seems the wire already has circuit protection via the circuit breaker.  I'm not sure I understand why two circuit protection devices are needed.  There are obviously lots of power wires (for other loads) longer than 6" with only one source of circuit protection, so I am not sure why this is different?



 	  Quote: 	
		  They don't want 

to be too short for thermal characteristics. 6" 

is a good number used on many cars. But certainly 

no longer. 	




6" is a max...2" is min?  3"?  



THanks,

Scott

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nuckolls.bob(at)aeroelect
Guest

Posted: Fri Mar 27, 2009 12:18 pm Post subject: Fusible Links and Bus Interconnects

At 12:54 PM 3/27/2009, you wrote:

 	  Quote: 	
		  



Hi Bob,



Thanks for the feedback...I re-reviewed 43.13 to try and better 

answer my own questions, but there was still a few things from your 

reply I was not clear on.

 > These notations suggest that connected devices should

 > be adjacent to each other. If you gotta go 10" for the

 > conductor, then so be it. But close proximity in

 > the airplane is a design goal.



Is this just from a circuit protection standpoint (want it to be 

like "one" device)?  10" is ok...what about 2'?


	


    You have to make the go/no-go decision as to how

    far you're willing to stretch contemporary

    conventions. The goal is to minimize risks

    to small gage feeders (by "small" we mean

    much smaller than the battery and cranking

    circuit wires). Occasionally we've needed to

    attach some device directly to the bus or

    other fat-wire feed power point in a TC

    aircraft where the amount of wire exposed to

    fault risk could be limited to 6" or so.



    Folks traditionally up-tight with the burning

    of ANY wire deduced that the hazards generated

    were small. Same philosophy was applied to

    fusible links in cars. Now, if you want to

    go for 12" or 10-feet, that's your decision

    and your responsibility for risk mitigation.

    Are you going to double insulate the wire?

    Extra special care to insure no damage that

    might create a fault? The choice is yours.

    I've recommended a fusible link as an upstream

    protection for the 5A crowbar OV shut down

    system. It's not a "rule" and would not perhaps

    even be mentioned in 43.13.



    My approach is to offer recipes for success

    based on my personal observations of history

    and experiences in analysis of failure mode

    effects. But the ultimate magnitude and

    nature of risk you're willing to assume is

    your choice.



 	  Quote: 	
		   > Z-13/20 was eliminated from the z-figures. After

 > some considerable, afer-the-fact consideration I

 > decided I could claim to have been hung over when

 > I did it. It's just too clumsy. Z-12 or Z-14 are

 > better integrations of an SD-20 with a larger main

 > alternator.



I'm now sure I understand why this is?  My problem with Z12 is that 

both alternators are feeding the same point.  If the connection from 

the main to ess bus fails, you've lost both alternators.  It seems 

preferable to have the standby alt feed the e-bus more 

directly.  Z14 has another battery, contactors, and other complexity 

I chose not to pursue.


	


    How do you loose connection between the main

    bus and e-bus? Your presumption of risk is

    no different than loss of connection between

    the main bus and avionics bus in contemporary

    TC aircraft. The weakest link in the legacy

    main/avionics bus architectures is the avionics

    master switch. As described in detail on the

    website, the whole premise for installing such

    a switch has evaporated based on new design goals

    and product improvements over the past 40 years.

    Further, even thought I've eliminated the AV master

    from the Z-Figures, I've provided a second,

    reduced power consumption pathway in the form of

    an alternate feed directly from the battery.



    Z-12 offers two power paths to the e-bus re-enforced

    by two alternators driving a main bus with a reliability

    history approaching that of prop bolts.



 	  Quote: 	
		   > Long lengths of wire (greater than 6" in the FAA

 > world) need some form of fault protection. Unless

 > your main bus is located within 6" of the breaker,

 > then protection is generally advised. Further, it

 > must be MUCH more robust than the 5A breaker which

 > is expected to operate FIRST during a crowbar ov event.

 >

I would be interested in learning the origin of the 6" rule (I could 

not find in AC 43.13), but it still seems the wire already has 

circuit protection via the circuit breaker.


	




    One might accurately say it was pulled from a place

    were the sun don't shine. Consider the wire protected

    by the fusible link runs from bus to breaker, the breaker

    protects wire(s) downstream AND complies with design

    goals established by the crowbar ov protection

    philosophy were a BREAKER is used in an airplane

    otherwise protected by FUSES. If you have some OTHER

    ov protection scheme, perhaps the 5A breaker, extended

    bus wire AND fusible link is eliminated. In this case,

    field power comes directly from the fuse block on a 5 or

    7A fuse.



 	  Quote: 	
		     I'm not sure I understand why two circuit protection devices are 

 needed.  There are obviously lots of power wires (for other loads) 

 longer than 6" with only one source of circuit protection, so I am 

 not sure why this is different?


	


    A single protective device suffices when a circuit

    branch from the bus (or fat wires) is located where

    that power leaves the bus as in rows of breakers or

    fuses. Once I tie onto the fat wires for extening power

    from a fuse block to a panel mounted breaker, THAT

    small wire becomes vulnerable. In observance of

    the "rule of dark origins", the fusible link seems

    easy and reasonable. Likelihood that the fusible link

    will EVER be required to operate . . . exceedingly

    low. But then, 99.99% of all breakers and fuses installed

    in cars, airplanes, other vehicles and indeed your

    house run the lifetime of the system never being

    required to operate in defense of a fault condition.



    So, knowing that . . . how far are you willing to

    depart from legacy design goals? I cannot advise

    you there. My design goals go to the simplest,

    lowest cost, most robust implementation of

    proven recipes for success.



    Bob . . .

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