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JOHN TIPTON

Joined: 17 Sep 2006
Posts: 239
Location: Torquay - England

Posted: Thu May 08, 2014 3:09 am Post subject: Fusable link

Hi Guys (Bob)



I see that Z11 is using a fuse block as opposed the CBs (no problem there), can you tell me why the feed to the 5a CB has a fuse able link, why not use a fuse in the distribution fuse block



Best regards



John

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JOHN TIPTON

Joined: 17 Sep 2006
Posts: 239
Location: Torquay - England

Posted: Thu May 08, 2014 3:12 am Post subject: Re: Fusable link

Ps: the 5amp CB did relate to the alternator feed circuit



John

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nuckolls.bob(at)aeroelect
Guest

Posted: Thu May 08, 2014 4:40 am Post subject: Fusable link

At 06:09 AM 5/8/2014, you wrote:

 --> AeroElectric-List message posted by: "JOHN TIPTON" <jmtipton(at)btopenworld.com>



 Hi Guys (Bob)



 I see that Z11 is using a fuse block as opposed the CBs (no problem there), can you tell me why the feed to the 5a CB has a fuse able link, why not use a fuse in the distribution fuse block



 Best regards



 John



 

    The length of wire from fuse block feeder terminal

    out to the circuit breaker is an EXTENSION of the

    bus. In larger aircraft the bus feeders will sometimes

    be protected by a VERY robust fusible device like

    the ANN/ANL series of "current limiters" . . .



   



  http://tinyurl.com/lrjm5y3



  http://tinyurl.com/k7o3l3q



   Note that these devices, while 'rated' at some

   value like 100A, they will CARRY 2x their

   rated current almost indefinitely. At the

   same time, when subjected to a hard fault

   of 5 to 10x rated current, they open in tens

   of milliseconds.



   Legacy design goals for aircraft suggest that

   this extension of the bus up to your panel

   mounted circuit breaker needs a 'current limiter'

   but not like the devices cited above. It's

   much smaller and designed to protect the longer

   run of wire to the breaker.  Protection with

   a response many times longer than the breaker

   it feeds. 



   This question came up on the List a few years

   ago . . .



 Q: Drawing Z24 shows how to add an OV protection module for an internally regulated power

 supply. It requires the use of a 5A circuit breaker and also of a fuselink attached to the main

 connection of the main bus. The question: Can we replace the fuselink by using one of the

 outputs on the main bus protected by a larger fuse (say 15amps)? I would like for sure the

 breaker to let go before the fuse.



 A: Some 5A breakers will open a 15A fuse. Fuses are MUCH faster than breakers. This is

 why I prefer the fusible link



   The risks of positioning fuses upstream of breakers

   is illustrated in the accident case I chronicled

   here



  http://tinyurl.com/msfmldj



   and demonstrated here



  http://tinyurl.com/ppdv5fg



   The automotive world has used fusible links

   in this capacity for decades and they've

   served well in the capacity of mini-limiters.



 

   Bob . . .    [quote][b]

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JOHN TIPTON

Joined: 17 Sep 2006
Posts: 239
Location: Torquay - England

Posted: Thu May 08, 2014 10:21 am Post subject: Re: Fusable link

So a ANL current limiter off the main battery protecting the whole system would not be a bad idea ?



John

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nuckolls.bob(at)aeroelect
Guest

Posted: Thu May 08, 2014 4:36 pm Post subject: Fusable link

At 01:21 PM 5/8/2014, you wrote:

 	  Quote: 	
		  

<jmtipton(at)btopenworld.com>



So a ANL current limiter off the main battery protecting the whole 

system would not be a bad idea ?


	


   Not a bad idea . . . but not a good one either. Current

   limiters have value for fault isolation in complex bus

   structures . . . which are generally not found in Part

   23 airplanes.



   The fat wires between battery, main bus and starter

   terminal are usually of such robustness that any

   fault to ground would be more likely to burn a hole

   in fault location as opposed to opening any limiter

   in the path.



   Single light TC aircraft have never been fitted with

   such protection that I know of.

   Bob . . .

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jluckey(at)pacbell.net
Guest

Posted: Thu May 08, 2014 7:17 pm Post subject: Fusable link

From: "Robert L. Nuckolls, III" <nuckolls.bob(at)aeroelectric.com>

 To: aeroelectric-list(at)matronics.com 

 Sent: Thursday, May 8, 2014 5:32 PM

 Subject: Re: Re: Fusable link

  

 

--> AeroElectric-List message posted by: "Robert L. Nuckolls, III" <nuckolls.bob(at)aeroelectric.com (nuckolls.bob(at)aeroelectric.com)>



At 01:21 PM 5/8/2014, you wrote:

 	  Quote: 	
		  --> AeroElectric-List message posted by: "JOHN TIPTON" 

<jmtipton(at)btopenworld.com (jmtipton(at)btopenworld.com)>



So a ANL current limiter off the main battery protecting the whole 

system would not be a bad idea ?


	


   Not a bad idea . . . but not a good one either. Current

   limiters have value for fault isolation in complex bus

   structures . . . which are generally not found in Part

   23 airplanes.



   The fat wires between battery, main bus and starter

   terminal are usually of such robustness that any

   fault to ground would be more likely to burn a hole

   in fault location as opposed to opening any limiter

   in the path.



Not sure what that means... ???  



If it is "robust" wouldn't it carry enough current to blow the limiter?

Are you suggesting that "welding/burning" aluminum does not draw a lot of current?

Have you ever tested that hypothesis?



Even if some of the above is true, a current limiter still could provide some protection

from a "hard" fault, could it not?



-Jeff

   Single light TC aircraft have never been fitted with

   lectric-List" target="_blank">http://www.matronics.com/Navigator?AeroElec   --> h                    &nbsphref="http://www.matronics.com/contribution" =======

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nuckolls.bob(at)aeroelect
Guest

Posted: Fri May 09, 2014 2:18 am Post subject: Fusable link

 	  Quote: 	
		  

Not sure what that means... ???



If it is "robust" wouldn't it carry enough current to blow the limiter?

Are you suggesting that "welding/burning" aluminum does not draw a 

lot of current?

Have you ever tested that hypothesis?


	




    Actually, it doesn't. Recall the anecdote about

    the C90 parting and elevator control cable with

    a soft fault to a wire. A 40A breaker didn't trip

    nor was the wire itself seriously damaged.



http://tinyurl.com/n3dqkb2

 	  Quote: 	
		  Even if some of the above is true, a current limiter still could 

provide some protection

from a "hard" fault, could it not?


	


    Define 'hard' fault . . . and hypothesize how

    one comes into existence.



    What kind of circuit closure has to happen to

    carry hundreds of amps of current? Consider

    how a contactor is built specific to

    that goal. Now, how does such a connection

    happen along the pathway for a fat wire as

    it travels about the airplane?



    The #1 failure in wires is simple disconnect.

    #2 is insulation failure by virtue of mechanical

    intrusion . . . abrasion or sharp edge cutting

    due to poor support combined with vibration.

    What is the quality of first contact? It's tentative,

    low force, high resistance event that produces

    some arcing but is generally far short of that

    required to put the wire at risk or even open

    it's upstream current limiter.



    Then there's the probability: #1 failure

    is rare, #2 several magnitudes more rare, and

    if a composite airplane, a few magnitudes more

    rare yet.



    The few times I've seen the result of a battery

    cable insulation chaffing and taking the conductor

    to ground, the fault self cleared when the offending

    aluminum burned away. When breakers open in

    airplanes, the most likely cause is failure

    in the powered device . . . not because of

    compromised insulation in the wire-run.



    If you want to put a current limiter in your

    battery line, by all means. I'm only suggesting

    that your airplane will be one of a very few

    in a constellation of tens of thousands.



   Bob . . .

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jluckey(at)pacbell.net
Guest

Posted: Fri May 09, 2014 7:55 am Post subject: Fusable link

From: "Robert L. Nuckolls, III" <nuckolls.bob(at)aeroelectric.com>

 To: aeroelectric-list(at)matronics.com 

 Sent: Friday, May 9, 2014 3:00 AM

 Subject: Re: Re: Fusable link

  

 

--> AeroElectric-List message posted by: "Robert L. Nuckolls, III" <nuckolls.bob(at)aeroelectric.com (nuckolls.bob(at)aeroelectric.com)>

 	  Quote: 	
		  

Not sure what that means... ???



If it is "robust" wouldn't it carry enough current to blow the limiter?

Are you suggesting that "welding/burning" aluminum does not draw a 

lot of current?

Have you ever tested that hypothesis?


	




    Actually, it doesn't. Recall the anecdote about

    the C90 parting and elevator control cable with

    a soft fault to a wire. A 40A breaker didn't trip

    nor was the wire itself seriously damaged.



http://tinyurl.com/n3dqkb2

 	  Quote: 	
		  Even if some of the above is true, a current limiter still could 

provide some protection

from a "hard" fault, could it not?


	


    Define 'hard' fault . . . and hypothesize how

    one  comes into existence.



Bob - I think you side stepped the question...



".. current limiter still could 

provide some protection

from a "hard" fault, could it not? "



regardless of how the hard fault happens.



Let's consider the worst case scenario where

we have a crash and a hard-fault occurs.



    What kind of circuit closure has to happen to

    carry hundreds of amps of current? Consider

    how a contactor is built specific to

    that goal. Now, how does such a connection

    happen along the pathway for a fat wire as

    it travels about the airplane?



    The #1 failure in wires is simple disconnect.

    #2 is insulation failure by virtue of mechanical

    intrusion . . .  abrasion or sharp edge cutting

    due to poor support combined with vibration.

    What is the quality of first contact? It's tentative,

    low force, high resistance event that produces

    some arcing but is generally far short of that

    required to put the wire at risk or even open

    it's upstream current limiter.



    Then there's the probability: #1 failure

    is rare, #2 several magnitudes more rare, and

    if a composite airplane, a few magnitudes more

    rare yet.



    The few times I've seen the result of a battery

    cable insulation chaffing and taking the conductor

    to ground, the fault self cleared when the offending

    aluminum burned away. When breakers open in

    airplanes, the most likely cause is failure

    in  the powered device . . . not because of

    compromised insulation in the wire-run.



    If you want to put a current limiter in your

    battery line, by all means. I'm only suggesting

    that your airplane will be one of a very few

    in a constellation of tens of thousands.



   Bobric-List"  target="_blank">http://www.matronics.com/Navigator?AeroElectric-Lista href="http://forums.matronics.com/" target="_blank">http://forums.mat                     &nbs//www.matronics.com/contribution" target="_blank">http://www.matronics.co==================

  

 

  

 [quote][b]

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