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Circuit protection for relay/contactor trigger circuits.

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andrew.d.zachar(at)gmail.
Guest

Posted: Tue Jan 11, 2011 3:28 pm Post subject: Circuit protection for relay/contactor trigger circuits.

Bob and all,



I notice in Z-10/8 (as an example) that some trigger circuits are protected and some are not.

Starter contactor trigger - shows 7A protection.

E-Bus alternate feed relay trigger - shows no protection (just 22AWG)

 Battery contactor trigger - shows no protection (just 22AWG)

Brownout battery relay (triggered from start switch) - shows no protection (just 20AWG)

I know the Z-figures are not to be taken as anything other than architecture guidance, but is there a rule of thumb on protecting some of my relay-closing switch circuits?

 



Thanks in advance.

-- 

Andrew Zachar

andrew.d.zachar(at)gmail.com (andrew.d.zachar(at)gmail.com)

 

  [quote][b]

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andrew.d.zachar(at)gmail.
Guest

Posted: Wed Jan 12, 2011 8:47 am Post subject: Circuit protection for relay/contactor trigger circuits.

On Wed, Jan 12, 2011 at 8:18 AM, Robert L. Nuckolls, III <nuckolls.bob(at)aeroelectric.com (nuckolls.bob(at)aeroelectric.com)> wrote:

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



	
 



<snip>

 

 	  Quote: 	
		     and in particular paragraph 23.1367

	
 



We use 23.1367 a lot in Flight Test, particularly "(c) Accessible to appropriate flight crewmembers;" and "(d) Labeled as to operation and the circuit controlled" in our Human Factors evaluations. 23.1367(c) is my favorite regulation. It's how us flight testers get to completely redesign the cockpit after someone with far more intelligence than us has put a lot of time into layout. Just kidding. (Lately, most designers have become much more tuned into the human factors aspects of designs. Everyone wins, especially customers.) Sorry for the digression.

 



 	  Quote: 	
		     Fuses and breakers protect wires from hard faults (LOTS OF CURRENT) and nothing more.

	
 



<snip>

 

 	  Quote: 	
		       Now, armed with this simple-idea, run the

     traps on what kind of fault would cause the

     contactor control wires you've cited to become

     exposed to high fault currents?

	




<snip>

 

 	  Quote: 	
		       Faulting a supply wire

     to ground exposes it to the potential fault

     current of a battery . . . perhaps 1000 amps

     or more. Faulting a ground-to-close control

     wire to ground exposes it only to the current

     it takes to close the contactor. Hence, no

     protection is called for in these lines.

	




To summarize to verify my own understanding...

I'll protect all wires over six inches who supply enough current to make bad things happen.

 



Why, then, does the starter contactor circuit in Z-10/8 show a 7A fuse? I see that it closes two relays...

 	  Quote: 	
		      Bob . . . 



 

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-- 

Andrew Zachar

andrew.d.zachar(at)gmail.com (andrew.d.zachar(at)gmail.com)

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