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Z19RB Fat wire protection, etc.

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Allen Fulmer

Joined: 10 Jan 2006
Posts: 79
Location: Alexander City, AL

Posted: Wed Nov 12, 2008 7:46 pm Post subject: Z19RB Fat wire protection, etc.

Okay Bob,



Airplane and configuration:

RV7 w/Eggenfellner Subaru H6 up front.

2 PC625 batteries in the very back of fuselage wired per Z19RB.

Fat wire (#4 CCA from Eric Jones) runs from battery contactors to a firewall

"pass thru stud" (West Marine).  Firewall side of the stud has 2 connectors:

one to an ANL on the way to a 75A IR alternator (supplied with engine

package); another to the starter solenoid mounted as part of geared starter.

I think this is all in accordance with Z19RB architecture.



My question concerns the cockpit side of this "pass through stud".  Could

you please reassure me that I am correct in planning a #6 wire from that

same stud (as the fat wire is connected to) to the MAIN POWER DISTRIBUTION

BUS (approximately 42 inches "as the wire goes") withOUT any fuse/breaker

protection?



Your picture:

http://www.aeroelectric.com/Pictures/Wiring_Technique/A36_Firewall_B.jpg

(cert. plane of course) shows a bunch of ANL's to avionics, etc., but in the

OBAM world I guess we do not need to imitate that kind of installation?



And could you also point me to the reference in AEC that speaks to the lack

of a need for ANL protection of the fat wire at the batteries?  I think I

have read that ANL's are not needed by the batteries but can not remember

where I read that.



Thanks for all you do, Bob.



Allen Fulmer

RV7 Wiring/Plumbing

Eggenfellner Subaru E6Ti on firewall

N808AF reserved

Alexander City, AL

256-329-2001

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_________________
RV7 QB Fuselage
Eggenfellner H6 on orde
N808AF reserved
Alexander City, AL

nuckolls.bob(at)cox.net
Guest

Posted: Thu Nov 13, 2008 4:03 am Post subject: Z19RB Fat wire protection, etc.

At 09:44 PM 11/12/2008 -0600, you wrote:

 	  Quote: 	
		  



Okay Bob,



Airplane and configuration:

RV7 w/Eggenfellner Subaru H6 up front.

2 PC625 batteries in the very back of fuselage wired per Z19RB.

Fat wire (#4 CCA from Eric Jones) runs from battery contactors to a firewall

"pass thru stud" (West Marine).  Firewall side of the stud has 2 connectors:

one to an ANL on the way to a 75A IR alternator (supplied with engine

package); another to the starter solenoid mounted as part of geared starter.

I think this is all in accordance with Z19RB architecture.



My question concerns the cockpit side of this "pass through stud".  Could

you please reassure me that I am correct in planning a #6 wire from that

same stud (as the fat wire is connected to) to the MAIN POWER DISTRIBUTION

BUS (approximately 42 inches "as the wire goes") withOUT any fuse/breaker

protection?


	


   That's correct.

 	  Quote: 	
		  Your picture:

http://www.aeroelectric.com/Pictures/Wiring_Technique/A36_Firewall_B.jpg

(cert. plane of course) shows a bunch of ANL's to avionics, etc., but in the

OBAM world I guess we do not need to imitate that kind of installation?


	


    I've never paid much attention to the fuse-patch on

    the A36. Just pulled up the wiring diagrams. That airplane

    has a variety of busses on the left cabin wall (c/b panel)

    pilots switch panel (switch breakers), copilots switch

    panel (miniature push-pull breakers), two alternators, etc.

    This is a mechanically busy architecture that I hope will

    never be implemented in an RV! The designers no doubt found

    it necessary to worry about failure modes grown out of

    complexity.



 	  Quote: 	
		  And could you also point me to the reference in AEC that speaks to the lack

of a need for ANL protection of the fat wire at the batteries?  I think I

have read that ANL's are not needed by the batteries but can not remember

where I read that.


	


    It's a convention that has been with us for a long

    time. Fat-wire feeders in small aircraft have never

    benefited from circuit protection due to the nature

    of the exceedingly rare, soft faults these wires

    experience. By "soft fault" I'm referring to a condition

    that will NOT product the hundreds of amps in current

    flow necessary to open an ANL or similar device.



    I've seen soft faults that parted elevator control

    cables while never causing a light to flicker, no

    smoke or bad smells, yet the feeder in question was

    protected by a 40A breaker in a 28v aircraft! These

    tend to be very high, total energy events delivered

    in small packets over long periods of time. The FAA

    acknowledges this in the crafting of FAR 23.1357 which

    I quote . . .



Sec. 23.1357   Circuit protective devices.



(a) Protective devices, such as fuses or circuit breakers, must be

installed in all electrical circuits other than--

(1) Main circuits of starter motors used during starting only; and

(2) Circuits in which no hazard is presented by their omission.

(b) A protective device for a circuit essential to flight safety may not be

used to protect any other circuit.

(c) Each resettable circuit protective device ("trip free" device in which

the tripping mechanism cannot be overridden by the operating control) must be

designed so that--

(1) A manual operation is required to restore service after tripping; and

(2) If an overload or circuit fault exists, the device will open the

circuit regardless of the position of the operating control.

(d) If the ability to reset a circuit breaker or replace a fuse is

essential to safety in flight, that circuit breaker or fuse must be so

located and identified that it can be readily reset or replaced in flight.

(e) For fuses identified as replaceable in flight--

(1) There must be one spare of each rating or 50 percent spare fuses of

each rating, whichever is greater; and

(2) The spare fuse(s) must be readily accessible to any required pilot.



    For the purposes of this discussion, paragraphs (a)(1) and (a)(2)

    apply. I might point out also that our implementation of fuse-blocks

    in a failure tolerant system design also meets the spirit and

    intent of the rest of 23.1357 as well. If your system is failure

    tolerant, then there are no single systems "critical" to safe

    operations and termination of flight. If your fuse panels are

    not accessible to the crew, then no spares need be carried

    aboard the aircraft.



 	  Quote: 	
		  Thanks for all you do, Bob.


	


    You're most welcome sir.



    Bob . . .

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