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Dual redundant electrical system

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terryml5c2p6(at)sympatico
Guest

Posted: Wed May 14, 2008 4:09 pm Post subject: Dual redundant electrical system

This is my first time on the AeroElectric list server, though I’ve been 

lurking in the forums for some time. I’m looking for comments/critiques on a 

dual alternator dual battery system I’ve designed. Right off the bat I have 

to acknowledge the contributions made in this area by Bob Knucolls, I’ve 

read his book several times, and his ideas and philosophies have strongly 

influenced my design. Greg Richter of Blue Mountain Avionics has also been a 

major source of information and inspiration.



My application is an ‘all-electric’ plane, an RV7 I’m building with 

Aerosance FADEC and an all-glass cockpit (AFS3400, GRT Horizon HX, and 

Trutrak EFIS). I’m an electrical engineer with 22 years experience in a 

large electrical power utility doing protection and control work. Duplicate 

redundant protection systems for ultra high reliability are the core work in 

this field. In 1988 I left to start my own company, designing and 

manufacturing complex expensive microprocessor-based test equipment for use 

in that field. I’ve flown my own Cessna 172 since 1992. I’m comfortable with 

electricity and flying, but I’m incorporating some non-standard ideas in 

this design and would appreciate comments. My prime concern is 

technical/safety aspects of the electrical system, but ergonomics is 

something I’ve also considered important, and feel is too often neglected in 

the design of complex hardware systems.



I’ve attached reduced size jpg files to respect the bandwidth guidelines, 

based on the original dxf files, but will verbally describe the unusual 

aspects as well. I can also make available the dxf files showing clearer 

detail if anyone wants them.



The first component after the battery in most aircraft systems is the master 

relay, which isolates both the starter solenoid and the feed to the main 

and/or essential buses. It’s usually a large heavy solenoid that must be 

rated to carry starter current but normally carries only a small fraction of 

that current, and it takes about an amp to keep it picked up. My design uses 

a 60A ANL fuse at the battery, feeding a heavy-duty automotive 70A relay in 

the cockpit next to the master switch and fuse bus. I expect the smaller 

lighter relay will be more reliable than a $17 solenoid, especially because 

it is in a cooler lower vibration location, plus it will draw only one-tenth 

of the hold-in current which means more flying time in the event of battery 

only operation. In my specific system this is not insignificant, the 2 amp 

draw of two master relays would be ~10% of the typical main bus load.



In a conventional system, there is no high-speed protection of the several 

feet of cable that can exist from the master relay through the firewall to 

the buses, other than how fast the pilot can smell smoking insulation and 

turn off the switch, by which time major damage can be done to the wiring 

bundle. Once I discovered ANL fuses, I saw they could offer fast automatic 

protection of this major cable run. There are some good photos at 

http://www.highrf.com/gallery/Power-Grids of a Legacy ‘power grid’ using ANL 

fuses. In my system, because I require separate FADEC buses, I’ve used an 

additional fuse plus relay and fuse bus mounted at the battery, to eliminate 

power wiring runs into and then back out of the cockpit, which cuts weight, 

simplifies wiring, and minimizes wiring exposure to damage.



A reason given for the conventional system layout is that it can isolate a 

stuck starter solenoid. In my experience however, properly sized starter 

solenoids are far more likely to fail open than closed, especially since 

there is often an additional solenoid internal to the starter, and in worst 

case it’s an on-ground event.



The master relays are controlled by double pole 3 position rocker switches 

with internal LED annunciators (see attached Switchgear.jpg). This allows 

replacement of the traditional split control switch by providing a battery 

only/alternator disabled configuration in the middle position, with the 

normal configuration in the upper position. Illuminated rocker switches, 

apart from being more visually appealing than the typical steel toggle 

switch, can provide rapid day/night visual confirmation of true operational 

status since the indicator is driven by the voltage of the actual controlled 

circuit. Arranging the switches in close functional groupings also makes it 

easy for pre-landing checks- just brush a finger across the tops of all the 

switches to verify they are in the upper position.



 	  Quote: 	
		  From there, power flows to the master bus via 60A Schottky diodes, which 

generate much less heat than conventional diodes, reducing heat sink 
	


requirements. There is only one main bus. I spent a lot of time trying to 

decide what would go on an essential bus and what would not, plus 

considering what kind of interlocks and switching would be necessary to 

operate from the essential bus. In the end, given my dual batteries, dual 

alternators, and better protection of the main battery feed lines, I 

realized that a large essential bus is just as secure as a small one, and 

not having to set up an emergency configuration during a high-stress 

situation is a big advantage. Greg Richter’s comments about unnecessarily 

complex emergency systems (page 7 of his Aircraft Wiring pdf) are spot on in 

my opinion. Modern avionics and LED nav & panel lights don’t draw a lot of 

power, so normal daytime flight requirements are well under 20 amps for my 

setup, which can be carried by the 20A backup alternator, never mind the 

dual batteries. Many engine monitor systems, like the AFS3400, can monitor 

alternator loading and bus voltage plus provide programmable high/low limit 

alarms to these parameters, helping to keep track of loading during loss of 

a power source.



I don’t have an avionics master switch. I know this is controversial, and 

apposed to common procedure, but here is my rationale:

1) Modern avionics are not susceptible to switching transients the way a lot 

of earlier commercial gear was, especially during the early days of discrete 

transistors. Additionally, though it’s not shown on my schematics, diode 

suppression is fitted to all relays, something that wasn’t usually done in 

the past.

2) A single avionics master switch provides a single contingency failure 

point for the avionics, after all the work to provide full redundancy.

3) Battery voltage drop during starting can cause some avionics to drop out, 

but because of the dual diode isolation of the main bus, the avionics will 

automatically be fed only from the normal-voltage alternate battery when the 

main battery sags during start.



My fuel boost pump switch is a bit different in that there is an additional 

(AUTO) position, controlled by the FADEC system. However, takeoff/landing 

(MANUAL) position is up, consistent with the master switches, and as with 

them LED annunciators provide additional status information.



There is no Pitot Heat control, this is a personal thing. I’ve found that 

after flying the same plane for a while I rarely look at airspeed, and my 

setup gives me dual airspeed info plus AOA. Also, I’m a VFR pilot. Doesn’t 

mean I won’t decide to add it if I hear compelling reasons to.



Lighting is traditionally done with a row of toggle switches, one for every 

circuit, and here again I have departed from tradition, but in this case 

mainly with an eye to improved ergonomics. Details are on a separate drawing 

(Lights.jpg). Note that the labels on the rotary switch on the above panel 

layout jpg are one revision behind the schematic. I have shown just a 2-pole 

rotary switch, as high-reliability sealed switches usually have fewer 

configuration choices than cheaper switches. A single rotary switch, common 

in automotive applications, makes more sense to me than individual toggles. 

Just turn clockwise for more lights. The first position BEACON gives nav 

lights and strobes. Because strobes can be disorienting in reduced 

visibility they can be disabled via the STROBE ENABLE rocker switch, but the 

LED nav lights are always on given their miniscule power draw. APPROACH as 

shown energizes the landing lights in a wig-wag mode for high visibility, 

though this wouldn’t appeal to everyone. TAXI and LANDING are the last two 

positions respectively. Taxi lights are on in both positions, but one 

rotation counterclockwise on the ground eliminates the long-range landing 

light to avoid glare to other pilots. The small rotary knob beside the 

larger lighting switch controls dimming of cockpit lights. The STROBE ENABLE 

rocker switch status LED gives instant visual confirmation of strobe status, 

and the tight group of 3 controls in their separate location can be operated 

by touch only, like the master and fuel boost controls. This is difficult to 

accomplish in the traditional row of identical toggle switches which are 

often mixed in with other identical toggle controls.



I don’t have panel controls for trim or flaps, these are provided on the 

pilot and copilot control stick handles. A junction box is provided as a 

convenient local plug-in terminus for the control stick switches, trim & 

flap servos and controller, radio control lines, and power/ground lines. I 

have a schematic of this if anyone wants to see it.



My instrument panel will obviously have a lot fewer controls on it than is 

now common. I know that banks of toggle switches look impressive to the 

uninitiated, but realistically this doesn’t contribute to safety by reducing 

the pilot’s workload. Colour coding and grouping can help a little bit, but 

it still leaves something to be desired for night operations and status 

checks by touch while keeping focused outside the cockpit.



That’s about it for the design highlights. One thing not evident but also 

important is, wherever possible, separate physical routing of the main and 

alternate system wiring, including firewall penetrations. I’ve tried to 

envision how the system would handle a variety of failures and haven’t come 

up yet with any realistic scenario that would bring the plane down before 

you could get to a reasonable destination point. My only minor concern at 

this point is how load sharing is controlled, since the secondary alternator 

is just 20A. I’ve talked to Bill Bainbridge at B&C, and he said that he sets 

the voltage regulator about a volt lower on the alternate. This seems like a 

lot to me, I’m not sure what effect this has on keeping the alternate 

battery in optimum condition. I couldn’t find much in Bob’s book or the 

forums on this point.



Anyway, I’m completely open to suggestions and/or criticism. Ego should 

never get in the way of safety. I’d appreciate anything in the way of 

comment. I’m now at the point of being almost ready to start wiring things 

up, and need a reality check that what I intend to do makes sense.



Thanks, Terry McMillan

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nuckolls.bob(at)cox.net
Guest

Posted: Thu May 15, 2008 4:58 am Post subject: Dual redundant electrical system

At 08:05 PM 5/14/2008 -0400, you wrote:



 	  Quote: 	
		  X-MIME-Autoconverted: from 8bit to quoted-printable by matronics.com id 

m4F09XD3009485

Content-Type: text/plain; x-avg-checked=avg-ok-7F1173FE; format=flowed


	


   <snip>

 	  Quote: 	
		  I've attached reduced size jpg files to respect the bandwidth guidelines, 

based on the original dxf files, but will verbally describe the unusual 

aspects as well. I can also make available the dxf files showing clearer 

detail if anyone wants them.


	


    Can you .pdf the .dxf drawings from your CAD application?

    The .jpg files are kind of "swiss-cheesy" difficult to

    deduce the details of your design. I'll .pdf them and repost.

 	  Quote: 	
		  The first component after the battery in most aircraft systems is the 

master relay, which isolates both the starter solenoid and the feed to the 

main and/or essential buses. It's usually a large heavy solenoid that must 

be rated to carry starter current but normally carries only a small 

fraction of that current, and it takes about an amp to keep it picked up. 

My design uses a 60A ANL fuse at the battery, feeding a heavy-duty 

automotive 70A relay in the cockpit next to the master switch and fuse 

bus. I expect the smaller lighter relay will be more reliable than a $17 

solenoid, especially because it is in a cooler lower vibration location, 

plus it will draw only one-tenth of the hold-in current which means more 

flying time in the event of battery only operation. In my specific system 

this is not insignificant, the 2 amp draw of two master relays would be 

~10% of the typical main bus load.


	


    <snip>



    Terry, you've obviously spent a great deal of time and thought

    on this. At first blush, the system you propose is a relative

    of Figure Z-14 as published at:



http://www.aeroelectric.com/PPS/Adobe_Architecture_Pdfs/Z14L_1.pdf



http://www.aeroelectric.com/PPS/Adobe_Architecture_Pdfs/Z14L_2.pdf



    This dual-alternator/dual-battery architecture was crafted over

    a period of 15 or so years and embodies practices well demonstrated

    in aircraft for over 60 years.

 	  Quote: 	
		  Anyway, I'm completely open to suggestions and/or criticism. Ego should 

never get in the way of safety. I'd appreciate anything in the way of 

comment. I'm now at the point of being almost ready to start wiring things 

up, and need a reality check that what I intend to do makes sense.


	


    What you've proposed will probably function as you envision it.



    It's more complex than the Z-14 drawings in many respects

    and may not meet design goals for aircraft systems design

    where we strive for low parts count and failure tolerance.

    This translates directly to reductions in weight, cost, and

    pilot workload. This approach also increases reliability . . .

    a part that isn't there cannot be a failure item in your finished

    system.



    You've cited rationale for features of your proposed

    system that are not a concern for most folks in aviation. It

    would be a much more direct path to "architectural nirvana"

    to build on what's gone before than to stir a bunch of

    parts into a totally new recipe.



    At the risk of sounding like I'm suffering a "not invented

    here" moment, could you take a bit of time and tell us where

    you perceive that Z-14 does not meet personal design goals?



    Bob . . .

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rjquillin

Joined: 13 May 2007
Posts: 123
Location: KSEE

Posted: Fri May 16, 2008 4:25 pm Post subject: Dual redundant electrical system

On Fri, May 16, 2008 at 8:51 AM, Robert L. Nuckolls, III

<nuckolls.bob(at)cox.net> wrote:

 	  Quote: 	
		   

 <nuckolls.bob(at)cox.net>



 At 11:27 PM 5/15/2008 -0400, you wrote:


	


 	  Quote: 	
		  

  The .pdf files are fine. I've posted them to my

  server at:



 http://www.aeroelectric.com/_temp/



  I'll invite folks on the List to go get them and

  participate in this thread as the spirit moves them . . .




	


Incomplete or incorrect url Bob?



Ron Q.

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rshannon

Joined: 05 Sep 2007
Posts: 62

Posted: Fri May 16, 2008 5:01 pm Post subject: Dual redundant electrical system

 	  Quote: 	
		  

 > http://www.aeroelectric.com/_temp/

 >

 >  I'll invite folks on the List to go get them and

 >  participate in this thread as the spirit moves them . . .

 >

 

 Incomplete or incorrect url Bob?

 

 Ron Q.

 	


The link works from here. Note there is an underscore character preceding "temp".



FWIW, as a former IT geek, I discouraged webmasters from using underscores in web URL's because when the whole link is automatically highlighted and the whole link becomes underscored (as it does in most email programs) the unaware reader may not realize there's a separate underscore character. If spacing is necessary, hyphens are better. Of course, spaces never work in a URL.

 

Ron



  [quote][b]

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rjquillin

Joined: 13 May 2007
Posts: 123
Location: KSEE

Posted: Fri May 16, 2008 5:23 pm Post subject: Dual redundant electrical system

About 10-15 minutes after the post, it also worked for me...

 Sigh.

 Also paid to be an IT geek at work.

 Ron Q.

 At 17:57 5/16/2008, you wrote:

 [quote]  >  http://www.aeroelectric.com/_temp/

  >

  >  I'll invite folks on the List to go get them and

  >  participate in this thread as the spirit moves them . . .

  >

  Incomplete or incorrect url Bob?

  Ron Q.

 The link works from here. Note there is an underscore character preceding "temp".

 FWIW, as a former IT geek, I discouraged webmasters from using underscores in web URL's because when the whole link is automatically highlighted and the whole link becomes underscored (as it does in most email programs) the unaware reader may not realize there's a separate underscore character. If spacing is necessary, hyphens are better. Of course, spaces never work in a URL.

 Ron

  	  Quote: 	

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Bob McC

Joined: 09 Jan 2006
Posts: 258
Location: Toronto, ON

Posted: Fri May 16, 2008 5:27 pm Post subject: Dual redundant electrical system

The link worked fine for me and there were two .pdf's there which both

opened fine as well. One labelled "lights" the other "master". Just clicked

on the e-mail link and away we went.



Bob McC



DO NOT ARCHIVE



---

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_________________
Bob McC
Falco #908
(just starting)

nuckolls.bob(at)cox.net
Guest

Posted: Fri May 16, 2008 6:30 pm Post subject: Dual redundant electrical system

At 05:21 PM 5/16/2008 -0700, you wrote:



 	  Quote: 	
		  



On Fri, May 16, 2008 at 8:51 AM, Robert L. Nuckolls, III

<nuckolls.bob(at)cox.net> wrote:

 > 

 > <nuckolls.bob(at)cox.net>

 >

 > At 11:27 PM 5/15/2008 -0400, you wrote:



 >

 >  The .pdf files are fine. I've posted them to my

 >  server at:

 >

 > http://www.aeroelectric.com/_temp/

 >

 >  I'll invite folks on the List to go get them and

 >  participate in this thread as the spirit moves them . . .

 >



Incomplete or incorrect url Bob?


	


   Don't think so. This is a link to a directory where

   you will find two separate .pdf files each of which

   needs to be downloaded independently.



   This is not unlike the general files archives

   on my server like:



http://www.aeroelectric.com/Pictures/



and



http://www.aeroelectric.com/Reference_Docs/



   where you don't get a particular document

   but a directory structure for many documents.



   You should be able to double-click the link

   cited and it should take you to the appropriate

   directory(ies).

        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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