Matronics Email Lists :: View topic - Alternate Feed Switch, is it a Master Switch??

Matronics Email Lists
Web Forum Interface to the Matronics Email Lists

Get Email Distribution Too! FAQ Search Memberlist Usergroups Register

Profile Log in to check your private messages Log in

Alternate Feed Switch, is it a Master Switch??

Matronics Email Lists Forum Index -> AeroElectric-List

View previous topic :: View next topic

Author Message

nuckolls.bob(at)cox.net
Guest

Posted: Sat Apr 12, 2008 7:36 am Post subject: Alternate Feed Switch, is it a Master Switch??

At 12:06 AM 4/12/2008 -0700, you wrote:



 	  Quote: 	
		  



Help! Am working on a new electrical system design (pretty much a standard 

Z-11) for a complete re-wire of a certified ship. A friend tells me that a 

certified aircraft can only have one Master Switch per FAA regs.


	


   Ask him to cite the regulation. I've never read it anywhere.

   We have lots of switches in TC aircraft OTHER than the DC

   POWER MASTER switch that re-organize bus structures and/or

   control alternative pathways to power sources. For example,

   many offerings in recent years at HawkerBeech have a "clearance

   delivery" switch that allows one to power up one radio directly

   from the battery for the purpose of getting ATIS and Clearance

   Delivery before powering up the whole airplane.



 	  Quote: 	
		    Is the "Alternate Feed" switch  for the Endurance Bus considered a 

 Master Switch? If so, how does it reconcile with any regs? Or, is my 

 friend wrong? I have read Sec. 23.1361, regarding Master Switch 

 arrangement, and I'm not totally comfortable in my interpretation of what 

 is being said. The section seems to infer that more than one switch can 

 be incorporated into a Master Switch arrangement, but with some 

 restrictions. Can someone shed some light on me regarding this matter?



I have been planning to locate the Alternate Feed switch immediately 

adjacent to the main Master Switch and under a switch guard. Presumably, 

both switches would only be on simultaneously for the short time after 

energizing the Endurance Bus via the Alternate Feed and turning the main 

Master off after an alternator or battery contactor failure.



Per 23.1361, am I restricted to no more than a 5 amp total load on the 

Endurance Bus (that should be plenty)? Would I also be restricted to no 

more than a 5 amp protective device for the bus feed? My bus feed wire 

will be about 8" long.



Thanks to any of you "Masters" for any enlightenment you can share.


	


   The 5-Amp rule presupposes that the battery feeder to the

   E-bus Alternate Feed switch is LONG . . . 5A is the rule-

   of-thumb for limiting always-hot feeders directly from

   the battery that might pose a post-crash risk of fire.

   It's pretty much a hip-shot number. Give me control over

   the conditions and I can light off gasoline with a 1A

   feeder and not open the breaker.



   If you NEED to protect the alternate feed at more than

   5A, then install a "mini-battery contactor" right at

   the battery bus as suggested in:



http://www.aeroelectric.com/Pictures/Schematics/E-BusFatFeed.gif



http://www.aeroelectric.com/PPS/DCPwr/E-Bus/E-Bus_Heavy.pdf



    This provides for local disconnect of feeders protected

    at more than 5A.

 	  Quote: 	
		  The FAR quoted above can be found at:



http://www.risingup.com/fars/info/part23-1361-FAR.shtml


	




   Okay, let's look at it:

Sec. 23.1361 - Master switch arrangement.



(a) There must be a master switch arrangement to allow ready disconnection 

of EACH electric power source from power distribution systems, except as 

provided in paragraph (b) of this section.



       Note that the rules allow for MORE than one power source, hence more 

than one switch . . .



The point of disconnection must be adjacent to the sources controlled by 

the switch arrangement.



       Yup, unless the feeder is limited to 5A or less as described below:



If separate switches are incorporated into the master switch arrangement, a 

means must be provided for the switch arrangement to be operated by one 

hand with a single movement.



       Yeah . . . this is a tad silly. If one is concerned about getting

       switching ALL potential hazards off with switches as a single movement

       then the magneto switches should probably be included under the

       one-movement-does-all mechanism. In the suggested switch layouts

       shown in



http://www.aeroelectric.com/PPS/Mechanical/Switches.pdf



        . . . switches that control all power including ignition are

       grouped at one end of the array. Easy to locate and easy to get

       turned OFF in a timely manner when circumstances call for it.

       For a TC aircraft, you can craft a hinged "crash bar" that

       provides the requested one-stroke operation of adjacent switches.



       I had a discussion some years ago with a 172 owner who was

       wanting to do the same thing you're doing. We discussed using

       switch guards over adjacent switches and ganging them together

       with a bonded plate so that slapping them down opened both

       switches.



       Now, this one-hand-operation is somehow magically ignored in

       some TC aircraft where emergency batteries are used to power

       up essential appliances . . . they are generally not fitted with

       or even arranged such that one-handed operation is possible.



(b) Load circuits may be connected so that they remain energized when the 

master switch is open, if the circuits are isolated, or physically 

shielded, to prevent their igniting flammable fluids or vapors that might 

be liberated by the leakage or rupture of any flammable fluid system; and



(1) The circuits are required for continued operation of the engine; or



(2) The circuits are protected by circuit protective devices with a rating 

of five amperes or less adjacent to the electric power source.



(3) In addition, two or more circuits installed in accordance with the 

requirements of paragraph (b)(2) of this section must not be used to supply 

a load of more than five amperes.



(c) The master switch or its controls must be so installed that the switch 

is easily discernible and accessible to a crewmember.



      Since your proposed changes are not part of the original

      TC on the as-built airplane, you're going to have to jump

      STC/337 hoops. So this is between you and your local

      regulators. What you're proposing has been done before

      and is not a really big deal. It's just slight rearrangement

      of the design goals that have been similarly addressed with

      additions of the "clearance delivery" switch on modern

      aircraft.



      Bob . . .

- The Matronics AeroElectric-List Email Forum -

Use the List Feature Navigator to browse the many List utilities available such as the Email Subscriptions page, Archive Search & Download, 7-Day Browse, Chat, FAQ, Photoshare, and much more:

http://www.matronics.com/Navigator?AeroElectric-List

Leo Holler

Joined: 29 Mar 2007
Posts: 7

Posted: Sat Apr 12, 2008 10:52 am Post subject: Re: Alternate Feed Switch, is it a Master Switch??

Thanks Bob for an exceptional and thorough response. You very clearly took the most of the "fog" out of my feeble interpretation of Section-23-1361. I'll be able to better make my 337(Field Approval) case to FAA now, as well as educate my friend. 

 	  Quote: 	
		  The point of disconnection must be adjacent to the sources controlled by 

the switch arrangement. 



Yup, unless the feeder is limited to 5A or less as described below: 	




Does the above mean that the 5A rule applies only if there is no disconnection adjacent to the source? Also, do you consider the 8" unprotected run from battery to Alternate Feed switch to be excessive? If so, I can fuse it, or add the mini-contactor as you have suggested. Also, I have heard that new Cessnas have Essential/Endurance buses installed. Have you or anyone else heard of this and, if so, how did they handle the Master Switch arrangement? A local Cessna dealer hinted that I could get their schematic by buying the CD for several hundred dollars or by buying a new aircraft.  This forum seems much more user friendly.  



Thanks again.



Leo

- The Matronics AeroElectric-List Email Forum -

Use the List Feature Navigator to browse the many List utilities available such as the Email Subscriptions page, Archive Search & Download, 7-Day Browse, Chat, FAQ, Photoshare, and much more:

http://www.matronics.com/Navigator?AeroElectric-List

Display posts from previous:

	Matronics Email Lists Forum Index -> AeroElectric-List	All times are GMT - 8 Hours
Page 1 of 1

Jump to:

You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot vote in polls in this forum
You cannot attach files in this forum
You can download files in this forum