Critique My Electrical System

[stickandrudder1]

Below is a draft electrical system based on Nuckolls' Z-13 electrical system drawing. Please offer comments and ideas for improvement. Note that I plan on using the internally regulated Plane Power 30A backup alternator, which is supposed to be out by Oshkosh. If not, I'll use B&C SD-20. The reason for the second 7.2 AH battery is to insure field current to the backup alternator in the event of main battery failure.

Other questions I have are:

Is an Avionics bus & switch unnecessary or is it still a good idea? I know the GTX 327 is recommended to be powered off during engine start. Also the GTN 6xx install doc says it should be on an Avionics bus. Others have told me that the Avionics bus is unnecessary with modern avionics, adds complexity and additional potential points of failure.

What are the failure modes of the EBUS Diode? Will it ever fail open? If it fails and allows reverse current, what is the best way to detect the failure?

My schematic has a battery bus drawn, but I really don't see a need for it. Other than a clock and interior lights, is there something that I have placed on another bus that really should be on a battery bus?


What about the location of the backup alternator?
I show it connected to the essential bus instead of the unswitched side of the battery contactor, in the event of a dead short in the battery. I'm not sure if that's a failure mode I need to even consider.

Thanks

Paul

 

[Bob'sRV6A]

Paul,

I'm thinking of an avionics bus, too. Maybe one EFIS, GTNxxx, and SL30. What does that add up to? Less than 20A? So maybe a switch instead of the relay to save parts? Or is the relay really needed? I don't know.
Also, is the 30A backup alternator really needed? I am a little under amped with an SD-8 which can put out about 10A max, but it doesn't need the extra battery and is probably much lighter than the 30 A. or the B&C 20 A. Have you done a load analysis yet?
I too like the Z13 for simplicity.
Bob

 

[Bevan]

Hi Paul,

I'll try to answer your questions. This is based on my opinions and is based on what I am doing, but not flying yet. YMMV.

You haven't stated what your aircraft's mission is, but assuming day VFR for fun, I would (and am) building my electrical system using the SD8 for a backup in case of main alt failure. Enough to comfortably get me to an airport. Much lighter and simpler than the 30 amp alt/ 2nd battery config.

Voltmeter and ammeter via the EFIS/EIS will tell me if the backup alternator is keeping up with the demand for power and if not, I can shed more loads for awhile.

I like the avionics switch/relay and have put one in. I think the benefits offset the additional complexity of wiring and doesn't add much weight.

Shorted battery? How rare is that? Just replace it regularly.

I'm using a battery bus for the convenience of wiring. It powers the Pmags, Endurance bus and the horn. Yes, I said horn. Shouldn't every airplane have one? The horn can honk even with the master switch turned off. I think this is a valid safety feature, but it is a relatively heavy item. I'm using a compact motorcycle airhorn compressor combo and looks like this one, see

http://www.twistedthrottle.com/trade/productview/2137/

I feel the aux alternator should be connected directly to the battery so as not to require the contactor to be on when trying to conserve power. Also, the batt bus is connected here too which feeds power to the endurance bus via the alternate feed path. Your drawing is missing this feed path by the way. I don't think it would work any other way.

I believe the diode's typical failure mode is to go open circuit. Ask Bob Nuckolls

My 2 cents YMMV

Bevan
RV7A Z13/8 with Pmags and horn Not flying yet. YMMV