Electrical Buses
 

I am finishing up the design of my electrical systems and would like some opinions on how people have set up their electrical busses. Basically I am putting in a main bus and an essential bus and am curious as to what devices people consider "essential". I am feeding my busses in pretty much the standard way by isolating the essential bus from the main bus with a doiode and providing a switch to connect he essential bus directly to the battery if I want to turn off the master switch to shut down the main bus. The question is what devices should I place on the essential bus so I am curious as to what others have done.

Good Question!
 

This should provide some interesting discussion....

I make no claim to having a "right answer" - just what I thought about in terms of an IFR essential panel. Note that I have a separate Avionics bus which can be switched between Main and Essential as sources....that's where the Navs, Comms and GPS's get their power....

Essential Bus Loads:

Trim
Flaps
Fuel Pump
Audio Panel
EFIS Displays (power feed B)
EIS
AHRS
Autopilot

Of course, for a VFR only plane, I'd probably think a lot different - heck, there isn't much that is essential for flight in that case!

Paul

Timely
 

This is timely and I'd like to chime in with my current plan and a request for feedback as well.

Battery Buss
Battery Contactor
Cigar Lighter Socket
Ignition - E-Mag / P-Mag incl. Maint Ann Light
Starter
Any avionics "keep alive" circuits
Panel flood light

Essential Buss
AHRS / Magnetometer
Audio Panel / Intercom
Autopilot
EFIS
EIS
Elevator Trim
Fuel Boost Pump
GPS(s)
Nav/Comm
Panel power for portable electronics
Transponder
Avionics fan / defroster

Main
Cockpit Light(s)
Add'l Panel Light(s)
Low Voltage Warning Light (also serves as a master switch reminder)
Flaps
Seat Heaters
Landing Light
Taxi Light
Position Lights
Strobes
Starter Contactor

Notes:
1. I have manual aileron trim.
2. I will be able to load shed individual items if needed.
3. This will be a "light" IFR bird meaning no planned enroute IMC, almost only to get up and down through a layer.
4. System design will likely be an Aeroelectric Z-11 or Z-13/8

Input is very much welcome.

George

Paul,

Thanks, I'm kind of using your panel as my model but there are a couple of breaker labels I couldn't read on the photos. If I can remember correctly from one of our discussions, your switchable avionics bus source was due primarily to not being able to get the switch you originally wanted and it just kind of ended up that way. If you were to do it again, would you do the same thing or would you just put all the avionics on the essential bus? I sometimes wonder which of the avionics are really essential as I worry about the load on the essential bus of all the avionics as I was only going to feed about 15 amps to the essential bus from the battery directly. I guess I could put my GNC300XL GPS on the main bus as my EFIS has a built in GPS that draws a lot less power and that will be on the essential bus.

Also, did you actually put a seperate breaker in for the AHRS? It seems to me that it could be fed off the same breakers as the displays since the displays are somewhat useless without the AHRS, so why power it seperately.

I'm also doing a small aux battery and 8 amp standby alternator from which I will provide emergnecy power to the EFIS (and AHRS) and my Pmags. I figure so long as I have an attitude display and ignition, I can get the plane down in a real emergency.

As I write this, I am beginning to wonder if an essential bus is really necessary as it seems that all the services that would be shed by shutting down the main bus have on/off switches of their own which could remove their load. Having a direct battery feed could still be useful to recover from a master relay failure, but it may not be necessary to seperate the bussess you can shed all the unnecessary things like lights individually.

As you said, this could create an interesting discussion.

Paul,

AS far as avionics loads go, they really aren't very large! Heck, when I am in cruise, with lights off, I am barely registering 6 amps with everything else (including strobes) powered on. The avionics load isn't that significant.

I would probably separate the avionics bus again if I repeated the design, because it does give me an option. I am conditioned by years and years of training to deal with "bus shorts", where you have to unpower a bus. Can't tell you that they are common - except in simulations! For that reason, I like being able to power down a shorted bus, and still be able o power the avionics. There are other ways to do it, but this is pretty simple. You're right that I originally was going to have an On-Off-On Avionics bus switch (wanted it lever locking), and settled for an On-On. No big deal.

I do occasionally power cycle the DU's without cycling the AHRS, so separate breakers are useful. Sometimes, if you want to do a hard reboot on the DU's, that saves an AHRS re-align. (Never had to do it in flight, but very useful for ground testing and software loading.)

If I were doing things again, I would give strong consideration to buying a bunch of power diodes and having all critical boxes powered through multiple feeds, like the GRT units are set up (they do it internally, but there is no reason you couldn't do it externally for other boxes). Then you'd have no systems management to perform at all...and where's the challenge in that?! ;)

Paul

George,

You have some interesting ideas. I like the idea of a panel light on the battery bus. I also find it interesting that you put your starter directly on the battery bus without going through the main contactor, I guess there is no real reason that it should. Other than that you came up with pretty much the same list as I did except that I have the Pmags on the essential bus as well as on my emergency battery. That is just a extra failsafe as they should work without power on their own.

My biggest problem is trying to keep from over-complicating things. I need to step back and decide what is really necessary.

Paul,

That is good to know. Without any real experience, I tend to use breaker size to estimate power consumption which I guess is not a very accurate method :-)

I agree with you on isolating bus shorts, but I guess you have to draw the line somewhere with respect to complexity.

The idea of diode isolated power feeds is a good one and I am going to power my Pmags through diodes off the essential bus and aux battery.

I think I'll create a avionics/essential bus and use a locking off/on/on switch as follows:

Off
Main
Main and Battery

This has the same effect as a seperate battery feed switch, while allowing an off position for the bus.

Too much to think about this late at night.

Electrical Buses
 

Paul:

When using a diode to connect two buses, remember that there will be a voltage drop. So be careful where you sense the bus voltage on your system, otherwise you may see it at .7 volts low and it may give you a warning depending upon which low voltage warning type of sensing you use. I originally used a diode in my split bus system, and changed over to a dual path relay which was rated at 4 times the power requirement that the essential bus draws.

I also have an essential bus power feed through a fuse wired directly to the battery. If the master contactor opens, all I need do is lift the essentail bus power feed switch guard and flip the switch.

The only advantage to having the spit bus is that it takes the decision making away from the pilot, only powering essential items by way of bus grouping. In reality, all the pilot need do is reduce the load by turning things off (or pulling breakers) and run with a reduced load until landing. Try to keep in mind that no two inflight distress situations are alike. There may be times when you may need a certain piece of equipment that is not powered by the essential bus. So the real need for installing an essential bus can be questioned, especially when put in the context of your comment about over-complicating things.

Simpler, lighter, cheaper and less prone to failure is often the best way to go.

Anyway, my .02c your mileage may vary.

If the starter current does not go through the main contactor, what is the procedure to use in the unlikely event that the starter relay ever sticks closed? If the starter current runs through the main contactor, you can simply select the Battery Master OFF, and this removes power to the starter. If the starter current does not go through the main contactor, your choices seem to be to let it spin until it drains the battery, which would likely result in the battery getting, very, very hot. Would the battery explode? Or, you could find some tools and unbolt the battery cable, but I wouldn't want to be the guy who pulls a battery cable off with over 200 amps going through it.

Here's mine and the reasons:

Essential:

• GPS - Need to know where I'm going

• NAV/COM - Need to talk to people to ask for help

• TPDR - I want Center to be able to find me

• INTERCOMM - Can't hear the radios without it

• EI - Need to spin the big prop out front

• EFIS - Need to keep greasy side down

• AH - Backup instrument, same as above

• Engine Monitor - Critical fuel information contained here

EVERYTHING else goes on the primary. I don't need it for continued flight ops. When I get ready to land, I can switch the Master back on to use the fuel pump, flaps, or trim motors.