Electrical System Planning - Inputs are welcome

YvesCH said:

Dear all

I am currently planning my Electric Power distribution and wanted to know what you think about it.

Basic infos: I am building a RV-8 for VFR and Night VFR flying.
Equipment: G3X 10" Touch, GNC-255, GTX23ES, GMA245, Garmin A/P and Electric Trim. As I will install a Catto Prop the Battery (PC680) is mounted on the Firewall. On the engine I have planned a B&C starter and a Plane power alternator "with OV".

As the Panel is glass I really want to have a Ground Power Jack.

My current Problem is that I do not know if I need a Avionics Bus to save the
avionics during engine start (Com, Audio Panel etc). During startup the G3X, GSU73 and the GEA24 will be kept alive with the GAD27.

Here is what I have so far. Am I missing something?

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skylor said:

Hi Yves,

I don't think I had a chance to discuss my electrical system when you visited last November, but here are some of my opinions on the subject:

1. When I designed my panel and electrical system, I chose to go without an avionics master because I feel a single avionics switch is potentially a single point of failure.

Yes that`s what I had in mind as well but in my current configuration some of the avionics are not switchable (e.g. GMC307)

2. Every item in my radio stack has built in power switches, which I use to power them up with after engine start.


Unfortunately not mine (see above)

3. My dual EFIS displays do not have internal switches so I installed a pair of miniature toggles next to the ignition switch to switch the displays. The AHRS/Air Data unit is also powered from these two switches so that turning on either display will provide it with power.

this could be a solution!

4. I chose to keep my lighting switches off of the panel and put them in the right side console. This leaves the panel a little less cluttered with only "flight essential" switches.

yes this I wanted to do as well, and the CBs like Pauls Dye below

Skylor

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Carl Froehlich said:

Some thoughts:
- Fr a 60 amp alternator use a 70 amp output fuse/breaker.

will do!

- If you want to run your panel on ground power, you need not install the clunky ground jump connection. As you should never run your panel on a battery charger, you can make a connection on the output side of your master to plug in a quality, 20amp or so regulated power supply. The point here is you leave the master off such that your battery is not connected.

Yes but as I live in Switzerland I want the ability boost my battery in cold conditions, therefore I decided to use this connector. As it is "aviation standard" I can find a suitable booster in nearly every FBO

- I agree about not doing engine start with all the panel up. You can solve this by putting a switch in line with the diode from the main power buss to the essential buss. As you have an essential switch anyway you have a second path to power the essential buss if this switch fails.

Hmm good Idea but I need the G3X and the GEA24 running to see if I get fuel flow and after starting if I get Oil pressure

- Put both pMags on the main buss. Power is only needed for engine start.

Ok will do!

- You have too much load on the essential buss for the single #14 wire feeding it.

I have a max of 13.33A therefore a #14 wire should do the job, no?

Carl

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Carl Froehlich said:

#6 wire and a 60 amp fuse for you main buss is way too much. Use the #6 on the alternator output.
True, I will use a #8 wire for the main bus and a #6 wire on the alternator output
You now have a diode preventing essential buss power from getting to the avionics buss. Considering you are a single alternator, single battery system, I'm afraid you added complexity without gaining reliability.
But this is exactly what I want, I do not need the audio panel or autopilot panel in the case of an alternator failure but I want the ability to switch them off during cranking the engine with one single switch.
There are a lot of ways to go. For example, if you divide your Essential buss and Avionics buss into "Avionics #1" and "Avionics #2" such that the left EFIS on #1, right EFIS on #2, Nav/Comm #1 on #1 and Nav/Comm #2 on #2, and so forth. Power these using a 30 amp relay connected directly to the battery. This allows you to independently control the avionics without having the master solenoid on. This comes in handy if you are using a traditional master solenoid as it draw ~3amps just to stay shut. So for a loss of alternator the first step would be to open the master and dump all the loads not on the avionics buss. You can always add them back (like fuel pump) when it comes time to land. By doing this you greatly extend the battery reserve for a loss of alternator casualty.
Now add a simple toggle switch to cross connect between Avionics #1 and Avionics #2 to use if one of these 30 amp relays fails.

To simplify, divide all your loads into three groups:
Avionics #1 (I call this "Left Vital")
Avionics #2 (I call this "Right Vital")
Main Buss (I call this "Non-Vital"). The Non-Vital buss gets the pMags, electric fuel pump, nav lights, strobes, landing lights, pitot heat, and any misc. stuff like seat heaters and USB connections.
The Vital Busses will include things like trim, flaps, autopilot and panel lights.
Hmm I have to rethink this but its also a bit complicated no?
If you want to get the biggest leap in capably and redundancy you will need to go to a two battery, single alternator set up. If you do this you can dump the clunky external power stuff as two PC-680 (or PC-625 like I'm using) will crank your engine under the most severe conditions. Two batteries will also provide ~3 hours of full IFR flight with no alternator.
I rather would install a SD-8 or SD-20 backup alternator
Carl

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vlittle said:

I submit that the concept of an essential bus is flawed. Predetermining what is essential means that you can foresee every possible failure scenario. Other than keeping the engine running, what is essential in VFR flight? What about IFR flight... A second engine?

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sportflyer said:

I decided not to have an essential bus and installed a backup 20A alternator from B and C http://www.bandc.aero/standby-regulators.aspx
All of my circuit breakers are pop-able, except the 20a backup one.
The idea is that should the primary alternator fail, once the battery voltage droops the backup alternator starts providing power. You then pull circuits till you get your current consumption below 20A.
Very simple, effective and does not complicate the bus with diodes and extra relays.
The B and C site has a good installation and operation guide.

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ariel_arielly said:

Dear Yves,
Looking at your schmatic, I found some issues I would recommend you to notice:
1. There are no wires from tha SHUNT to the Garmin ammeter.
True, I forgot those!
2. The diode between the main bus to the essnttial bus should be a low voltage drope schotkey diode.
Yes I would use this one:
http://www.bandc.aero/essentialbusdiodew8wattheatsink.aspx

3. I would add an additional fuseof 60A as close as practical to tha alternator "B" lead (this will protect the wire between the alternator and the SHUNT in case when the engine is running and a short circuit between the SHUNT and the A/C ground occures).
Ok, I check that again!

4. You better choose C.B's to protect electrical wires in accordance with AC43-13 (don't forget that fuses or circuit breakers protect electrical wires not equipment).
Yes except the ANL there are only CBs in the system. The Ground PWR and the E-BUS Alt Feed switch are Switch breaker

Ariel Arielly, Israel
RV8a S/N 80295 200 hours

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BillL said:

There are several "right" ways to measure amps, and the buss condition. I put the shunt in the power side from the battery to all its demands, except starting. That way I know the amp draw of all the components with the alternator functioning or not. Buss voltage is used to assess the health of the alternator and charging of the battery.

My system lacks the alternator output change during the start recovery to the battery, but yields voltage/amps for managing the alt-out flight condition.

Some measure one or the other, some measure both. Builders choice.

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Parky109 said:

Hi Yves

How did you draw your wiring diagram , what software package ?

Looks good !

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I-TERA said:

Hi Yves,
I will try to introduce an argument adding more reliability to aircraft electronics.
Using diodes in the power supply path ( series) will subtract some tension to the equipment. The relevant data of Shottky diodes in this application are Vf ( forward voltage drop at the max current used ) Vr ( reverse voltage supported by the diode ) If ( direct current supported ).
The most frequent failure of diodes is related to spikes ( inductive load switched, the worst is the alternator and alt. field in case of alternator failure ). Spikes are defined in max Voltage and max energy ( sorry for the radical simplification ).
If your choice is a diode capable of withstand a hi voltage spike, the consequence is a diode with a high Vf. During the discharge of a Pb accumulator the voltage drops below 10 Volt if you use all the capacity ( when you calculate the time of operation under battery remember to considerate not only the nominal capacity in Ah but also age and temperature ), so subtracting one other volt ( Shottky diode with a Vr of 600V ) may fall under the min supply voltage of the electronics.
If your choice is a low drop Shottky (STPS60L45CW) ( less than 0.3V at 30A ) the spikes can destroy the diode itself (also a high reverse voltage diode can be affected )
The answer is to add a capacitor ( 10.000 microFarad, 50V, 125 °C ) and a spike suppressor ( 2 x BZW5015, max clamp voltage 35V, max spike energy 5KW) both in parallel to the main bus.
They are sigle failure components but the security factor of more than tree gives the compliance with the norm.
Adding the protections permit also to start and stop the motor and alternator with the electronic on. ( alternator field absorbs 5A ).
The component codes refers to a 12V circuit.
Claudio

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terrye said:

Yves,
You are showing 2 x ANL 60 amp fuses between the starter contactor and battery contactor. An ANL fuse is more properly used as a "current limiter". According to Bob Nuckolls, the purpose of this device on the alternator B lead is to protect the system from a catastrophic alternator failure that will cause many hundreds of amps to flow from the battery. The ANL fuse is a device that still behaves like a fuse but has a much longer time constant. So I think the proper place for the ANL fuse is on the alternator B lead as you have shown.

However you also show another one going to the main power bus. I'm not sure this is a good application for this device with a long time constant. In addition, with the mounting base it needs, it's heavy and bulky. I think you need "normal" fuse or a 60 amp circuit breaker here with a short time constant.

Here's some more information from Bob Nuckolls:
http://www.aeroelectric.com/articles/anl/anlvsjjs.html

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YvesCH said:

Hi Terry,

thank you for your Reply. In most certified Aircraft I have seen, the line between the battery contactor and the main power bus is not protected at all. But my build advisor suggested that I should put at least a ANL fuse in this line. A standard CB also has a trip time from 2-35sec at 200% of the rated current. Not much longer than a ANL (at least from the datasheet I have seen). My Idea was to protect the line as close to the battery contactor as possible as the possiblities to make a short is higher where it penetrates the firewall and gear Tower than on the main power bus.

Or do you have some datasheet to compare a CB and a ANL Time-current curve?

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