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Please critique my electrical design

Nihon_Ni

Well Known Member
I've been working on the electrical architecture for my RV-8 and I'm getting close to a solution, but before I start buying and installing components, I'd like to get some feedback on my deign.

I'm planning an IFR-capable RV-8 with a Titan IO-370 and G3X system with G-5, GTN-650, GTR20, et al. I'm planning to use the Earth-X ETX680 battery as the main, and a TCW 3 Ah backup battery. The main alternator will be a belt-driven B&C BC-460H (60 Amps) with a gear-driven SD-8 backup (<8 Amps).

I've read Bob Nuckolls book several times and I'm sold on two alternator one battery set up he described in Z-13. My architecture is based on that, and only has minor changes such as, no battery buss fuse block, addition of avionics switch, dual P-mags, deletion of the low voltage package (G3X has that feature), and addition of a back-up battery. After talking with a pro in my chapter, I deleted Bob's fusable link leading to the alternator field CB, and just placed that CB on the main bus. I also removed the diode between the main bus and essential bus, because I inserted an avionics switch and didn't see the need for a diode there.

I've come up with a battery buss, main busses and endurance busses. The battery bus isn't really a bus, but a couple of items hooked to the battery. I didn't think there were enough things there to warrant a battery fuse block. Main Bus has been divided into three: MB1 is fuses, MB2 is circuit breakers, and MB3 is a planned expansion of an additional fuse block. I gave this block a load of 5 amps as a planning figure. I won't construct it, but I will have a place to put it in case I need it in the future. Essential Buss 1 is fuses, and EB2 is CBs. I've mostly used fuses, except for a few items where I want to be able to use the fuse as an alternate means of disconnecting power to an appliance.

I've worked tirelessly on my load analysis, and I think it's pretty solid. I've had to make some assumptions of appliance power requirements when I couldn't find the exact power requirements of a particular unit.

I've used the "Night IFR" load analysis for my continuous flow value, and "Typical Max" load as my intermittent flow value when doing my analysis for sizing wires, using figures 11-2 in AC 43.13 as a reference.

I welcome any comments, suggestion or critiques of my design.

Thanks for looking.
Rob

Edit: Please see the updated schematic before diving into this one. I solved some of the issues that people pointed out with the schematic below.

https://www.dropbox.com/s/nrmieqbdi9egavm/Electrical System Overview (v13.15).pdf?dl=0

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Rob,
I used same setup.

Check using 2 LR3C units

Check amps on SD-8 @2500 rpm
I would recommend 410 model @2500rpm

Good job,
Boomer
 
A few comments:

The oil pressure switch only has two terminals, the normally open and the normally closed terminal. The switch has the common terminal connected to ground through the threads of the sensor. You'll need to move the switch to the other side of the Hobbs meter to switch the ground side of the meter.

Follow the flow of operation if you have the master switch off, the essential bus breaker closed, and the essential bus switch grounded. Current will flow through the NC contacts to power the essential bus... which then powers the essential bus relay, causing the NC relay contacts to open... which will remove power from the essential bus causing the NC contacts to close again... Lather, rinse, repeat. You end up with a buzzer circuit.

- What happens if your avionics master crowbar circuit somehow does see an overvoltage and tries to trip? A crowbar circuit operates by temporarily shorting its power supply inputs causing an upstream fuse to blow. In your case, the only upstream fuse is a 60 amp slow blow fuse. That's a pretty stout fuse for a small crowbar circuit to try to blow. Are you sure that the crowbar can blow that fuse before it blows itself up? There's a good chance the crowbar itself will blow up before the fuse blows.
 
Thanks for the feedback!

The SD-8 should make 7.6 amps at 2500. I failed to mention that I'm going to use a constant speed prop, so that will give me a little more control over the power generated by the SD-8. I looked at using the BC410 (20 Amp) in a Z-12 architecture and went down that road for a while, but after a lot of reading and study of Bob's designs, I opted for the Z-13/8 as a simpler option to cover my electrical needs. Bob recommends the Z-13 as the best option, and it took me a while to understand why. The Z-13 doesn't support a 20 Amp backup alternator, so changing it would dictate reverting to the Z-12 architecture.

My design philosophy is that the SD-8 will cover my electrical needs in the cruise phase, perhaps except when transmitting on the radio. When I start the approach phase the difference between the higher electrical demand and lower SD-8 production will be made up by stored power in the battery. Although my load analysis indicates 14.5 Amps for the approach and landing phase, that includes the fuel pump, which won't run during the entire phase. I could also turn off my Comm 2 and transponder if not needed, and that will reduce my approach and landing demand to 7.9, with the SD-8 generating ~3.5 @ 1700 RPM, those additional 4.4 amps will come from the battery. With a 12.4 Ahr battery, that should give me plenty of time to make the approach and land before the battery goes flat. Thus, the fuel on board becomes the limiting factor in an alternator failure condition, not the battery.

Rob,
I used same setup.

Check using 2 LR3C units

Check amps on SD-8 @2500 rpm
I would recommend 410 model @2500rpm

Good job,
Boomer

Have you been happy with your setup, or do you wish you had installed a 20 amp backup alternator?
 
A few comments:

The oil pressure switch only has two terminals, the normally open and the normally closed terminal. The switch has the common terminal connected to ground through the threads of the sensor. You'll need to move the switch to the other side of the Hobbs meter to switch the ground side of the meter.

Thanks, I'll correct that in the next version.

Follow the flow of operation if you have the master switch off, the essential bus breaker closed, and the essential bus switch grounded. Current will flow through the NC contacts to power the essential bus... which then powers the essential bus relay, causing the NC relay contacts to open... which will remove power from the essential bus causing the NC contacts to close again... Lather, rinse, repeat. You end up with a buzzer circuit.

Wow, I missed that. Would you recommend adding a diode between the avionics relay and the endurance bus, or moving the power source for the endurance bus relay? I was attempting to design in an automatic switching feature of the essential bus, but I could achieve the same effect if I moved the power wire to the main bus or to the Comm terminal of the avionics relay. I had it on Main Bus 1 in an earlier version but changed my mind. I now see the problem I've created.

Or, perhaps I eliminate the E-Buss Feed relay entirely and simply use the E-Bus CB switch as the feed control instead of the armed function as I have drawn it? Bob suggested a E-Bus Feed relay in his design so that's what led me to use it, and a chapter member suggested adding circuit protection between the relay and the battery contactor. I decided to make that circuit protector into a switch as part of my automatic E-Bus switching design, but maybe I'm overly complicating things?)

- What happens if your avionics master crowbar circuit somehow does see an overvoltage and tries to trip? A crowbar circuit operates by temporarily shorting its power supply inputs causing an upstream fuse to blow. In your case, the only upstream fuse is a 60 amp slow blow fuse. That's a pretty stout fuse for a small crowbar circuit to try to blow. Are you sure that the crowbar can blow that fuse before it blows itself up? There's a good chance the crowbar itself will blow up before the fuse blows.

Another good point I hadn't considered. I guess the first question is, do I even need a crowbar on the avionics relay? I added it patterned after Bob's design on the B/U Alternator relay, but I didn't think about its effect on the upstream CB. I could use a spare fuse on the Main Bus 1 to protect the avionics relay power supply.

I considered a CB switch in lieu of the avionics relay, but if I did the calculations right, I'd need a 30 Amp CB switch. I decided on a relay mostly for cost considerations, but perhaps a CB switch would be a better feature?

Thanks for the feedback!
 
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What's the logic of a crowbar circuit on the avionics relay? Crowbars in a/c are to prevent overvoltage events from an alternator/generator, which you already have; that's the only source of OV.

If you have an OV event on the main alt, the crowbar has high odds of blowing the 10A fuse feeding the main2 bus before the 5A CB trips. It's the nature of time constants built into typical CBs vs. fuses, and likely one of the drivers for using a fusible link to feed the CB in Bob's original design.

Any fault on the main2 bus could remove power to both Pmags. If you have to go to idle (turbulence, landing configuration, etc), can the internal generators reliably keep the Pmags alive?

Are the various buses physically separated by more than a few inches? If they are within 6-8 inches of each other, you could eliminate the fuses in the feeders to the bus2s. Blade fuses are very reliable, but you're adding the fuse & several contact points in each path. In any case, soldered-in fusible links would virtually eliminate any risk of 'nuisance trips' in the feeders.

Charlie
 
Work through basic failure analysis. Wires and devices fail short or open. Make a wire list, mentally short or open each of them, and think about the result.

For example, let's short wire #6. The 60A current limiter opens. With no power on wire 22, the e-bus alt feed relay switches to NC, which feeds electrons to the short via 10. The e-bus alt feed relay might buzz, but if it has much ON time (or simply welds its contacts), the 30A e-bus arm breaker will pop. The automatic system automatically unpowered everything.

Is there a recovery? SOP would have the pilot first switching the alternate feed, then the master. Given an open 30A breaker, doing so would net nothing. Add a 30A breaker reset, and it just pops again. Ok, so now throw the avionics master switch, then reset the breaker. Maybe the avionics relay opens, or maybe it doesn't. With a dead short on 6, relay coil voltage is probably too low to operate, and even if it works, it's just another buzzer circuit, like the one Gerald described for the alt feed relay with its switch at 2-3.

Gotta do the homework.
 
Get rid of that 30 amp breaker feeding the
E-Bus relay. It requires an always hot wire
running into the cockpit. In case of a
forced landing, that hot wire will ignite
fuel. Mount the E-Bus relay within 6 inches
of the main battery contactor. Double
insulate always hot wires where they pass
through the firewall.

How can you energize the E-Bus relay if the
main bus has no power? Connect the coil to
the always hot terminal.

Do NOT connect the E-Bus to the normally
closed terminal of the relay. The only way
to shut off the E-Bus is to energize the
relay which will run the battery down when
not flying.

Notice that the avionics relay is connected
in parallel with the battery contactor. The
relay contacts will carry starter current (and main bus current)
causing something bad to happen. Replace
that relay with a diode.

It is a bad idea to have two fuses in series.
Chances are both will blow, not just one.
Or the wrong one could blow. Get rid of that
15 amp fuse between the two E-Buses.

Why not use Bob Nuckolls' Z-13/8? Many have
tried to improve upon his architecture but
few have succeeded.

The problem with modifying a proven design is
that you might miss a problem that will only
show up when something else fails.
 
Thanks!

I really appreciate everyone's input. I'll address everyone's questions and comments individually, and your comments are helping me better understand what I've done wrong.
 
Charlie,

This is really good insight, thanks!

What's the logic of a crowbar circuit on the avionics relay? Crowbars in a/c are to prevent overvoltage events from an alternator/generator, which you already have; that's the only source of OV.

One thing I've learned from this post is that I didn't clearly understand the purpose and function of the crowbar. I have a better understanding now, and realize I don't need one on the avionics master.

If you have an OV event on the main alt, the crowbar has high odds of blowing the 10A fuse feeding the main2 bus before the 5A CB trips. It's the nature of time constants built into typical CBs vs. fuses, and likely one of the drivers for using a fusible link to feed the CB in Bob's original design.

Thanks, that makes sense. When I ran it by guys in my chapter we couldn't figure out why the alternator feed supply needed two circuit protections, but now I get it. I'll put that fusable link back in.

Any fault on the main2 bus could remove power to both Pmags. If you have to go to idle (turbulence, landing configuration, etc), can the internal generators reliably keep the Pmags alive?

That's a really good point. P-mags stated capability is to generate their own power at 750+ RPM. I don't foresee a condition in flight where I'd have that low of an RPM aside from the landing flare. Bob's design didn't have dual P-mags, so this was an area where I adventured out on my own. I see the logic in moving one of them to the Battery Bus.

Are the various buses physically separated by more than a few inches? If they are within 6-8 inches of each other, you could eliminate the fuses in the feeders to the bus2s. Blade fuses are very reliable, but you're adding the fuse & several contact points in each path. In any case, soldered-in fusible links would virtually eliminate any risk of 'nuisance trips' in the feeders.

Main Bus 1 & Endurance Bus 1 are next to each other on the avionics access panel cutout I made in the forward baggage hold. Main Bus 2 and Endurance Bus 2 are on the F-865 Switch Console, down by the pilot's right leg, across the cabin from the throttle. The wire distance there is about 3 feet from the fuse blocks, so that why I added fuses to those wires. I wasn't sure how far I could go with how many amps and not need a fuse, so I erred on the side of adding fuses. Thanks for the info on fusable links, I'll change out those fuses for links.
 
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Work through basic failure analysis. Wires and devices fail short or open. Make a wire list, mentally short or open each of them, and think about the result.

For example, let's short wire #6. The 60A current limiter opens. With no power on wire 22, the e-bus alt feed relay switches to NC, which feeds electrons to the short via 10. The e-bus alt feed relay might buzz, but if it has much ON time (or simply welds its contacts), the 30A e-bus arm breaker will pop. The automatic system automatically unpowered everything.

Is there a recovery? SOP would have the pilot first switching the alternate feed, then the master. Given an open 30A breaker, doing so would net nothing. Add a 30A breaker reset, and it just pops again. Ok, so now throw the avionics master switch, then reset the breaker. Maybe the avionics relay opens, or maybe it doesn't. With a dead short on 6, relay coil voltage is probably too low to operate, and even if it works, it's just another buzzer circuit, like the one Gerald described for the alt feed relay with its switch at 2-3.

Gotta do the homework.

Dan, as always, your posts are very insightful and helpful. I had thought through appliance failure modes, but I didn't think through all the dead short modes. The one you point out is pretty bad, and I totally missed that one.
I'm going to redesign the E-Bus feed, but even the changes I was thinking about fixing the problem Gerald pointed out wouldn't have addressed this short. I'll review my design wire by wire. Thanks!
 
Get rid of that 30 amp breaker feeding the
E-Bus relay. It requires an always hot wire
running into the cockpit. In case of a
forced landing, that hot wire will ignite
fuel. Mount the E-Bus relay within 6 inches
of the main battery contactor. Double
insulate always hot wires where they pass
through the firewall.

Bob's Z-13 and his Avionics Switch designs both have the E-Bus feed on a fuse, so that's why I added circuit protection to mine. I decided to change it from Bob's fuse to a CB switch to keep the appliances on the E-Bus from draining the battery when the engine was off due to the way I've wired the relay as NC. That's a good point about keeping an always hot wire out of the cockpit.

How can you energize the E-Bus relay if the
main bus has no power? Connect the coil to
the always hot terminal.

The E-Bus relay is NC. That was to enable auto switching function in the event of an alternator causality.

Do NOT connect the E-Bus to the normally
closed terminal of the relay. The only way
to shut off the E-Bus is to energize the
relay which will run the battery down when
not flying.

Which is why I put the E-Bus Arm CB Switch upstream of the relay.

Notice that the avionics relay is connected
in parallel with the battery contactor. The
relay contacts will carry starter current (and main bus current)
causing something bad to happen. Replace
that relay with a diode.

I guess I don't understand the benefit of adding a diode there. I understand a diode to be like a check valve in function. How will that protect starter current from reaching the E-Bus?

I just checked Bob's design for an Avionics switch, and he does in fact have a diode between the Main Bus and the Avi switch. I replaced the switch with a relay because my E-Bus max draw is 29 amps, so that means I need a switch rated for 35 amps, correct? I thought a relay would be easier than that size of a switch. (The distance between those two buses is only inches.)

It is a bad idea to have two fuses in series.
Chances are both will blow, not just one.
Or the wrong one could blow. Get rid of that
15 amp fuse between the two E-Buses.

Those buses are about 5 feet apart. As Charlie suggested above, I'll look into fusable links there instead of the fuse.

Why not use Bob Nuckolls' Z-13/8? Many have
tried to improve upon his architecture but
few have succeeded.

The problem with modifying a proven design is
that you might miss a problem that will only
show up when something else fails.

I did use his design. However, I needed to modify it because it didn't completely suit my needs. I wanted dual P-mags, which his design didn't have. I don't need the LV module that he designed because that capability is in my G3X. I also wanted an avionics switch, so I followed his optional design for that, but used a relay instead of a switch. Unfortunately, his avionics switch design is more generic and doesn't use a relay for the E-Bus, so I combined the two designs as I thought best. I strayed from his design in other areas that I will undo (Alt field circuit). So I couldn't simply follow his design without modification and end up with an airplane that I wanted.

Thanks for taking the time to offer your insights. I knew nothing about electrical when I started this project and I've learned a lot in the process, but clearly I have a lot more to learn.
 
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The reason for a diode is to prevent current from flowing from the battery
through the E-Bus to the main power bus and then to the starter motor when
cranking the engine. Yes, starter current will flow via the normal path, but part
of the starter current will also flow through the E-Bus circuit. Either a diode is
required, or else an automatic way to open the avionics relay during starting.
It is hard to say how much current will flow through those relays during
starting. But a starter can draw hundreds of amps. Chances are that the relay
contacts will be overloaded.
 
Schematic below uses a double pole momentary push button start switch
to disable the avionics relay during engine start. The diode in your drawing
is not wired correctly. Actually the diode is redundant because there is
already one connected to the start contactor. But if you want a second
diode, connect it to ground as shown below.
enhance
enhance
 
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* I've got an RV-9A that I fly IFR and an RV-8 that I don't. The -8 has a very responsive Dutch roll mode in turbulence, so I don't think it would be a good IFR plane, regardless of what equipment is in it. Double check that you really want to fly your -8 IFR;
* When time comes to update your databases, you'll want to be able to power the GTN650 and the G3X touchscreens without powering up the whole system and draining the batteries. Trust me, really worth doing;
* Rear seat heater? You'll end up with a really heavy plane if you add in all kinds of stuff like that;
* Consider the GTN750 instead of the GTN650 if you are going to go IFR, as the keyboard is nicer;
* Seriously consider having a separate switch to disable the servos and the electric trim, just in case there's a runaway condition.

Enjoy!

Ed
 
The reason for a diode is to prevent current from flowing from the battery
through the E-Bus to the main power bus and then to the starter motor when
cranking the engine. Yes, starter current will flow via the normal path, but part
of the starter current will also flow through the E-Bus circuit. Either a diode is
required, or else an automatic way to open the avionics relay during starting.
It is hard to say how much current will flow through those relays during
starting. But a starter can draw hundreds of amps. Chances are that the relay
contacts will be overloaded.

Thanks Joe. In the schematic I posted, as soon as power is applied to the main bus by closing the battery contactor, the avionics relay would close breaking the path between the main bus and the endurance bus for engine start. After start, I would remove the ground by throwing the Avionics Master switch. That's why I didn't think I needed a diode. That normally closed feature of the avionics relay was part of my design goal, which I patterned after a type certified airplane, but I didn't get the details right so I'm going to change that function.

The schematic you posted of the starter diode isn't showing up. Could you PM me the link?

Thanks again!
 
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* I've got an RV-9A that I fly IFR and an RV-8 that I don't. The -8 has a very responsive Dutch roll mode in turbulence, so I don't think it would be a good IFR plane, regardless of what equipment is in it. Double check that you really want to fly your -8 IFR;
* When time comes to update your databases, you'll want to be able to power the GTN650 and the G3X touchscreens without powering up the whole system and draining the batteries. Trust me, really worth doing;
* Rear seat heater? You'll end up with a really heavy plane if you add in all kinds of stuff like that;
* Consider the GTN750 instead of the GTN650 if you are going to go IFR, as the keyboard is nicer;
* Seriously consider having a separate switch to disable the servos and the electric trim, just in case there's a runaway condition.

Enjoy!

Ed

Thanks for the input Ed.

Yes, I am going to fly the -8 IFR. I appreciate your perspective, but I'm way too far down this path to switch horses. All planes are a series of compromises, and the -8 has the right set of compromises for the missions I want, which includes the ability to fly IFR.

That's a good point about being able to power up the system on the ground. I know that during the build I'll want to have the ability to do ground power ops for extended periods of time, and I haven't exactly figured out how to do that yet. For database updates perhaps I could just use the back up battery and let it recharge during the next flight. Does the database update take long enough that I need to be concerned about the 3Ah backup battery going flat during an update cycle? If so, perhaps I could retain the ability for longer ground power runs that I'll figure out for the build phase. I'll have to work on a solution to that issue.

The rear seat heater is 14 ounces. I doubt that will make the airplane so heavy that I won't enjoy it, but it certainly will make the plane warm enough that my GIB will enjoy it. I'm not doing a full interior, but I definitely need some creature comforts in the back seat, such as USB ports and seat heat. Happy wife, happy life! ;)

The keyboard is definitely nicer on the GTN750. I've used both a -650 and -750 in flight on a friend's dual equipped airplane. For me the price and panel space are more important features than the ease of functionality.

The reason for all of my CBs is because I want a way to disable individual appliances. In the G3X, the trim wires through the servo, so if I get a runaway trim situation, I'll disable the trim motor by pulling the auto pilot servo CB.
 
* Double check what the G3X has for a Hobbs meter equivalent. If you don't have an oil pressure switch, you'll get register lots of time spent fiddling with the avionics;
* Add in wires for a trickle charger or a Genius battery charger. That $99 charger saved a dead battery that was killed by leaving the avionics on too long during a database upgrade. And that should provide enough power for the database updates without drawing on your batteries. I don't recall the times for the updates, but IIRC the longer ones can be the better part of an hour -- 4 GB can take a while;
* Sure, you can do runaway trim with a CB, but it's easier to find a CB switch. Your mileage etc...

Ed
 
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Version 10

Thanks again to everyone who offered feedback. The discussion brought out weak points in both my design and my knowledge and I'm working to fix both of them. I've created a new version of my schematic to solve the issues brought up so far. Although I haven't had a chance to do all the homework that Dan suggested, I thought I'd update the post with the corrections I've made to date. I seriously doubt this is the final version, but it's definitely better than the last.

I've given up on the automatic switching feature of the E-bus relay I was attempting to design. I've also given up on the NC feature of the avionics master and replaced it with a CB switch. I think that's a simpler solution, and adds a layer of fault protection for the example Dan highlighted, but it doesn't have a closed circuit failure mode. In this design the E-Bus Alt Feed relay could serve as a back up Avionics Master in the event of a Avi Mstr switch failure.

I've changed the Bus 1 to Bus 2 circuit protection to fusible links, but I'm not certain I've done that correct. Looking at my load analysis, E-Bus 2 uses 8.2 typical max and 16.7 max. That wire is 3 ft long, so sizing that wire for 10 Amps continuous indicates 20 AWG. This size becomes the fuselink, and I step up the wire 4 AWG to 16 AWG, correct?

I revived the Battery Bus and moved the right P-Mag to it. That adds about 8 feet of extra wire to run from the Battery Bus to the CB panel so I still have to weigh that decision. That length of power supply wire probably needs a fuse at the hot end even though that doubles protection on that circuit. The instructions dictate 18 AWG power supply wire, so I added a 10A fuse on the Battery Bus. I need to chat with the folks at E-Mag about that set up. Their material specifies a CB, so I need to check with them about the upstream fuse, or if a 3A fuse will be sufficient in lieu of a CB. I also need to discuss the idea of placing both P-mags on the same bus for the reason that Charlie pointed out. I could easily move the right P-mag to E-Bus 2 as it's already located under the CB Console, but I'm not sure that's the best solution.

I'm considering moving all the CBs from the F-865 Console to somewhere on the panel. That would shorten the distance between buses 1 and 2 as well as the distance between the CBs and the Battery Bus (probably on the front side of the fire wall). I'll need to work out panel space issues and see what other ripples that change will have.

Thanks again for everyone's input.

Edit: Updated 3/27 with the most current version.

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For those who are wondering, I used PowerPoint to produce this drawing as well as the individual component drawings. I experimented with a couple of the CAD programs others have used, but I had trouble getting those programs to do what I wanted. I'm familiar with pptx and use it nearly daily at work, so I just switched to it for my electrical drawings.
 
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* Double check what the G3X has for a Hobbs meter equivalent. If you don't have an oil pressure switch, you'll get register lots of time spent fiddling with the avionics;
* Add in wires for a trickle charger or a Genius battery charger. That $99 charger saved a dead battery that was killed by leaving the avionics on too long during a database upgrade. And that should provide enough power for the database updates without drawing on your batteries. I don't recall the times for the updates, but IIRC the longer ones can be the better part of an hour -- 4 GB can take a while;
* Sure, you can do runaway trim with a CB, but it's easier to find a CB switch. Your mileage etc...

Ed

Ed, thanks for the additional input.

Good point about the G3X Hobbs function. I just checked and it does have a total time record feature that can be set to oil pressure, engine RPM >100, or GPS determined flight. I'll have to think about if I want that to be the only source of aircraft time, or if I want an analog meter too (I already bought a Hobbs meter). At some point, the G3X will be upgraded for something else whereas the analog Hobbs meter would likely be in service for the life of the airplane. I know Hobbs meters fail too, and total time could be recorded in the logbook when it's switched from one meter to another, so I guess it's just a matter of preference. I'm generally in the "Less is More" camp, so my inclination is to sell the Hobbs meter and just use the G3X time record. I'll have to stew on that one for a while.

I know the Earth-X battery has a specific charger requirement, but I don't know what it is. I'll have to figure that out and see if I can use it as you described. The backup battery probably won't last for an hour, and it doesn't power the GTN-650, so after more thought it's not really an option.

Good point about finding a CB vs a switch. More to think about!
 
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One thing to keep in mind is that fuses/breakers are intended to protect the wire; not the device. (Contrary to cheesy mfgrs that expect you to 'protect' their device instead of building it into their device.) For your P mags: If the mfgr insists on you providing 'protection' for the mag itself, then fuse the line at the source. The switch provides the means to shut the mag down if there's an operational malfunction, so you shouldn't need a fuse + a breaker. Only variable is if they specifically spec a breaker because they need the longer time constant (high inrush currents), but that seems doubtful on that device.

On fusible links: size the wire for the load, then size the fusible link 4 sizes under the wire size. Since 22 is the smallest practical size that's physically rugged enough to work with, 22 would protect 18, 20 would protect 16, etc. The link should be ~5-6" long, be on the 'supply' end, and have some sort of flame proof protection. B&C sells a kit with fiberglass tubing. I've experimented with thick wall silicone tubing (a lot cheaper for a virtually unlimited supply of different sizes), and with a catastrophic fault (which is what you're really worried about when using a fusible link), the link vaporizes so quickly that the silicone tubing has no trouble containing the event.

Charlie
 
The reason for all of my CBs is because I want a way to disable individual appliances. In the G3X, the trim wires through the servo, so if I get a runaway trim situation, I'll disable the trim motor by pulling the auto pilot servo CB.

But if the trim servo is the problem who creates the runaway you won`t stop the trim servo by pulling the CB for the GSA-28 Servos as they have a relais inside. Unless you have the trim and autopilot servos on the same CB:

We answered Mike's question offline, but wanted to post here as well in case others were waiting on an answer.

As shown on the drawing on page C-6 in the Rev. K G3X Installation Manual, the servo is installed inline with whatever you would normally use to drive your trim motors.

When the autopilot is engaged, the autopilot immediately takes control and trims up the airplane to zero the forces on the servos and keeps doing this as long as the autopilot is engaged.

When you disconnect the autopilot, you restore manual electric trim, but the servos still drive the trim motors and do speed scheduling per your configuration mode setup.

If you remove power from the servos (not normal) a relay drops, and you can continue to run the trim motors from your trim switch, albeit without speed scheduling.

Thanks,
Steve
 
One thing to keep in mind is that fuses/breakers are intended to protect the wire; not the device. (Contrary to cheesy mfgrs that expect you to 'protect' their device instead of building it into their device.) For your P mags: If the mfgr insists on you providing 'protection' for the mag itself, then fuse the line at the source. The switch provides the means to shut the mag down if there's an operational malfunction, so you shouldn't need a fuse + a breaker. Only variable is if they specifically spec a breaker because they need the longer time constant (high inrush currents), but that seems doubtful on that device.

Thanks Charlie, that's really helpful.

On fusible links: size the wire for the load, then size the fusible link 4 sizes under the wire size. Since 22 is the smallest practical size that's physically rugged enough to work with, 22 would protect 18, 20 would protect 16, etc. The link should be ~5-6" long, be on the 'supply' end, and have some sort of flame proof protection. B&C sells a kit with fiberglass tubing. I've experimented with thick wall silicone tubing (a lot cheaper for a virtually unlimited supply of different sizes), and with a catastrophic fault (which is what you're really worried about when using a fusible link), the link vaporizes so quickly that the silicone tubing has no trouble containing the event.

Charlie

So this is what is confusing me about fusible links. So if I started with the wire sized at 20 AWG for the load, and stepped it down 4 sized for the fuselink, that would give me 24 AWG fuselink. Bob's book and website both say a 24 AWG fuselink should be limited to 3 Amps, whereas I need 8.2 Amps continuous for E-Bus 2. Am I looking at this wrong?

Thanks again,
Rob
 
But if the trim servo is the problem who creates the runaway you won`t stop the trim servo by pulling the CB for the GSA-28 Servos as they have a relais inside. Unless you have the trim and autopilot servos on the same CB:

Thanks for the info, I didn't realize that was the case. I do have autopilot and servo CBs next to each other, so I guess in a runaway trim situation I'd need to pull both of them. I'll have to think about this one some more to decide if I'm happy with a multi-step immediate action response against a likelihood/severity analysis. I suppose I could run the auto pilot servo to electric trim motor wire back to the CB Console and put a switch in it and then run the wire back to the tail to meet the electric trim motor, but that's more complexity and more potential points of failure. Hmmmm....
 
Thanks for the info, I didn't realize that was the case. I do have autopilot and servo CBs next to each other, so I guess in a runaway trim situation I'd need to pull both of them. I'll have to think about this one some more to decide if I'm happy with a multi-step immediate action response against a likelihood/severity analysis. I suppose I could run the auto pilot servo to electric trim motor wire back to the CB Console and put a switch in it and then run the wire back to the tail to meet the electric trim motor, but that's more complexity and more potential points of failure. Hmmmm....

Thats why i placed my trim CB on top right so I can reach it blind:
epq978.jpg
 
Thanks Charlie, that's really helpful.



So this is what is confusing me about fusible links. So if I started with the wire sized at 20 AWG for the load, and stepped it down 4 sized for the fuselink, that would give me 24 AWG fuselink. Bob's book and website both say a 24 AWG fuselink should be limited to 3 Amps, whereas I need 8.2 Amps continuous for E-Bus 2. Am I looking at this wrong?

Thanks again,
Rob

I went back and reviewed what he wrote about links, and you're right. But...sometimes reading his book is a little like reading FAA regs; you sometimes have to look in multiple places to get a complete picture.

If you look at the link ratings in that section, they are assuming a 10*C temperature rise (found in the 'Wire' section of the book). This is a *very* conservative rating, especially for a 4"-6" length of wire that will almost certainly be in free air (not buried in a bundle).

From the 'wire' section, I offer this quote:

Now that we've laid out the "rules" we can discuss how
and why they are sometimes broken. If you were to
run 10 Amps through a length of 22 gauge wire, does
this imply that you are going to come spinning out of
the sky trailing black smoke like the victim of a dogfight?
No, specially if the wire is short for low energy
loss in spite of overload, well ventilated to control
temperature rise, or is subject to an overload for only
short durations.


There's more in that section, addressing intermittent loads, etc.

So if you want to be *super* conservative, pick your link for that 10*C temp rise at continuous load and go 4 sizes larger for the wire. If your circuit will have intermittent loads, or you're willing to accept slightly higher temps in the link (remember, it's only 4" long so not much resistance), you could lean toward sizing the *wire* to the load and go 4 sizes smaller for the link.

Charlie
 
Rob and folks with diagrams,
I don't want to derail the discussion, or change the topic, but can you share what tools you used to come up with your electrical diagrams? They all look great! I am about to start doing electrical (just for the wings) but I'd like to create some diagrams like you guys have done.
 
Rob and folks with diagrams,
I don't want to derail the discussion, or change the topic, but can you share what tools you used to come up with your electrical diagrams? They all look great! I am about to start doing electrical (just for the wings) but I'd like to create some diagrams like you guys have done.
For those who are wondering, I used PowerPoint to produce this drawing as well as the individual component drawings. I experimented with a couple of the CAD programs others have used, but I had trouble getting those programs to do what I wanted. I'm familiar with pptx and use it nearly daily at work, so I just switched to it for my electrical drawings.

Looks like Rob is using pptx as well. I did the same Thing as I did not have access to a CAD software at the time I did mine.. It works not to bad!
 
Rob,
The attached drawing shows the correct way to connect the diode. The way
that the diode is connected in your drawing does no good at all. In fact, it
adds an unnecessary failure point. Capacitors resist any change in voltage.
Inductors (contactor coil) oppose any change in current. When the current to
an inductor is shut off, the inductor opposes any change in current and tries
to keep the current flowing in the SAME direction. Therefore, the diode
connected as shown in your diagram does nothing to prevent current from
flowing in the same direction that it was flowing. The induced current could
have a high voltage that will arc across the opening switch contacts,
eventually causing the switch to fail. Since your starter contactor already
has a diode across the coil, another diode is not required. But it is OK to
have a second diode as long as it is connected correctly, which is in parallel
with the contactor coil with the diode arrow pointing towards positive.
Below is a picture showing the correct way to protect the switch.

I0CYGQiYzJoEt_QBdht7AL7rU5okP2vLzy5EViji6riRT2zZ7Bk2NmprRij0yOMGRnEk1NqyWoTZjMNMkePNgKd_3mbnUrrMBVPfXT90ch7YiNq5tTw2056g6VZOHRXQCkh1w-A_BrXnVpzadDF5xw9a_I0rkg-Ih7mpReijzO4nduAnBQWBCt_-zIDMzj7dDt5zNBNU4sxZ9WMCrMZIFDv6apriffTFo_u7CEud277V5ZRt6r5jPfm_3tYFvKTpefUfXbfskOIThs2HHa6oY3Scf-Om-nchqiLHsRdZCSv508dbkbL-rJo58ok7fqvw69DKdxRvtzFZ18iPB1l_NUBApUu0W_sDsNIh1Rzd4T8cB7v7qPkI3oke1h5g4jmxd_rJ6QTZWtoPM_EAaGPQRJFH-JiZ0NMKRiqSIH8MjmO6aqFezMrRYn1Xsn-n9UofK6ub9BF2tYiZDFO9pXG1nFcrkSgOaclSp7-5AG-BMlCy0N8j4e3vX1hvaTTqJIknr6o3Yx23QC2qidTjxtxCpgfvAOL08ZZxo2J4q7xNJ4ikBubRoEBIPbagDChmX8NyIrgYAhUdENki9opdIll4keew2cDv6sfhnJ0hPt87bEj7Rbhq6Pkelqs0LgQQ4ZLkgxBMDqXZORy-MBxtaVDcWvwFtHqzEa4q=w649-h408-no
 
Administrator - why is it that this page is fixed width, whereas the others are variable width? I have to scroll horizontally to see each line, but don't with posts in the forums section.
 
Quote: "For those who are wondering, I used PowerPoint to produce this drawing as well as the individual component drawings. I experimented with a couple of the CAD programs others have used, but I had trouble getting those programs to do what I wanted. I'm familiar with pptx and use it nearly daily at work, so I just switched to it for my electrical drawings."

Rob, would you post your schematic in .pptx format (for those of who are searching for the correct symbols to use in our PowerPoint schematics.

Best regards,
Jim Heekin
 
Administrator - why is it that this page is fixed width, whereas the others are variable width? I have to scroll horizontally to see each line, but don't with posts in the forums section.

I'm not an admin, but look at the images. This seems to be common when an image is linked that's wider than the screen; it drives the entire page to load at that width.
 
There is a program built into MS Windows called "Paint".
Use "Paint" to resize and crop pictures so that they are
not so wide before uploading them to the internet.
 
Rob,
The attached drawing shows the correct way to connect the diode. The way
that the diode is connected in your drawing does no good at all. In fact, it
adds an unnecessary failure point. Capacitors resist any change in voltage.
Inductors (contactor coil) oppose any change in current. When the current to
an inductor is shut off, the inductor opposes any change in current and tries
to keep the current flowing in the SAME direction. Therefore, the diode
connected as shown in your diagram does nothing to prevent current from
flowing in the same direction that it was flowing. The induced current could
have a high voltage that will arc across the opening switch contacts,
eventually causing the switch to fail. Since your starter contactor already
has a diode across the coil, another diode is not required. But it is OK to
have a second diode as long as it is connected correctly, which is in parallel
with the contactor coil with the diode arrow pointing towards positive.
Below is a picture showing the correct way to protect the switch.

I0CYGQiYzJoEt_QBdht7AL7rU5okP2vLzy5EViji6riRT2zZ7Bk2NmprRij0yOMGRnEk1NqyWoTZjMNMkePNgKd_3mbnUrrMBVPfXT90ch7YiNq5tTw2056g6VZOHRXQCkh1w-A_BrXnVpzadDF5xw9a_I0rkg-Ih7mpReijzO4nduAnBQWBCt_-zIDMzj7dDt5zNBNU4sxZ9WMCrMZIFDv6apriffTFo_u7CEud277V5ZRt6r5jPfm_3tYFvKTpefUfXbfskOIThs2HHa6oY3Scf-Om-nchqiLHsRdZCSv508dbkbL-rJo58ok7fqvw69DKdxRvtzFZ18iPB1l_NUBApUu0W_sDsNIh1Rzd4T8cB7v7qPkI3oke1h5g4jmxd_rJ6QTZWtoPM_EAaGPQRJFH-JiZ0NMKRiqSIH8MjmO6aqFezMrRYn1Xsn-n9UofK6ub9BF2tYiZDFO9pXG1nFcrkSgOaclSp7-5AG-BMlCy0N8j4e3vX1hvaTTqJIknr6o3Yx23QC2qidTjxtxCpgfvAOL08ZZxo2J4q7xNJ4ikBubRoEBIPbagDChmX8NyIrgYAhUdENki9opdIll4keew2cDv6sfhnJ0hPt87bEj7Rbhq6Pkelqs0LgQQ4ZLkgxBMDqXZORy-MBxtaVDcWvwFtHqzEa4q=w649-h408-no

Thanks Joe. I knew diodes used in that fashion prevent arcing, but didn't understand it completely until you explained it better. When I had an electrical whiz from the chapter he told me I should/could have a diode to protect the starter switch, I put it in similar to the way the relays and contactors were wired, but I see how that's different.

I updated my drawing to correct the diode placement, but if it's not required I'll probably just leave it out.
 
I went back and reviewed what he wrote about links, and you're right. But...sometimes reading his book is a little like reading FAA regs; you sometimes have to look in multiple places to get a complete picture.

If you look at the link ratings in that section, they are assuming a 10*C temperature rise (found in the 'Wire' section of the book). This is a *very* conservative rating, especially for a 4"-6" length of wire that will almost certainly be in free air (not buried in a bundle).

From the 'wire' section, I offer this quote:

Now that we've laid out the "rules" we can discuss how
and why they are sometimes broken. If you were to
run 10 Amps through a length of 22 gauge wire, does
this imply that you are going to come spinning out of
the sky trailing black smoke like the victim of a dogfight?
No, specially if the wire is short for low energy
loss in spite of overload, well ventilated to control
temperature rise, or is subject to an overload for only
short durations.


There's more in that section, addressing intermittent loads, etc.

So if you want to be *super* conservative, pick your link for that 10*C temp rise at continuous load and go 4 sizes larger for the wire. If your circuit will have intermittent loads, or you're willing to accept slightly higher temps in the link (remember, it's only 4" long so not much resistance), you could lean toward sizing the *wire* to the load and go 4 sizes smaller for the link.

Charlie

Thanks again Charlie! The one thing I'm not comfortable with is knowing when I could break the rules. If the super conservative method you've suggested provides the same circuit protection and the only penalty is a bigger wire (more weight and cost), I'll probably go that route. I'll go reread that section of Bob's book.
 
File Link

Several people have asked for a copy of my file. Below is a link that will take you to the current version, which is still a working draft with issues I'm correcting that are brought up in this post. Sorry for posting the large photos. I tried using smaller ones that wouldn't skew the width of the thread, but when I did you couldn't read any of the detail and the photo was worthless.

https://www.dropbox.com/s/jtoe8xdci2xiyjh/Electrical System Overview (v13.10).pptx?dl=0
 
Rob,
The downloaded Power Point drawing is much easier to read.
Flip the E-Bus-relay diode over so that the arrow points towards positive.
It will blow the fuse (or itself) the way it is now.
 
You might consider using a 2 or 3 amp circuit breaker instead of that 5 amp fuse
on the ground side of the aux alternator relay. Sometimes alternator output voltage
is unstable and momentarily bounces above the over-voltage module set point.
It might be desired to reset the breaker. If the aux alt voltage continues to be too
high due to aux regulator failure, just turn on some more electrical loads.
If the load on the aux alternator is at its maximum capability, then increasing the
load even more will decrease the voltage output.
 
Rob,
The downloaded Power Point drawing is much easier to read.
Flip the E-Bus-relay diode over so that the arrow points towards positive.
It will blow the fuse (or itself) the way it is now.

Thanks!

You might consider using a 2 or 3 amp circuit breaker instead of that 5 amp fuse
on the ground side of the aux alternator relay. Sometimes alternator output voltage
is unstable and momentarily bounces above the over-voltage module set point.
It might be desired to reset the breaker. If the aux alt voltage continues to be too
high due to aux regulator failure, just turn on some more electrical loads.
If the load on the aux alternator is at its maximum capability, then increasing the
load even more will decrease the voltage output.

Bob's design had a 2A CB there, but I was trying to avoid a round trip to the CB console. I suppose I could mount a CB somewhere other than the CB console since I wouldn't reset it in flight if it popped anyhow, and I don't need the CB as an alternate switch; or I could use a CB switch instead.

I haven't had time to consider relocating my CBs from the F-865 Console to somewhere on the panel, but I have 8 hours in the thinking machine (airliner) tomorrow, so hopefully I can work through those decisions/implications.
 
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Alternator field circuit breaker

... was trying to avoid a round trip to the CB console... I wouldn't reset it in flight if it popped... I could use a CB switch instead.

B and C Aero, that's basically Bob Nuckols, regulator installation instructions say:
  • Use KLIXON or Mechanical Products circuit breakers... avoid combination switch-circuit breakers for reliability reasons.
  • A panel location within the pilot's field of vision and reach is recommended.

I didn't find a source at the moment but as I recall it is Bob Nuckols and the reality is that the crowbar fires falsely on occasion so you want 1) Not a fuse but a resettable device... a circuit breaker. 2) Easy access to the circuit breaker in flight.
 
What John said. You want a really short time constant for OV protection to protect your avionics, which can lead to occasional nuisance trips. The one breaker you'd want to try resetting (once) while in flight. It's detailed in the book.

Charlie
 
I think I've reached a solution!

I wanted to post an update to my design. I really appreciate all the help folks have given me with my design, and all the time they have spent to review my architecture and offer input. Special thanks to Joe and Carl, who sent me tons of info in email and helped me figure all this out.

I contacted EMag and got an answer to my dilemma about where to put the P-mags:

The P-mags should be put on the main bus because we only require a battery for starting. After 800 RPM or so we switch to running off our internal generator and do not require an external source. In the event of an internal generator failure we would switch back to the external power (main bus). Be sure to do the minimum cut-out test to see where the ignitions drop off and note it so in the event of an alternator failure you wont run the risk of the having to low of RPM for our ignitions to continue sparking.

If you do put us on the battery directly you run the chance of leaving us on and draining your battery.

With that advice, I've once again removed the battery bus and put the P-mags back on the main bus.

I also posted my design on Matronics and got some additional feedback, including a few comments from Bob Nuckolls himself.

Joe pointed out that I had the voltage regulator wired incorrectly, and I realized I had switched part numbers from the generic Ford regulator on the Z-13 to the LRC3 on the Z12, but hadn't updated the icon or wiring. Bob Nuckolls also gave me some advice to remove a redundant crowbar on the master switch because it's included in the LR3C.

I've moved the starter switch from a CB to a fuse. I had it on a CB so that I could pull it in the event the starter ever hung up. After considering it anew, I realized I could get the same result by just turning off the master, and I'd be a lot more apt to find the master switch that I use on every flight, than find the starter CB. I also moved the LV Warning line to the fuse block, so now I have 8 CBs total: Main Alt Fld, Aux Alt Fld, L Ign, R Ign, AP Servos, Auto Pilot, Com2, Transponder. (The last two are there so I can reduce the load on my E-Bus if needed during an alternator failure, and I'm still thinking about moving them to a fuse and using a switch instead.)

I also removed the alternator's fuselink between Main Bus 2 and the Alt Fld CB. Having two fuselinks in series, as I did at one point in my design, doesn't give me the protection I thought it would. I now have fuselinks between the Bus 1s and Bus 2s. After some more thought I've moved the CBs to the right side of the instrument panel, which reduced the wire run to about 18" from the fuse blocks of Bus 1 (both Main and Essential), so I've kept the fuselink to protect those two wire runs.

I moved the trim and fuel gauge power to the E-Bus. Someone suggested that the power they consumed was a good trade off for having that information on the E-Bus, and upon further thought I agreed with that suggestion.

I also changed the inline fuse from the SD-8 to the 30A that Bob recommends in his Z-13/8. I'm not sure how I got a different value for that fuse, but someone pointed it out to me.

I removed the current limiter between the battery contactor and Main Bus 1 upon advice that a second current limiter was just introducing additional points of failure without any added protection.

I removed the OV light and will use the G3X annunciator for that function.

I've been reading through the G3X install manual to check for inputs from my electrical architecture to its monitoring devices. I changed the shunt on the SD-8 circuit to the part number called out by that manual.

Right now I still have three advisory lights in my design (battery fault, aux alt warn, and starter engaged). I'm not sure if I'll use all of those or use the G3X annunciator instead.

I think that's a round up of all the changes I've made since the last post on this thread.

I'm sure there will be more changes as I start building the electrical system, but I think this plan is solid enough to start installation.

I saw many builders suggest planning everything out before you start installing a single wire, and I took that advice to heart. I probably spent waaaay more time on this subject than most people do. I'm a slow builder, and I'm okay with that. Although my electrical journey was probably overkill for a lot of people, it's what I needed to get an understanding of the airplane I'm building.

I still have a few decisions to make. Namely, the EarthX battery. I reviewed the RV-8 accident last fall and have been giving that a lot of thought. I haven't made up my mind yet, but I feel comfortable moving forward with that decision still tentative.

Dan and Carl pointed out that I had been fixated on a main alternator failure as the most likely electrical causality, which it is not. Having just completed my PMP certification, I took a page out of my textbook and did a risk analysis of various electrical failures (components, failed connections, shorts of every wire, etc.). It's still a work in progress.

One of the main things I learned is you can't just take good ideas from one design and incorporate them into another design without thoroughly understanding the implications/functions of each component, which I do not. I've read Bob's book more than twice and spent way too many hours working with my plan, and I'm still a novice. Thanks again to everyone who helped further my education.

I'm looking forward to FINALLY installing my first wire!
 
Last edited:
Nice diagram and load charts.

I have been watching your system design progress and most of the problems I had seen have been addressed. I have a few ideas, not that they must be followed.

1. SD-8 alternator. You might call B&C. The regulator needs good cooling to achieve maximum available output. Under the cowling may be too hot for the best results.

2. The main alternator current limiter is too small. Most alternators easily put out more than advertised current. Recommend 70 amp.

3.The Auxiliary Alternator warning light will always be on with the switch turned off. Unless you plan to leave on, the solution would be a double throw switch that would also turn off the light circuit when not being used. A warning light on all the time would be a nuisance.

4. The 20 gauge essential buss jumper is too small for a continuous 15 amp load of more than 2 seconds. I would recommend a 14 or 16 gauge jumper with no protection. Same for the 22 gauge main buss jumper. Protection is unnecessary, especially if a very short run.

5. The 30 amp inline fuse for the E-buss alternate feed relay is just adequate for full load. Would put a 40 amp with the wire size used or would recommend a 40 amp current limiter. (same thing different package)

6. The main alternator and auxiliary field breakers should be easily accessible and identifiable by feel.

7. Recommend all Honeywell TL quality and/or type switches. The Carlin can give problems, especially on inductive loads. Fast-on are nice, but a quality switch is more important.

8. Have no provision to power up the avionics prior to engine start. Would be nice for IFR. Set up radios, check ATIS, insert flight plan, call for clearance, etc.. A small/medium Lithium could be used.

9. I would not use a Lithium for the main battery, especially IFR all electric plane. Time will tell. (I'm sure to start a fire storm for that suggestion)

10. Would recommend a Honeywell double throw switch for the avionics switch. (extra contact area). Why the diode, unnecessary voltage drop and can control current with the avionics switch. Breaker switches can be problematic. Does the avionics switch circuit between the busses need protection? You could put a 14 gauge fuse link on each end, but I would not bother.

11. Would run the strobes through a breaker rather than a fuse. Have seen multiple strobe fuses melted. This is documented, but not fully explained.

12. You have fuses for the GTN650. There is no way to reboot the radio. This is done with the breakers. Suggest easily accessible, small breaker panel with the necessary breakers.

You have the diagrams, now for the install. Be sure and use high quality connectors. Be careful of chafing areas and protect well. With the large number of connections a Daniels crimper, used red/blue crimper, tefzel strippers would be worth the investment, but not necessary with verification of proper results. Get the proper solder sleeves. Work on perfecting your techniques. Follow techniques in the manuals. The install is only as good as the harness and connection quality.

Looks like you are on the right path. Hope I am helping and not being too critical.

George Meketa
RV8, 16 years
 
1. Yes, heat is an enemy of electronics.
2. A 60 amp ANL current limiter will conduct 70+ amp indefinitely without blowing. Read Bob Nuckolls' post:
http://forums.matronics.com/viewtopic.php?t=16762449
3. Using a double pole switch is a good idea. One half of it can disable the aux alternator warning lamp.
4. The E-Bus jumper is 16 AWG. The #20 is a fuselink. It will conduct more than 50 amps before it melts. See this chart:
http://www.litz-wire.com/New PDFs/F...ure_Copper_Aluminum_Magnet_Wire_R2.011609.pdf
The main bus jumper is 18 AWG (not labled). The 22 AWG fuselink will melt if current exceeds 40 amps.
5.
6.
7. Yes, switches that use rivets as conductors (through plastic) are prone to fail.
8. There is a TCW 3 AH battery to prevent brownout during engine cranking.
9. Lots of aircraft use a Lithium battery including Van's new version of the RV-12 But who knows? I plan to keep my Odyssey PC680.
10. The diode is necessary to block starter current if the builder or future pilot inadvertently leaves the E-Bus switches turned on. The diode will also block reverse main bus current if the battery contactor fails or if the pilot turns on the E-Bus before turning on the master switch.
11.
12.
 
I will attempt to explain

2. When designing an electrical system one normally uses manufacturer supplied specs to provide the necessary data required to make informed choices. In choosing the current limiter size there are multiple selection factors, with current value being only one. For temperature the standard fuse re-rating is -25% for temperatures over 25C. That means that your 60 amp fuse would now be de-rated to 45 amps under the hot cowling. We have a 4 gauge B-lead cable that is capable of handling more than 80 amps (125 amp free air)(333 amps wire melting point from Joe's supplied chart) under the worst conditions being protected by a 60 amp current limiter. I stand by recommending a higher than 60 amp current limiter (80 is what is available). Not that the 60 will not work, maybe. I will disagree with Bob K. on this one. Do what you want.

4. The small gauged fuse link buss jumpers are without doubt too small for the application. At times there are reasons to design a circuit with high voltage drop, minimum wire size and tight overload protection, but this is not the place. The max E-bus amp load is 15 amps. A 16 gauge wire is rated at 12.5 amps (free air) and a 20 gauge at 7.5 (free air). The buses are powered by an unprotected 10 gauge cable. The #1 and #2 E-buses are likely next to one another requiring a short jumper that has no realistic chance of a short. I will up my recommendation to a 14 gauge or larger jumper. Same for the main bus jumper.

8. I still highly recommended an E-bus battery to power up the avionics, including the radio. The brown out battery does not provide this function. I assume, with this panel, the plan is to fly IFR. With all the items requiring boot up and set up time this is a feature that is closer to being required than just nice to have. My 8 (IFR) has a E-bus battery (lithium) that is the first thing turned on every flight when entering the plane.

10. The diode is a band-aid for a design flaw. Why would you want to drop voltage to the E-bus under all normal conditions. The switch wiring flow could easily be designed in a way to have the diode isolate the E-bus only when using the Alternate E-bus switch. If left alone at least monitor main bus voltage with the EFIS and not just E-bus voltage.

These changes are not that difficult.

I will work on suggestions for Rob's "How Do I Get Electrical Experience" thread. Design is the hard part, now just need to work on techniques.

George
 
This is a great exercise, but personally I question the need for an "E Buss" at all these days and the overall complexity of your design.

With most of the glass panels we now have dual power inputs for backup battery power sources, so you can basically shut off the master and continue to fly safely for over an hour with all necessary flt instruments.

Running everything through diodes/relays/multiple busses with fuse links is just adding extra fault sources, not to mention the diode is generating wasted heat while dropping the voltage to all your avionics.

I'm in the let's keep this really simple camp, a few switches and some breakers and we're good to go with a very high reliability system.

For an IFR panel add a back up alternator (not necessary but a nice to have item).
 
Last edited:
This is a great exercise, but personally I question the need for an "E Buss" at all these days and the overall complexity of your design.

With most of the glass panels we now have dual power inputs for backup battery power sources, so you can basically shut off the master and continue to fly safely for over an hour with all necessary flt instruments.

Running everything through a diodes/relays is just adding extra fault sources, not to mention the diode is generating wasted heat while dropping the voltage to all your avionics.

I'm in the let's keep this really simple camp, a few switches and some breakers and we're good to go with a very high reliability system.

For an IFR panel add a back up alternator (not necessary but a nice to have item).

Thank you for this breath of fresh air :)

I'm in the process of planning my electrical system and was getting concerned that my system was too simple.

Back-up battery for the EFIS, back-up iPad...I think I'm good to go ;)
 
Thank you for this breath of fresh air :)

I'm in the process of planning my electrical system and was getting concerned that my system was too simple.

Back-up battery for the EFIS, back-up iPad...I think I'm good to go ;)

That might well be true for your a/c and your mission. It works fine for my day VFR -4, too. But you can't apply Walt's 'at all' philosophy to every a/c and mission, and as a 'pro', he should know better than to make a statement like that.

IFR and electrically dependent panel?

Electrically dependent engine?

One alternator or two? If two, can the backup alt carry the full electrical load?

If you know anything at all about Bob Nuckolls, you know his decades of experience in the certified world, and his desire to make available more useful (and when possible, simpler) electrical system designs than have been available to those locked in the certified world. If one simple system would suit everybody, I'm very confident we wouldn't have the Aeroelectric Connection book; it would be a one-page diagram.

Charlie
 
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