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Old 03-23-2018, 04:38 PM
Nihon_Ni's Avatar
Nihon_Ni Nihon_Ni is offline
 
Join Date: Jun 2011
Location: Fredericksburg, VA
Posts: 244
Default Please critique my electrical design

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/jtoe8xdci2...0%29.pptx?dl=0





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Rob K
真喜志友幸
RV-8 under construction as of Sep 30, 2012
Tail kit completed on Feb 8, 2012 (180 hrs)
Wing kit mostly completed, fuselage under construction as of March 6, 2014 (750 hrs)
Paid up for 2018

Last edited by Nihon_Ni : 03-28-2018 at 09:06 PM.
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  #2  
Old 03-23-2018, 08:24 PM
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Bruce Bruce is offline
 
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Default

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
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  #3  
Old 03-24-2018, 12:10 AM
GeraldC GeraldC is offline
 
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Location: Buda, TX
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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.
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  #4  
Old 03-24-2018, 04:45 AM
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Nihon_Ni Nihon_Ni is offline
 
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Location: Fredericksburg, VA
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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.

Quote:
Originally Posted by Bruce View Post
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?
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Rob K
真喜志友幸
RV-8 under construction as of Sep 30, 2012
Tail kit completed on Feb 8, 2012 (180 hrs)
Wing kit mostly completed, fuselage under construction as of March 6, 2014 (750 hrs)
Paid up for 2018
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  #5  
Old 03-24-2018, 05:02 AM
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Nihon_Ni Nihon_Ni is offline
 
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Location: Fredericksburg, VA
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Quote:
Originally Posted by GeraldC View Post
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.

Quote:
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?)

Quote:
- 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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Rob K
真喜志友幸
RV-8 under construction as of Sep 30, 2012
Tail kit completed on Feb 8, 2012 (180 hrs)
Wing kit mostly completed, fuselage under construction as of March 6, 2014 (750 hrs)
Paid up for 2018

Last edited by Nihon_Ni : 03-24-2018 at 05:53 AM.
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  #6  
Old 03-24-2018, 06:55 AM
rv7charlie rv7charlie is offline
 
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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
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  #7  
Old 03-24-2018, 07:10 AM
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DanH DanH is offline
 
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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.
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  #8  
Old 03-24-2018, 09:50 AM
Mich48041 Mich48041 is offline
 
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Location: Riley TWP MI
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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.
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  #9  
Old 03-24-2018, 06:29 PM
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Nihon_Ni Nihon_Ni is offline
 
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Default 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.
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Rob K
真喜志友幸
RV-8 under construction as of Sep 30, 2012
Tail kit completed on Feb 8, 2012 (180 hrs)
Wing kit mostly completed, fuselage under construction as of March 6, 2014 (750 hrs)
Paid up for 2018
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  #10  
Old 03-24-2018, 07:25 PM
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Nihon_Ni Nihon_Ni is offline
 
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Charlie,

This is really good insight, thanks!

Quote:
Originally Posted by rv7charlie View Post
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.

Quote:
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.

Quote:
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.

Quote:
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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Rob K
真喜志友幸
RV-8 under construction as of Sep 30, 2012
Tail kit completed on Feb 8, 2012 (180 hrs)
Wing kit mostly completed, fuselage under construction as of March 6, 2014 (750 hrs)
Paid up for 2018

Last edited by Nihon_Ni : 03-26-2018 at 04:02 AM.
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