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Request review of electrical system diagram

dmn056

Well Known Member
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This is a first cut at the main power system diagram for my RV-8, with dual alternators, dual batteries aft-mounted, SDS electronic fuel injection and electronic ignition. It's drawn more or less in accordance with Australian standards, so please forgive any differences from USA usage.

I'm not an electrical expert, but I've spent a fair bit of time going over AeroElectric Connection, lots of other people's wiring diagrams, and of course DanH's recent works.

Please review what I'm planning and let me know what problems you see or how I could do it better.

Thanks in advance.
 

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  • ES001 Batteries Alternators Starter.pdf
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I don’t see the benefit of powering the EFI/EI by the batteries-only (if I’m interpreting this correctly). Why not power EFI/EI from the main bus? The main bus is supplied by 4 sources (2 alts and 2 batts).
 
This is minor, I think there is normally a diode on the starter contactor, like there is on the main contactor(s).
 
A quick look, first thought. Don’t know if you’ve procured components yet.

B&C products have a well earned good reputation.

Consider the Monkworkz for your back-up (or dual depending on your intent). Well designed, well packaged. Well applied “automation” and removes the need for excitation. I’m really enjoying learning this product and get smarter every time I correspond with the designer/company owner.

Some will point to price delta but it’s an~ wash when everything is considered. Better if you value your time IMO.

Just a thought. Lots of good ways to achieve your intent.
 
I don’t see the benefit of powering the EFI/EI by the batteries-only
In case of smoke in the cockpit, the pilot will shut off the master switches and the engine will quit if powered via the master contactor.
 
Don't use a dual diode. Avoid a single point of failure.
Use separate diodes connected to opposite ends of the engine bus.
 
It's drawn more or less in accordance with Australian standards, so please forgive any differences from USA usage.

Like your layout, communicating basic physical location as well as connections.

Don't use a dual diode. Avoid a single point of failure. Use separate diodes connected to opposite ends of the engine bus.

Two independent diodes merely share a base. What am I missing? https://www.st.com/resource/en/datasheet/stps24045.pdf

Only one EI/EFI buss?

Kinda moving that way myself. I had originally put ECUs and fuel pumps on separate busses, then fed a third through diodes to drive the injectors. The idea was to reduce power loss by not powering the fuel pumps (the power hogs) through diodes. In retrospect, the power savings is not a big deal, and with a single bus, both ECUs remain live (i.e. all spark plugs) if one of the power sources goes down.

A dedicated EFI/EI bus with dual battery feeds is what SDS specifies in their dual ECU power drawing.
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Two independent diodes merely share a base. What am I missing?
You are right about the diode. I didn't realize that it had 4 terminals.
But each diode should have its own independent connection to the engine bus.
 
Now that I can see better on a Laptop vs phone:

I'd strongly consider independent "buses" for redundant critical engine equipment; only diode-bridge together non-redundant items (injectors). Pumps, coils, ECUs separated as much as practical. I'd personally revise the drawing to show these components to get some better feedback here.

I'd consider two, independent diodes and also bridge the anodes and cathodes of each (versus only the anodes as depicted). While dual path, it's still a single component. This also would spread the heat(sink) load; very cheap insurance.

Some more quick thoughts for your entertainment.
 
Kinda moving that way myself. I had originally put ECUs and fuel pumps on separate busses, then fed a third through diodes to drive the injectors. The idea was to reduce power loss by not powering the fuel pumps (the power hogs) through diodes. In retrospect, the power savings is not a big deal, and with a single bus, both ECUs remain live (i.e. all spark plugs) if one of the power sources goes down.

A dedicated EFI/EI bus with dual battery feeds is what SDS specifies in their dual ECU power drawing.
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This assumes the only failure of the EI/EFI buss is the loss of power input. If there is a fault on the single buss itself (e.g a component has failed and is bringing the buss down), multiple diode protected power inputs still results in the loss of the engine.

I suggest for any electrically dependent engine each EI be powered directly from a separate battery via a dedicated breaker. For EFI do the same for all “dual” components. For the single components the dual diode protected feed is about all you can do - but have each feed on a separate breaker.

Multiple power feeds do little good if power does not get to what you need.

Carl
 
I'd consider two, independent diodes and also bridge the anodes and cathodes of each (versus only the anodes as depicted). While dual path, it's still a single component. This also would spread the heat(sink) load; very cheap insurance.

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Just on this point. I don't think parallel diodes help much. Thinking as follows:

You can't detect a failure of one, so it isn't a redundancy. When one fails you just fly until the second one fails.


As with most silicon, current goes up with heat, so as soon as one is warmer than the other more of the current will flow through that one and make more heat mitigating the heat benefit.

It has limited value as a reliability increase offset by more connections being more places for failures.
 
But each diode should have its own independent connection to the engine bus.

The typical fuse bus will have a single connection point. That said, sure, run a wire from each anode to the terminal, i.e. independent paths and terminal crimps.

If there is a fault on the single buss itself (e.g a component has failed and is bringing the buss down), multiple diode protected power inputs still results in the loss of the engine.

The supplied components are individually fused. A component failure can't take down the bus.

Just on this point. I don't think parallel diodes help much. Thinking as follows:

You can't detect a failure of one, so it isn't a redundancy. When one fails you just fly until the second one fails.

Note the feeds to the diodes are switched...which means they can be checked at runup, much like we should be checking P-mag internal power. Just like the P-mag, failure after takeoff is possible. However, see below.

As with most silicon, current goes up with heat, so as soon as one is warmer than the other more of the current will flow through that one and make more heat mitigating the heat benefit.

The inverse heat-resistance relationship is why each diode is always sized to accept the entire current. For example, if we expect 20 amps, we can't pair two diodes with 10 amp ratings. Sooner or later one will get warmer than the other and exceed its rating.

Here Mr. Newman has selected a very robust device...120 amps per diode, 45V, with a typical forward voltage of 0.52, and a 150C temperature limit. I suspect it's pretty reliable in this application...less than 20 amps, 14V, screwed to aluminum sheet inside the cabin.
 
Just on this point. I don't think parallel diodes help much. Thinking as follows:

You can't detect a failure of one, so it isn't a redundancy. When one fails you just fly until the second one fails.

Not following you.
(lack of) Fault indication has nothing to do with functional redundancy.

A single, dual diode shares a base, heatsink/contact area, output leads (as depicted).

Yes, I'm aware the following logic can be taken to extreme but two independent dual diodes is cheap insurance here. Let's see how our Aussie (Aussie Aussie Oi Oi Oi) friend goes forward with his design.
 
Note the feeds to the diodes are switched...which means they can be checked at runup, much like we should be checking P-mag internal power. Just like the P-mag, failure after takeoff is possible. However, see below.

The inverse heat-resistance relationship is why each diode is always sized to accept the entire current. For example, if we expect 20 amps, we can't pair two diodes with 10 amp ratings. Sooner or later one will get warmer than the other and exceed its rating.

Here Mr. Newman has selected a very robust device...120 amps per diode, 45V, with a typical forward voltage of 0.52, and a 150C temperature limit. I suspect it's pretty reliable in this application...less than 20 amps, 14V, screwed to aluminum sheet inside the cabin.

The design as presented is robust and the device is good no concerns there. I'm a dual bus guy, but nothing wrong here.

I was addressing the concept of doubling up on the "diode or" part of the circuit feeding the EFII and trying, perhaps not clearly, to express that redundancy without the ability to detect a failure, either by indication or periodic inspection leaves you in the position of not knowing if you have redundancy or not.

Rereading Freemasm I think he may have been proposing connecting the anodes in the drawing as drawn. This would defeat the purpose of the isolation provided by the diode or.

Edit: see his response now... take that back, now just the comment about ensuring if you build in redundancy make sure you have a way of knowing the redundancy is actually here.
 
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now just the comment about ensuring if you build in redundancy make sure you have a way of knowing the redundancy is actually here.

Could you measure the battery voltage for each battery between the diode and the bus? This would tell you the battery voltage minus the diode voltage drop however, but would show if a diode has failed?
 
Thank you everyone for your comments. You've given me plenty to think about.

Just clarifying a few items:

I don’t see the benefit of powering the EFI/EI by the batteries-only (if I’m interpreting this correctly). Why not power EFI/EI from the main bus?

See comments from Joe Gores. The EFI/EI bus will run from batteries with the battery contactors open. When the contactors are closed, the alternator(s) can also supply the EFI/EI bus.

I think there is normally a diode on the starter contactor

The S811-1 starter contactor has an internal diode.

Consider the Monkworkz for your back-up (or dual depending on your intent)

I'd already ordered the alternators with my engine before I first saw Monkworkz (supply chains these days are very long). Monkworkz looks like a good system, but the BC410-H alternator combined with the SB1B-14 regulator provides similar functionality. According to the documentation, it will automatically bring the backup alternator online if the bus voltage drops below set point (13V default, user configurable).

Only one EI/EFI buss?

I followed pretty much the same logic as DanH set out below.

with a single bus, both ECUs remain live (i.e. all spark plugs) if one of the power sources goes down.

A dedicated EFI/EI bus with dual battery feeds is what SDS specifies in their dual ECU power drawing.

Two entirely separate EFI/EI bus feeds mean that with any power failure you automatically lose half the system. With a single bus, it's possible to cross-feed everything and continue running as normal.

The bus itself will be protected by fuses on the driven components.

each diode should have its own independent connection to the bus

consider two, independent diodes and also bridge the anodes and cathodes of each (versus only the anodes as depicted). While dual path, it's still a single component.

As commented by DanH, the diodes are fairly robust for this application, and are unlikely to be a failure point. Adding connections might even increase the chances of failures. Also, pre-takeoff checks with EFI/EI switches will detect a single diode failure.

Could you measure the battery voltage for each battery between the diode and the bus? This would tell you the battery voltage minus the diode voltage drop however, but would show if a diode has failed?

A diode failure can be detected at any time by an EFI/EI switch check.

One thing I haven't decided yet, and haven't included in the diagram, is Hall effect current sensors. I'm thinking about how many I need, what information I require from them in flight, and what is the best location for them. Any suggestions about this would be welcome.
 
Could you measure the battery voltage for each battery between the diode and the bus? This would tell you the battery voltage minus the diode voltage drop however, but would show if a diode has failed?

I think the scenario we are talking about is having two diodes in parallel from each bus to the engine bus.

Since they are tied on both sides, you'd need to add some more components to isolate them to be able to capture the drop. E.g.another diode and then measure between them.

The other more practical solution since these things are long lived would be to put them somewhere that is could be inspected annually. E.g. detach each diode and put a multimeter on them. Not a fan of the reliability impact of opening the circuit every year, or the extra components needed for annunciation, hence my reticence. Math works better since you only fall down if they both fail in one year, rather than two failures over the life of the plane. The diodes are already redundant as drawn, either diode keeps the electrons flowing and when solidly speced as you have done very reliable.

If we are just talking about the system as you have drawn it then I defer to Dan's answer. Turn off the power on one side and you know the other side is good.

If we are talking about just putting two packages rather than one to make the two components more independent and not trying to achieve redundancy then we don't care.

Derek
 
Thank you everyone for your comments. You've given me plenty to think about.

One thing I haven't decided yet, and haven't included in the diagram, is Hall effect current sensors. I'm thinking about how many I need, what information I require from them in flight, and what is the best location for them. Any suggestions about this would be welcome.


Its a good question. While trying to avoid the entrenched alternator wire, or battery wire positions...

I'm B-wire guy and you could likely get by with just the one on the 2AWG to the main bus.

If you put one on both alternators, I'd have trouble how to come up with useful labeling for the Alt 2 since it will be zero most of the time. You may have power on both if the volts get pulled down to the lower set point. (Think idling after start up with depleted batteries, but more a curiosity than something to action).

If you are running on Alt 2 - you will see it in the voltage and the light so lots of ways to catch that.

Putting them on the batteries (and capturing all the Amps) means additional components between the engine and the battery, which would be a no go for me. Just putting them on the AWG2, and ignoring the EFI load, would need an additional physical post since right now you can just merge the AWG2's on the output post of contactor 2, so was a nah for me too.

Derek
 
Z101 puts the vacuum pad alternator on the battery. In the fire-in-the-cockpit-main-master-off scenario, flight endurance is not defined by battery capacity. With rear-mounted battery a 40A automotive relay on the B lead is advisable.

Engine ground strap is not shown in post #1 "ES001 Batteries Alternators Starter.pdf" but recognize that one strap is an SPOF for both alternators, tested at each engine start but still an SPOF. If you add a second strap, failure of one will not be electrically detectable so it becomes a periodic inspection item.

If an injector bus is implemented separate from buses for the other EFI/I items you still have the issue of what if the injector bus itself fails internally so why not put all EFI+I items on one bus as shown in the OP's schematic.

Bob Nuckolls suggest 13.5V for the vacuum pad alternator. Regulator can be LR3.

Monkworkz will start and run with no battery present.

Hall Effect sensor, only one IMO, run both B leads through it. Acutally, a current sensor is not required, voltage tells you if an alternator is offline or overloaded. If you connect the optional SBK-14 Hall Effect sensor to the SB1 it will alarm at 20A but the 410 alternator is capable of more, 29A at 2,300 RPM and 32A at 2,700 RPM.

In-flight stress tests are advisable in addition to preflight functional tests of EFI/I items... alternators, diodes, pumps, backup ECU injector drivers, coilpacks.
 
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I have spent a lot of time recently reviewing Z diagrams to try and understand them and i really like yours here - very simple.

One query however - am i understanding correctly, that the only way for the alternators to feed your EFI/EI bus is with the master contactors on?

I am trying to think of a scenario where you would have to turn off both battery contactors (maybe smoke in the cockpit?). This would limit your flight time to the battery duration, even though you have 2 alternators online?

Would a feed from the main bus through a diode to the EFI/EI bus be prudent?

Another query: you have separate switches for the batts and alternators. I understand that you are supposed to have a battery in the system if you have an alternator online. I was initially thinking progressive switches (aka you must have a battery on to get the alternator on) - but in your diagram you could have one battery of the 2 online, and there would be no reason you couldn't have both alternators. Is this why you have used separate switches? How do you ensure you don't end up with both batteries off, while having an alternator on? (i was thinking smoke in the cockpit - you would need to turn off both batteries and both alternators). Then bring each one back online one by one i guess to troubleshoot where the problem is.
 
I'm not an electrical expert, but ...

1. Yes. As I understand it, an electrical system with alternator(s) requires a battery in order to remain stable over an extended period. Think of the battery(s) as accumulators for the whole system, and the alternator(s) as a supply to top up the accumulator. Therefore, turning off both battery contactors will destabilise the system, and probably lead to the alternators dropping offline very quickly. The reason for having two batteries is to avoid this situation, since the probability of a dual battery failure is very low. If I have two batteries, either capable of feeding the EFI / EI bus without the rest of the system functioning, I don't want to add a feed of possibly poor quality and short duration from the other side of the battery contactors.

2. Yes. Having a switch for each battery and alternator is meant to maximise options when problems arise. Keeping the EFI / EI supplied with power while the battery contactors are off keeps the engine running (I hope) while sorting out the other problems. If I ever saw smoke in the cockpit, I'd turn off the alternators and the batteries, then start troubleshooting from there, meanwhile aiming for the nearest possible landing area. With the SDS system in minimum power consumption mode, two 15.6AH batteries gives me plenty of margin to find a reasonably clear, flat space 200m across (oceanic crossings excepted :)).

I'm not sure any of that helps your system planning, but hope so.
 
I'm not sure any of that helps your system planning, but hope so.

It really does actually. Thinking through failure scenarios is difficult! (For me)

I agree that 2 battery failures is probably pretty rare to happen in the air. In the case of one battery failure, I guess you would close the ecu power switch on that side?

What would happen if the alternator regulator went haywire and one alternator suddenly put out 100 volts. Could both earth x batteries come off line in that case at the same time? If that happened, you might have unstable power from the alternators but worse if you turned off the batteries your engine would stop?

I think in theory the b&c regulators are supposed to act faster in an over voltage scenario to shut off the alternator than the battery would shut itself off but I’m not certain on relying on this theoretical function for an electrically dependent engine - what if the regulator totally fries? (I assume you lose this OV protection). Or if using earth x batteries is there a seperate OV protection device to the regulators?

I wonder if the Monkworx alternator needs a battery in the system? If it didn’t then it would be a good candidate perhaps for a direct feed to the EFI/EI bus should both earth x batteries come offline?

Or perhaps one pc680 and one earth x is a good idea?
 
The SDS documentation suggests a 30 amp fuse for the battery to engine bus connection. That makes me wonder if the 20 amp fuses in your diagram are large enough in case one has to power the bus by itself due to a short or switch failure in the other line.

And on a related note, are the 1TL1-2D switches sufficient in that case since they are only rated for 15 amps?
 
The SDS documentation suggests a 30 amp fuse for the battery to engine bus connection. That makes me wonder if the 20 amp fuses in your diagram are large enough in case one has to power the bus by itself due to a short or switch failure in the other line.

And on a related note, are the 1TL1-2D switches sufficient in that case since they are only rated for 15 amps?

Cannot say for certain but am guessing the larger fuse is needed for the surge related to the initial coil charge which can be significant.

Regardless, there is no reason not to trust the OEM here. 30 is the right answer until they say otherwsie.
 
If I am interpreting the MANUAL correctly, it looks like the SDS system draws less than 10 amps.

That manual comes from the automotive section of the website. The info there might still be accurate but there is a different installation manual and more documentation on the aircraft page: http://www.sdsefi.com/aircraft.html#docs

I'll restate my concern rather than pointing out differences in fuse sizes, since that doesn't really mean much. Here is a chart that lists current draw ranges and recommended breaker sizes for the various components:
http://www.sdsefi.com/sdscurrentchart.pdf

Using the middle of the current draw range for each item, and assuming a dual ECU setup on a 4 cylinder engine (since OP is building an RV-8), and assuming a critical phase of flight where both fuel pumps are turned on it looks like this:

ECU 2 x .095 amps
Fuel pumps 2 x 5.25 amps
Injectors 4 x 0.85 amps
Coil pack 2 x 2 amps
Total 18.09 amps

I'm also ignoring the check engine light, advance switch power, RPM switch relay power, and fuel pump relay power because I'm not sure that all of those are commonly used.

Using the middle of the ranges from that table doesn't leave a lot of margin if it is all powered through a single 20 amp fuse. Maybe it's more common for the components to be at the lower end of the current draw ranges listed in the table but it seems like it would be safer to use a fuse that could handle all of those components running at the max current draw given, which for the items listed above is 25.06 amps.
 
Avoid single point of failure.

This is easily done with a Bussmann 15600 fuse block.

The engine bus is fed at opposite ends to eliminate an FMEA SPOF, loose nut on stud. Bussmann 15600 blade-type fuse panels are easily modified for the 2nd stud; snap the cap off, remove the bus strip, find the 2nd hex hole in the base for the stud head, notch the cap for the 2nd stud you add, reassemble.

Photos here
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It really does actually. Thinking through failure scenarios is difficult! (For me)

I agree that 2 battery failures is probably pretty rare to happen in the air. In the case of one battery failure, I guess you would close the ecu power switch on that side?

What would happen if the alternator regulator went haywire and one alternator suddenly put out 100 volts. Could both earth x batteries come off line in that case at the same time? If that happened, you might have unstable power from the alternators but worse if you turned off the batteries your engine would stop?

I think in theory the b&c regulators are supposed to act faster in an over voltage scenario to shut off the alternator than the battery would shut itself off but I’m not certain on relying on this theoretical function for an electrically dependent engine - what if the regulator totally fries? (I assume you lose this OV protection). Or if using earth x batteries is there a seperate OV protection device to the regulators?

I wonder if the Monkworx alternator needs a battery in the system? If it didn’t then it would be a good candidate perhaps for a direct feed to the EFI/EI bus should both earth x batteries come offline?

Or perhaps one pc680 and one earth x is a good idea?

Some thoughts (and I suspect I”m repeating myself):
- Assuming good maintenance and no abuse (e.g. master left on) AGM batteries are very reliable. I suggest they represent the most reliable component in your electrical systems. The rub is the stuff between the battery(s) and where you want power to go. This is the Achilles’ heel in many systems I review and I suggest the overriding reason two batteries are required for electrically dependent engines.
- Most dual battery designs demonstrate advantage in backup power modes if two identical batteries are used. Small backup batteries can provide a false sense of security.
- For any electrically dependent engine no engine electrical power should come from the downstream side of the master solenoid(s). I build my panels (pMag engine) so that the avionics also come from the battery side of the master solenoids. The test is can IFR flight be continued after you open your master(s) in the event of smoke in the cockpit?
- If running EarthX battery(s) and buss voltage went high (as you your OV protection failed to protect) the batteries would isolate themselves from the buss, the buss voltage would spike at whatever it does while you fumble to find the alternator switch, and your panel will be fried. Here is a VAF thread describing such an incident.
https://vansairforce.net/community/showthread.php?t=154137
The lesson learned is verify your OV protection and know how to turn off your alternator(s).
- The MonkWorX generator (not alternator) does not need any external component to come on line. Personally I would be uncomfortable running it without a battery but I’m good to list this as another backup mode in my plane.

Some common themes:
- A two battery, single alternator design provides for more redundancy backup modes than a two alternator, single battery. Battery size is based on your mission profile. I have ~3 hours of IFR flight on battery power alone.
- I consider my standby generator as a means to get home if the primary alternator fails. This was demonstrated last week by the current owner of my old RV-10.
- Carefully examine the path power goes to get to your critical components (avionics for the IFR mission and electrically dependent engine). Assume any single component (switch, relay, buss, connection, etc.) fails and evaluate what happens. Is this acceptable? If not then you need a mitigation plan.

Carl
 
The SDS documentation suggests a 30 amp fuse for the battery to engine bus connection. That makes me wonder if the 20 amp fuses in your diagram are large enough in case one has to power the bus by itself due to a short or switch failure in the other line.

And on a related note, are the 1TL1-2D switches sufficient in that case since they are only rated for 15 amps?

The schematic attached to post #1 shows two each ANL 20s. An ANL current limiter is a Bussmann device and the smallest one made is placarded at 35A. I say placarded because, unlike a fuse, an ANL current limiter will carry much more than its nameplate number. The only ones I can imagine using in RVs are 35, 40, 50, and 60 and all those will carry a little over twice their nameplate rating. They are designed to be placed close to the battery to protect a feeder to a fuse block or alternator B lead in case of a hard short. Fuselinks do the same thing but are even more robust.

As a fan of FAR 23.1361, I would put the engine bus FWF, make the feeders short and un-fused, and use automotive relays (20A rating is fine or if you're feeling more conservative 40A) remotely controlled.

If the fuses are retained they could be Littelfuse MIDI 30A fuses which as a bonus are smaller than Bussmann ANL current limiters. ATC blade fuses are also available in 30A rating.

And IMO it's important to periodically stress test redundant things like the engine bus feeds in flight in addition to preflight functional tests.
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I say placarded because, unlike a fuse, an ANL current limiter will carry much more than its nameplate number
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Looks like I need to go learn more about ANL current limiters. Thanks for pointing that out.

re SDS dual EM-5F four cylinder current requirement... here is my load analysis. Note the total currents to run the engine on row 22.
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It's nice to see that some of the components might not need as many amps as I was estimating. Where did you get the current vs GPH numbers for the injectors?
 
Looks like I need to go learn more about ANL current limiters. Thanks for pointing that out.

I learned a lot about a host of subjects over time by subscribing to the Aeroelectric List hosted by Matronics. Bob Nuckolls hangs out there.

It's nice to see that some of the components might not need as many amps as I was estimating. Where did you get the current vs GPH numbers for the injectors?

Without getting too deep into it and without being super precise... the injectors that came with my EM-5-F O-360 system in are Magnetti Marelli part number IWP 069. They have a specified coil resistance and a specified static flow rate at a specified pressure. I recalculate the static flow rate for a system pressure of 45 psi (it's actually a very small difference), assume opening and closing times are the same (which will make my result a little low, especially at low pulse widths), neglect current rise delay due to coil inductance (which will make my result a little high, especially at low pulse widths). Ref injector calculations starting at row 46 in my calculations measurements and specs spreadsheet. I imagine the spreadsheet is cryptic because it's not designed for public consumption. The detail I get into here is not necessary, it's just what I do.
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Thank you all for your comments. You've given me lots of good data, and I have plenty to think about. Clarifying a couple of minor items:

Bussmann ANL limiters only go down to 35A, but various other manufacturers produce compatible devices in lower amperages. The 20A ones I included in my drawing are widely available here in Australia, since they are used on domestic solar photovoltaic installations, of which we have millions. If you don't like them, fusible links should be fine.

My mission requirements are generally for VFR only, since I don't enjoy recreational aerobatics in bad weather. Luckily, most of Australia has much better flying weather most of the time than most of the rest of the world. Also, fewer and much smaller mountains :). I expect that for most electrical problems I could simply turn off the alternators and battery contactors and continue flying for more than an hour, giving me plenty of options at RV-8 speeds. On my infrequent cross-countries, I intend to have my Android tablet and smartphone loaded with AvPlan EFB, so there will be a couple of layers of backup navigation available before I need to revert to charts, protractors and pencils.

Generally, the philosophy underlying my proposed design was to keep it simple to understand, so that I don't need to go through complex analysis when something goes wrong, and simple to build, minimising the number of devices to fail. Also, to keep the EFI / EI on a single bus directly connected to either or both batteries and isolatable from the rest of the system.

The system failure protections depend heavily on the ETX900 battery management systems and the B&C regulators. Monitoring their health is important.
 
... Bussmann ANL limiters only go down to 35A, but various other manufacturers produce compatible devices in lower amperages. The 20A ones I included in my drawing are widely available here in Australia, since they are used on domestic solar photovoltaic installations, of which we have millions...

Now I see I already have the data sheet for Renology ANL bolt-down fuses for instance who offer a 20A product on the same 61 mm bolt center as Bussmann ANL current limiters.

Renology says their ANL will carry 150% of rating for 1 hour minimum.

The Renology ANLs open significantly quicker than the Bussmann ANLs. Comparing 35A devices since Renology does not show a curve for the 20A one... Bussmann opens in 100 seconds at 90A and Renology opens in 100 seconds 60A (these are averages). Maybe that's why Bussmann calls it a current limiter and Renology calls it a fuse.

The Renology ANL is less robust than a Bussmann ANL and more robust than a Littelfuse MIDI (which is a smaller device on 30 mm centers).
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Without getting too deep into it and without being super precise...

That's even more detailed than I was expecting. I figured maybe you found a chart that plotted current usage a various flow rates. Still, that's great info. Thanks for sharing.
 
What are the 20a anl for in this case? To protect the wires to the switch from a short? Would 60amps through a 10awg for 100 seconds not damage the wire? (It would destroy the switch wouldn’t it?). Is a fuseable link or an inline fuse better (quicker)?

For an aeroplane with FW mounted batteries, if you mounted the busman fuse box (EFI/EI bus) on the firewall could you just run the feeds from the battery with no anl / fuseable link? And activate the feed with a 30amp relay to the diode then the bus? I know the relay is a fail point but so is a large amp load through a small Honeywell switch?
 
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