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Switch selection

rmartingt

Well Known Member
Now that I've settled on a general power architecture, I'm really starting to sit down and plan out my electrical system in detail. Part of that involves choosing switches. I have some aesthetic and functional preferences in mind*, but some technical requirements are driving me away from what I'd really like. I've been chasing my tail on the subject for quite a while so I figured I'd throw it out to the community.

The biggest obstacle I'm facing is current. I'm going to have an electrically-dependent engine, with power provided by a dedicated engine bus fed directly from each battery through diodes. The trouble has been finding switches with enough of a current rating to handle this feed. The nominal average draw for the engine bus is about 10A (which isn't that bad, and I wouldn't normally be cycling this switch under that kind of load), but peak loads could be higher and I'd like some margin on that given how critical this switch would be. I'm thinking 30A is a good target, based on Ross's description of an alternate feed for the engine electronics (30A ATO fuse and a heavy duty switch).

However, there are relatively few switches out there rated for 20A, and even fewer rated for 30A. Almost none of them have any kind of locking mechanism, and I'd really prefer to avoid the big red flip-up milspec switch cover for multiple reasons.

NKK makes a locking toggle rated for 20A: https://www.digikey.com/product-detail/en/nkk-switches/S6AL/360-1927-ND/1007005

The Carling Contura rockers can handle 20A as well, but I've only found one supplier in the US that carries a version that can lock in the "on" position: https://www.onlinecomponents.com/carling-technologies/vdd1hw611kv000-16995989.html

Past that, I haven't found any viable options that don't take up tons of panel space (e.g. marine battery disconnects).



I kicked about the idea of using automotive relays on these feeds, but I'd prefer to minimize components and connections on the critical items.


Any other suggestions or ideas?

Beyond this I have fuel pump and coil loads to worry about (also things I want with locks or guards), but those could use the same switch if necessary.





* If I could really have what I wanted, I'd use illuminated square pushbuttons like the Korry switches on airliners, with flip-up transparent guards on the critical switches but those are expensive... cheaper options exist but I'd have to drive everything with relays, and that's too complicated.

Failing that, I'd prefer the Carlings over the traditional toggle switches.
 
Stein Air

I too have an electrically dependent aircraft.

For switches that "should" be operated on the ground I chose to go with Stein's Locking Toggle Switches (LT-001 & LT-002). They are manufactured by Honeywell; are a bit $$, but are very durable / dependable and should handle your requirements.

I say "should" be operated on the ground because it is my intent those switches only be turned on at start up and not touched until back on the ground and shutting down. The locking function requires a conscious effort to operate the switch from one position to the other - therefore preventing an accidental turning off of a critical circuit in the air.

For the "air operations" I went with the Stein Air Carling switches. They do not lock and can be bumped - but I don't believe an accidental turning off of the strobe lights, landing lights, etc. will be a safety issue in the air; thus no reason for the locking style.

Hope that helps.
 
Switches are not like fuses that blow apart if the current rating is exceeded. But the life of a switch will be reduced if it switches too much current. Suppose that a switch is rated for 10,000 cycles at 15 amps. If the load on that switch is 20 amps, its life will be shorted. Suppose that it only lasts for 2,000 cycles at higher current. That switch will still last as long as the aircraft. I am not saying that a switch should be operated above its rating. But if the nominal load is 10 amps and the switch is rated for 15 amps, that is good enough. Many switches fail due to lack of use. Switches need to be exercised to wear away corrosion on the contacts.
 
Switches

You really should size the switch properly. It isn?t any different than properly sizing a wire; you can almost always use a wire larger than required and almost never smaller. Switches are the same.
 
If the switch will be interrupting an inductive load, the switch can be protected against contact arcing by connecting the banded end of a diode to the load side of the switch. Connect the other end of the diode to ground. The diode will short induced voltage spikes to ground.
 
I say "should" be operated on the ground because it is my intent those switches only be turned on at start up and not touched until back on the ground and shutting down. The locking function requires a conscious effort to operate the switch from one position to the other - therefore preventing an accidental turning off of a critical circuit in the air.

That's my thinking as well--locking switches for engine bus power, coil power, and fuel pump power. I don't need everything else to be locking.

You really should size the switch properly. It isn?t any different than properly sizing a wire; you can almost always use a wire larger than required and almost never smaller. Switches are the same.
That's my intent. Part of the problem is all I have for the current draws are average numbers (approx 10A); it can vary a bit. Plus, I believe things like the coils and injectors aren't constant current, but draw higher current intermittently (e.g. instead of 2A continuous, it pulls 8A on a 25% duty cycle or something). I don't know those exact figures. Therefore I want to be conservative with a switch that is rated for 30A.... but I can't really find any.

To clarify... the total engine bus average load is expected to be about 10-11A continuous, and maybe 16A for takeoff/landing if I run both pumps. That's the biggest driver. I'd like the switches to be rated for at least the fused values for the fuel pumps and coils (15A and 10A, IIRC).

I recommend a relay for your 20 or 30 amp loads.
I'm definitely considering it. I've found several automotive ones that seem like they'd work fine for this. I'm just wary because if I did go with relays, those relays would be carrying the current that keeps the big fan turning. I know a switch would be doing the same; it's just another failure point.

Though, the big airplanes at work use contactors and the like for all their primary power... and that's why I'd have two...
 
A far bigger issue is running an electrically dependent engine from a fuse fed bus where thr fuse rating is only a few percentage points higher than your max calculated load. Honestly, if I knew someone had done that, I wouldn't fly in that plane, and I'll happily fly in alt engine planes. Do some reading here:
https://www.google.com/search?client=ms-android-google&ei=-yn6W9SFI4ubzwLAyLqIDw&q=30a+ato+fuse+time+constant&oq=30A+ATO+fuse+time+con&gs_l=mobile-gws-wiz-serp.1.0.33i299l3j33i160l2.35022.44679..46506...0.0..0.207.1509.0j8j1......0....1.........0i71j0i22i30.EVDuuKghAJ4
Specifically about ATO fuses.

I want my engine bus feed protection to activate only on a truly catastrophic fault. A nuisance trip of engine bus protection would be far more than a nuisance. :)
 
Every car on the road has has it's engine electronics (and everything else) powered by relays.

Starter relays very occasionally fail, but when was the last time you had a relay failure going down the road, and you had to pull over because your engine stopped.

Two of my vehicles are '86 and '87 VW's. Every relay is original, other than the 90's Subaru ignition and fuel pump relays powering their respective loads.
 
That's my thinking as well--locking switches for engine bus power, coil power, and fuel pump power. I don't need everything else to be locking.


That's my intent. Part of the problem is all I have for the current draws are average numbers (approx 10A); it can vary a bit. Plus, I believe things like the coils and injectors aren't constant current, but draw higher current intermittently (e.g. instead of 2A continuous, it pulls 8A on a 25% duty cycle or something). I don't know those exact figures. Therefore I want to be conservative with a switch that is rated for 30A.... but I can't really find any.

To clarify... the total engine bus average load is expected to be about 10-11A continuous, and maybe 16A for takeoff/landing if I run both pumps. That's the biggest driver. I'd like the switches to be rated for at least the fused values for the fuel pumps and coils (15A and 10A, IIRC).


I'm definitely considering it. I've found several automotive ones that seem like they'd work fine for this. I'm just wary because if I did go with relays, those relays would be carrying the current that keeps the big fan turning. I know a switch would be doing the same; it's just another failure point.

Though, the big airplanes at work use contactors and the like for all their primary power... and that's why I'd have two...

Multiple issues here.
1st, you need to know each load, and the sum of all loads (needed for proper load analysis, wire sizing, circuit protection, etc etc).

2nd, virtually all devices have some power-up surge, and if it's extreme, the device's published specs will detail it. (The only item that comes to mind that might have serious 'issues' is the old style strobe power supply, which can hit the switch with heavy pulse overloads every time it fires. That one-per-second overload has shortened the life of many switches.)

3rd, while 'going big' usually doesn't have any downside (other than money), in an a/c the equation can change, due to weight considerations. ex: few would want their RV to have 18G wings. 50% margin over 6G for safety? Of course, but a 100% margin over *that* would turn the plane into a dog of a performer, at best.

4th, remember that quality switches also have margins built in (kinda like your wing spar). And most have ratings for high-surge use, too.

5th, there really aren't many modern loads in our a/c that will exceed the abilities of a 15A switch. An auto-style engine control system+fuel pump, and *maybe* the flaps (but take a look at the switch Van ships), are the only things that come to mind at the moment.

You obviously want to use products adequate for the task, but you should never assume that you can oversize or 'quality' your way into 100% reliability. You should assume that any one component can fail during any one flight, and if it is critical to flight, have a stress-free way to trigger your built-in plan B to work around it.

FWIW,

Charlie
 
I'm using the NKK switches in your link on my H6 Subaru for the ignition and fuel pumps. Been flying for 10 years with no problems. My architecture is close to the AeroElectric Z-14 (I had mine before Z-14 existed) - dual batteries, one alternator. Two ignition and two fuel pump switches - one for each battery. The ignition power goes through diodes to the single feed to the ECU.
The big loads on those circuits occur when the switches are already closed (full power). At start up and shutdown the loads are minimal so the switches are not stressed a lot when opening/closing.
 
Multiple issues here.
1st, you need to know each load, and the sum of all loads (needed for proper load analysis, wire sizing, circuit protection, etc etc).

I have a sheet built up with all of these. I'm also using this:
http://www.sdsefi.com/sdscurrentchart.pdf

As well as these references from the SDS installation manual:
Current draw of the pump, ECU, injectors and coils would be around 7.5 to 10 amps.
We?ve found the most reliable, simple and light way to get backup battery power to the engine electrics is a single 12 to 14 gauge wire running from the backup battery, through a 30 amp ATO fuse, to a heavy duty switch, to an emergency buss where all the engine electrics can receive power.


2nd, virtually all devices have some power-up surge, and if it's extreme, the device's published specs will detail it. (The only item that comes to mind that might have serious 'issues' is the old style strobe power supply, which can hit the switch with heavy pulse overloads every time it fires. That one-per-second overload has shortened the life of many switches.)
That's part of my trouble; I don't know what that power-up surge is. I know what the breaker sizes are (see above) and if I played it super-conservative I'd be sizing for 80 amps (sum of all the breaker values). The breakers seem sized for about 3x average load, so using that number gets me 30A.

3rd, while 'going big' usually doesn't have any downside (other than money), in an a/c the equation can change, due to weight considerations. ex: few would want their RV to have 18G wings. 50% margin over 6G for safety? Of course, but a 100% margin over *that* would turn the plane into a dog of a performer, at best.
That's very true--an engineer I know is very fond of the saying "WEIGHT IS THE ENEMY." That said, I'm talking about a couple of slightly larger wires with length under 3 feet (not every wire on the plane, just the bus feed), a larger fuse (no additional weight, really), and maybe a pair of relays (ok, a pound or two there).


5th, there really aren't many modern loads in our a/c that will exceed the abilities of a 15A switch. An auto-style engine control system+fuel pump, and *maybe* the flaps (but take a look at the switch Van ships), are the only things that come to mind at the moment.
Precisely what I'm talking about here :) Though I think pitot heat might draw around that, too, for an old Piper blade.

You obviously want to use products adequate for the task, but you should never assume that you can oversize or 'quality' your way into 100% reliability. You should assume that any one component can fail during any one flight, and if it is critical to flight, have a stress-free way to trigger your built-in plan B to work around it.
Again, exactly my plan. I'm just trying to minimize the number of places where something can fail and knock out one of my two engine power feeds.

I guess the trade I'm facing is, will a relay/contactor plus a switch carrying minimal current, both well within their design limits, be less likely to fail despite having more places a failure could possibly occur, vs. direct routing through a switch that carries higher currents but has fewer failure points?

I think that using some sort of relay/contactor at this point is probably slightly more reliable, but I have no numbers to back that up. At that point it becomes a tossup--more aviation-standard contactors, or industrial sealed units like this one that are lighter and draw less power?


I also thought about a pair of heavy-duty marine battery disconnects, but those are freaking huge and I don't have panel/console room for them.
 
I knew I was forgetting one more big load. :)

FWIW, there are choices out there of quality 20A switches. I found some 30A
(overkill) toggles made for heavy trucks. I chose to feed my engine bus direct from battery through one of those switches. I ran the rest of the plane, including the start circuit, on one bus, through a conventional master contactor. I used another of those 30A switches as a cross-feed switch between the engine & aircraft buses.

I do have the luxury of two identical alternators, so no essential/endurance bus for me.

Charlie
 
Just one comment on Online Components. I've purchased parts from them before and everything went extremely well. If I remember correctly they have the Honeywell toggle switches at an attractive price too.
 
If you like toggle switches, you can't go wrong with Honeywell Microswitch TL series switches, most are available with a locking feature that obviates the need for switch guards.

https://sensing.honeywell.com/switches/standard-toggle-switches/TL-series

https://sensing.honeywell.com/honeywell-sensing-micro-switch-tl-toggle-product-sheet-005430-3-en.pdf

It takes some wading through the latter link to decide what you want; comparing to Bob Nuckoll's switch info may help decipher some of it. I believe some of the variants are available through Stein, etc. Otherwise Digikey, Mouser, Allied and even Ebay are other sources. Not cheap, not very sexy, but reliable.
 
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