Master Contacter

[Bryan Cotton]

I'd also like to stop wasting 1 out of my 10 precious alternator amps on the master contactor. There are solid state "relays" that use essentially no current, something like https://www.mouser.com/ProductDetail/Cr ... 7kvuhYo%3d. Anyone tried this route?

The thing you have to watch for with solid state devices is that they're sufficiently protected from voltage spikes, these have maximum ratings of at least 50Vdc, but it would be possible to add an extra overvoltage protection on top.

Interesting idea, I have seen a few of these at work. You would need an x870 from the datasheet. I see a few potential pitfalls:
1) Leakage current .1 mA max. Not a lot, but for any resistive load left on the bus it will flow some current when you have it off.
2) Voltage drop of 0.8V max. So you could be burning up some watts there. What will happen to your loads, and how well will your battery charge with 0.8 less volts?
3) Will this flow current backwards so you can charge your battery or is it meant just to turn on loads?
4) You will need a heat sink plan. Maybe the stainless firewall will be good enough.

[Bryan Cotton]

Hi Bryan,

How does the shaft for the battery switch get around the fuel tank? I don't have a front fuselage yet so I can't see for myself. I am contemplating the Flaming River battery switch that comes with an extension handle. But I already have a Sky Tek master contactor in the hangar that I am tempted to use out of simplicity sake. The one in my RV-6 has been holding up for almost 18 yrs & 960 hrs.

Still trying to finalize my power circuit.

Rick
Xenos 0057

My latest picture with the tank installed.

[inventor]

I don't understand why anyone would want to build a single point of failure into their electrical system. I have had a lot of failures with these contactors in liftgates, boats and in a jeep that I installed one in. Try driving on the Turner Turnpike in the middle of the night with no moon when all your lights go out. I would suggest using fuses or circuit breakers on all loads and use a contactor in series with the starter to guard against a short in the starter system if that is a worry. I might use a master switch in my Onex and I would welcome any input on a decent switch to use.

Richard

[Jgibson]

Why re-invent the wheel?
Part 23 aircraft have a pretty good record using the 'master' contractor and 'starter' relay since the '30s. Whenever I'm unsure of a fix or idea on how to wire up or repair, 43:13B will steer me in the proper direction for a safe method.
JMHO

Joe Gibson

[NWade]

I don't understand why anyone would want to build a single point of failure into their electrical system

As an IT professional I spend a lot of time thinking about redundancy and points of failure. I see the sentiment quoted above on the Sonex forums a fair amount and it baffles me because there are *lots* of "single points of failure" throughout the entire electrical system for most aircraft; at least until you get up to Biz-jets and Airliners.

• Replacing the contactor does nothing to solve the fact that there's probably a single wire between your battery and your main bus. So that's a "single point of failure".
• Do you have multiple parallel wires running from each fuse or CB to each item that it powers? If not, then all of those "single string" wires are single points of failure.
• Do you have multiple fuses or CBs providing power to each item? If not, then each of your circuit-protection devices is a single point of failure.
• Do you have fully redundant instruments? If not, then each one of those is a single point of failure (whether electrical or driven by pitot/static inputs).
• Do you have multiple wires feeding each ignition coil? How about grounding each magnetron?

Each one of these items, when installed and maintained correctly, is highly unlikely to fail. If you add redundancy for these items you're adding extra parts, complexity, and new maintenance items (& failure points) that you will likely never need - yet you carry around the cost (in money, in weight, and in labor) on every single flight.
And if you only add redundancy to one of two of these things, how much have you really improved your odds of never having a failure; given that each scenario is so very unlikely to happen? Yet by adding that redundancy have you lulled yourself into a false sense of security? And have you fully taken all of the additional necessary maintenance into account, and are you performing it diligently?

In the IT world we focus a lot on being "fault tolerant" and having "risk mitigation" factors. That is to say, rather than just buying 2 of everything and trying to run with perfect redundancy we architect our systems so that a small failure does not cause a major problem, and we try to make sure that any failure scenario merely degrades services in as safe and graceful a manner as possible.

There are lots of things that can fail in an VFR aircraft and you can still have a perfectly safe conclusion to your flight. For example: as long as you're not flying in the clouds, losing your artificial horizon is mitigated by the fact that you can visually orient yourself. Heck, even losing your voltage regulator (assuming you can isolate it from your electrical bus) is a manageable fault that simply results in landing sooner than you might've intended.

Adding parts and redundancy does not necessarily raise safety. Most of us by now have been exposed to the topic of safety in light twins versus single-engine GA aircraft, and that example can serve us well in this instance.

If you want to add things to your experimental aircraft, that's totally fine and is the right of any builder. But as Bob N (author of "The AeroElectric Connection") has said in his interviews on the SonexFlight podcast: its important to think through the positive and negative effects, and understand all of the failure modes of each item you add to your aircraft.

--Noel

[inventor]

As an IT professional I spend a lot of time thinking about redundancy and points of failure. I see the sentiment quoted above on the Sonex forums a fair amount and it baffles me because there are *lots* of "single points of failure" throughout the entire electrical system for most aircraft; at least until you get up to Biz-jets and Airliners.

[/quote]



There is more than one type of "single point of failure". One that takes down the entire electrical system of an aircraft is a big one. This type of failure is probably just as dangerous to a pilot as data loss is to an IT professional. As far as I am aware the only way to prevent data loss is with redundancy and without data, a computer is a boat anchor.

Richard

[sonex1374]

...[snip] I have had a lot of failures with these contactors in liftgates, boats and in a jeep ... [snip] ... I would suggest using fuses or circuit breakers on all loads and use a contactor in series with the starter to guard against a short in the starter system if that is a worry.

Using a battery contactor, a manual battery switch or a high-quality master switch are various ways of doing the same thing - incorporate a way to isolate the battery from the electrical system with a simple pilot action. This isn't required of us for experimental airplanes, but it's good practice that has been proven over the years. Wiring all loads directly to the battery thru fuses or circuit breakers is also a viable approach, and for simple airplanes and simple electrical systems it's not a bad way to go. Look at most 2-stroke airplanes and how they are wired - the lighting coil connects to the voltage regulator and the output goes directly to the bus or battery (if there's even a battery present). No switches or contactors are used, and the loads are driven off the voltage regulator output directly. It's simple (crude, really), and it works.

When you start stepping up the size of the battery (e.g. amount of stored energy) and the complexity of the electrical system, it starts to become convenient to incorporate more control over the system, and eventually the battery has so much capacity that it becomes a liability in a crash - such that we really want it to be spark-free when the structure starts deforming. Is the Odyssey 625/680 or EarthX a worry? - you'll have to make up your own mind, but I wouldn't mis-handle a pair of jumper cables connected to one of these batteries cause they can deliver a wallop!

Using fuses or circuit breakers on the loads connected directly to the battery can accomplish much the same thing, though. Should there be a short circuit in any particular wire, the fuse/CB will open and stop the flow of current that might cause sparks. You'll still have the possibility of parasitic loads draining the battery (because they're all directly connected to it), but there's ways to manage that if need be (e.g. frequent flying, float charger, etc).

The bottom line is that we have options on our planes on how we do things. There are pro's and con's to the various approaches, but none so extreme that there is a clear winner or clear looser. You'll have to consider these yourself and decide accordingly.

Jeff

[N190YX]

Suggest we not stray from: 1) aircraft quality components and hardware; and 2) Standard practice and how things are done on certified airplanes. I read numerous aviation publications, aviation safety publications, and accident reports, and I don't recall reading about failures of master contactors contributing to accidents or dispatch reliability. Try not to over think this. The failures Richard mentioned may not have been with aircraft quality contactors. It is important to be able to isolate the battery (and the alternator) should a problem happen with the electrical system, to prevent a fire. Also important to use aircraft quality wiring, so if there is a fire, the fumes will not be toxic. Etc. etc. Aircraft quality, standard typical aircraft systems, keep it simple. The place to be creative is in cosmetics, not on the airframe or systems. The airframe and systems must be aircraft quality and standard practice, anything else may not be safe.