electrical questions

[iowaflyer]

I've got 3 electrical questions that I hope someone can help me with:

1. I'm a little confused about the concept of an e-bus and the redundancy it provides. I thought an e-bus was primarily to protect against a failure of the battery contactor. But, when going through old posts I see alot of discussion of load requirements and amps. Why is electrical load a factor? Wouldn't total amps on the e-bus only be an issue if there was an alternator failure in addition to a battery contactor failure?

2. Is there any way to wire an avionics master switch with an e-bus (if the e-bus doesn't include all of the avionics)?

3. Some people here on the forum have argued against using an avionics master because it could provide a single point of failure. I understand this, but how much of a concern is this? I've never had any switch in an aircraft fail and I've never heard of an accident resulting from an avionics master switch failure. Is this more of a theoretical concern?

 

[Mike S]

Andrew, this is gonna be a bit hard to answer, without knowing more of your system, but that aint gonna stop me

The concept of "E" buss is not so much for redundancy, as it for load shedding in case of an alt failure. Some things are necessary to keep the noise happening up front, and some are not.

For a single battery system it is going to be different than a dual battery system, so that also comes into play here.

For my electrical system (dual batt/alt) I did not use a contactor on the Aux----what I call my "E" buss. I wired the second battery through a fuse, sized to the anticipated load+ a safety factor, and then also feed the main battery power to that buss through a diode. This does provide a bit of redundancy.

All my EFIS and radio equipment is powered directly from the Aux buss, and indirectly from the main buss through the diode.

I use an Avionics Master, powered from my Aux buss.

You are right about a single point of failure, but I agree with you about the failure rate of switches being pretty low, and I am using a circuit breaker switch for added protection.

Good luck, expect additional comments/opinions on this----read them all, evaluate the pros and cons of each, and then make the best decision you can.

 

[rv8ch]

Aeroelectric Connection

All your questions will be answered here:

http://www.aeroelectric.com/

There is an excellent book you can buy there that will make all the electrical work on your aircraft very clear, very simple, and a lot of fun.

There is also a Matronics forum that the author participates in to answer any questions.

A truly amazing resource.

 

[iowaflyer]

So, some people use an e-bus for load shedding in the event of an alternator failure. That explains the concern for electrical load. But, it doesn't really add redundancy, does it? And, no offense, why not just turn components off individually?

But, an e-bus will also protect against a battery contactor failure, in which case the electrical load is not an issue (given that the battery contactor alone fails). Right?

I've read Nuckolls' book. Great stuff. I'm a newbie, though, and it lead to as many questions as it answered. Good tip about the forum, though. Thanks.

Andrew

 

[erich weaver]

The protection against contactor failure is not the purpose of the ebus - thats more of a happy coincidence than anything else. The ebus simply allows a very quick and simple way to shed electrical load that has all been thought out in advance. It prevents you from having to make numerous decisions about what to turn off and what to leave on in what may be a very stressfull situation. If you have read the Aeroelectric connection book and missed the ebus explanation you have missed its heart and soul. No disrespect intended here - I think I went through it 5-6 times before the philosophy really hit home: ASSUME failures will happen and have a simple and practiced Plan B ready to go.

best regards
erich

 

[Ironflight]

So, some people use an e-bus for load shedding in the event of an alternator failure. That explains the concern for electrical load. But, it doesn't really add redundancy, does it? And, no offense, why not just turn components off individually?

The other thing that I personally think of when I think "Essential Bus" is really to protect against a short on the main bus - something that makes smoke to the point that you have to kill the Main. If you feed the essential stuff from both busses, isolated with diodes, you have protection from this event.

I don't really use the multi-bus system for load shedding - I have color coded rings on my circuit breakers that tell me what to pull quickly. I mean, let's face it, even the most well-equipped RV is pretty simple electrically. You have a few big-draw items that you can easily switch off. It is rare when you have to load-shed in a super hurry - you usually have 30 seconds at a minimum, which is all it takes to turn off the landing lights and pitot tube.

While it is nice to think like a Boeing driver, don't build your smaller airplane with the intent of making it more complicated than it needs to be.

Paul

 

[SMO]

Batt Contactor

One of the points Bob K makes is the batt contactor itself takes power to operate, and when you are looking for every amp you can get, the ability to shut it off and run only the essential stuff off the ebus just might save your hide.

 

[dahugo]

http://www.supremecourt.gov/opinions/09pdf/08-1521.pdf;lm';lm'l;m'lm'l;m;'m';lm'lm'l''ml'l

 

[az_gila]

Paul...

The other thing that I personally think of when I think "Essential Bus" is really to protect against a short on the main bus - something that makes smoke to the point that you have to kill the Main. If you feed the essential stuff from both busses, isolated with diodes, you have protection from this event.
.....

...the terminology now used by Aeroelectric Bob is "Endurance Bus" not an "Essential Bus" -

http://www.aeroelectric.com/PPS/Adobe_Architecture_Pdfs/Z11M.pdf

This gives more credit to the load shedding ability/useage of the circuit as mentioned previously.

I'm not sure what you mean by "a short on the Main Bus". A physical short to ground on the Main bus would be covered by the Z-11 circuitry, but by definition, those circuits can be dumped anyway.

A true physical short on the Essential bus has no recovery, jsut like a short on a traditional (old-fashioned?) single bus system.

If you are suggesting that primary/essential/endurance electrical items are fed by two physical wires from two fuses through diodes from both buses, then that failure could be taken care of - however that is not what the Z-11 circuitry does.

Without thinking like Boeing (or NASA......) the extra wiring involved in duplicating the simple Z-11 circuitry for a one alternator, one battery system is pretty minimal. Basically it involves splitting the bus into two parts and adding a switch and a diode - minimal extra work, especially since Bob publishes the schematic for you...

It does allow for the more probable failures in the master contactor - alternator charge circuitry - battery, with only a simple response required by the pilot. I personally have had an in-flight alternator failure, and just replaced a friends master contactor, which luckily failed on the ground.

With modern EFIS units having built-in back-up batteries, the problem of knowing how the plane is flying is taken care of...

 

[Wayne Gillispie]

Thanks Paul

While it is nice to think like a Boeing driver, don't build your smaller airplane with the intent of making it more complicated than it needs to be.

Paul

I am going to finish in less than 3 years and it sure helps to keep things simple on our little rocket ships.

 

[Ironflight]

...the terminology now used by Aeroelectric Bob is "Endurance Bus" not an "Essential Bus" -

Oh yes Gil, I am well versed in Bob's book, and as I have said many times, I am generally in about 95% agreement with him (pretty astounding for engineers to agree like that!). Terminology aside, my point was that folks spend an awful lot of time trying to recreate the "automatic load shed" capability of complex, multi-engine turbine equipment, when all you really have to do is turn a few things off - it takes seconds!

By "Short on the Main Bus", yes, I mean to ground, with all the associated smoke. That is when it is nice to have a backup bus that can provide power to essential items. I worry about it because I have had it happen, in airplanes and "other craft". In truth, my favorite power architecture for critical equipment is to have everything powered from multiple sources through isolation. Some EFIS equipment is built with this in mind. Using this scheme, there is simple a "Bus 1, Bus 2" scheme, and one can go away with no problems.

But a forum is no place to write a book on the topic....

 

[az_gila]

Now you are making...

Oh yes Gil, I am well versed in Bob's book, and as I have said many times, I am generally in about 95% agreement with him (pretty astounding for engineers to agree like that!). Terminology aside, my point was that folks spend an awful lot of time trying to recreate the "automatic load shed" capability of complex, multi-engine turbine equipment, when all you really have to do is turn a few things off - it takes seconds!

By "Short on the Main Bus", yes, I mean to ground, with all the associated smoke. That is when it is nice to have a backup bus that can provide power to essential items. I worry about it because I have had it happen, in airplanes and "other craft". In truth, my favorite power architecture for critical equipment is to have everything powered from multiple sources through isolation. Some EFIS equipment is built with this in mind. Using this scheme, there is simple a "Bus 1, Bus 2" scheme, and one can go away with no problems.

But a forum is no place to write a book on the topic....

...it much more complex... with the paragraph on true redundancy with dual buses.

But, if we have fairly simple planes, and don't have dual Electronic Ignitions, and have built-in back-up EFIS batteries, do we really have any "critical equipment" that is essential for flight? We should have no critical life support issues in our planes....

You first paragraph on Bob's Z-11 version is correct, but for the sake of a diode and a switch, all of the thinking on what equipment to shut down to preserve battery life has been previously conducted over the kitchen table with little stress. This seems like a good pay off for the addition of a few parts....

No book writing involved...