[RVbySDI]

I have read several threads on the issues of turning on/off the alternator before and/or after the engine is running (here is one of the recent threads, another one here). I have also been working on cars for 30 some odd years now and have never seen a car that had a switch to turn off the alternator separate from the battery.

My question, can someone give me the reason why this setup exists in the aviation world and why I need it in my 9? Why have this setup when automobiles do not?

I am interested in just having a MASTER switch to shut off power to everything and other individual switches for nav lights, strobe lights, etc. The starter will be a standard key switch. Can anyone give me reasons why I need the split alt/bat switch?

 

[Mike S]

Here is one reason,

My question, can someone give me the reason why this setup exists in the aviation world and why I need it in my 9? Why have this setup when automobiles do not?

Your chances of dying from an electrical fire in an airplane are a lot higher than a car.

 

[RVbySDI]

Your chances of dying from an electrical fire in an airplane are a lot higher than a car.

Ok, I will not argue that point but why does this split switch save me from an electrical fire? I am trying to understand why it is needed? What does it do differently than a standard "TURN EVERYTHING OFF" switch.

 

[hecilopter]

If you had an overvoltage

situation, you could turn off the alternator and still function off of the battery. I also don't know why it has to be a switch, I would think a pullable breaker would work just fine and only have a master switch. I have 2 switches, but always turn them ON and OFF in pairs (before start and after shutdown). If I were to do it again, I would probably have a master switch and a pullable breaker for the alternator field.

 

[Mike S]

What does it do differently than a standard "TURN EVERYTHING OFF" switch.

Lets you keep running on the battery, at least until it goes dead.

Radio, xpndr, lights, boost pump, ETC.

But, by then, you should be able to make a safe landing.

 

[RVbySDI]

Lets you keep running on the battery, at least until it goes dead.

Radio, xpndr, lights, boost pump, ETC.

But, by then, you should be able to make a safe landing.

Well this leads to another question. So If I have all the "important" items to keep me flying, navigating, communicating on their own switch then a separate ALTERNATOR switch to turn it off if need be, is this going to accomplish the same thing as a split ALT/BAT switch?

Oh yeah, same question as above. Why does the ability to turn off the alternator mitigate the risk of fire? Is it that the alternator can still generate power to some electronic component that may be shorting out and causing the cockpit fire or is it that the alternator itself can be the cause of the fire?

 

[kentb]

Steve, I always start with alternator on.

And shutdown with alternator on.

My switch will let me select BAT only or BAT/ALT. I have had the alternator go over-voltage before (bad regulator) and I just set the switch to BAT.
Of course I do have a standby alternator so I just set that one on and kept flying.

Kent

 

[Mike S]

The items listed are an example of what would help make an emergency landing, all of them are on the master switch, not on "their own" switch.

The master is just that----a master. It controls power to anything that is switched. Even though there may be other switches in line, after--downstream of-- the master

The alt switch just shuts off the charging circuit.

 

[L.Adamson]

Well this leads to another question. So If I have all the "important" items to keep me flying, navigating, communicating on their own switch then a separate ALTERNATOR switch to turn it off if need be, is this going to accomplish the same thing as a split ALT/BAT switch?

Oh yeah, same question as above. Why does the ability to turn off the alternator mitigate the risk of fire? Is it that the alternator can still generate power to some electronic component that may be shorting out and causing the cockpit fire or is it that the alternator itself can be the cause of the fire?

The alt/bat switch is designed, so that the battery can be turned on by itself. If the alt side is pushed, that battery side automatically goes on too. This is because an alternator requires a reservoir for excess current. You can have individual switches if you want, but the battery side MUST be on when the alternator is running.

As to turning the alternator off; there is the possibility of a runaway alternator that will cause over voltage. As to weather they can actually be turned off or not, varies with the different alternators that are available.

Personally, I prefer the red Cessna type split switch over individual switches.

L.Adamson --- RV6A

 

[az_gila]

The overvoltage situation...

......
As to turning the alternator off; there is the possibility of a runaway alternator that will cause over voltage. As to weather they can actually be turned off or not, varies with the different alternators that are available.

Personally, I prefer the red Cessna type split switch over individual switches.

L.Adamson --- RV6A

....is a bit of a red herring when talking about switches....

Alternators and/or voltage regulators certainly can fail in a "high voltage" mode, but by the time you know it - no matter how good your instrumentation is, and how fast your finger moves to the switch - it is too late, and the damage will have been done. All it takes is milliseconds to blow your electronics.

The best solution for this is a crowbar circuit that will electronically sense an overvoltage and switch off the alternator. This is usually built into higher end aircraft voltage regulators, or can be installed as a seperate module.

...so, what switches are needed if an electronic crowbar circuit is in the system?

 

[aerhed]

Al's pretty right, but most alternators are gonna saturate around 18V or something and most of todays glass stuff will handle that no problem. Worst thing on an alt. overvoltage is usually a cooked battery & an acid belly. Now for laughs, an old duster told me once that he shuts off the alt. on heavy takeoff to gain the extra 1/2 HP or whatever.

 

[noelf]

A good read on the pros / cons...

... of the split ALT / BATTERY switch is Bob Nuckolls book "The AeroElectric Connection".

Bob goes into the history of using the combo switch, the different alternator choices (auto vs aircraft), and what one "should" expect from a designed-in-features of our electrical system.

It is left up to the aircraft builder to chose what approach gets implemented, but I feel he gives a lot of information that make the (your) decision at least rational.

If you do not have his book, get it. Also, Bob has just updated the chapter 2 on Batteries and chapter 3 on Alternators and Generators. These are available for down load on aeroelectric.com

Also on his website are a number of stand alone articles that address a lot of items that we kitbuilders get faced with when designing our electrical systems and panels.

 

[RVbySDI]

Thanks guys for all the great replies.

I truly am not looking to start any "primer" debates about split switches. I am just trying to wrap my brain around why it is a necessity. I currently am flying a '66 Cessna 172 that does not have a split switch. All it has is a pull on/push off MASTER switch. I also have never driven a car no matter how old it was that had a switch to turn off the alternator. Now I know there are all kinds of differences between a car and a plane but in this instance is there that much difference in the electronics of a car and the electronics of a plane?

Inquiring minds want to know?

P.S. Reading Bob Knuckles' information is on my to do list.