split alt/bat switch question

[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]

Originally Posted by RVbySDI

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]

Originally Posted by Mike S

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]

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]

Originally Posted by RVbySDI

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]

Originally Posted by Mike S

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]

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]

Originally Posted by RVbySDI

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]

Originally Posted by L.Adamson

......
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?