Alternator field switch required ?

[Larry DeCamp]

Installing B&C system with regulator, OV function etc. The field is protected by a 5A circuit breaker and the output is protected by an ANL current limiter which I assume would protect the buss from an internal short in the alternator. So, my queustion is: Why would I need a FIELD switch ????

[Mich48041]

There needs to be some way of shutting off the alternator in case of smoke in cockpit. If the circuit breaker also functions as a switch, then a separate switch is not necessary.
Shutting off the master contactor will NOT shut off the alternator once it is running because the field will be powered by the alternator output, not by the battery. In fact, once the engine is running at high enough RPM, everything is powered by the alternator, not by the battery. The battery is still required after engine start to help stabilize the electrical system voltage.

[krw5927]

Originally Posted by Mich48041

There needs to be some way of shutting off the alternator in case of smoke in cockpit. If the circuit breaker also functions as a switch, then a separate switch is not necessary.
Shutting off the master contactor will NOT shut off the alternator once it is running because the field will be powered by the alternator output, not by the battery. In fact, once the engine is running at high enough RPM, everything is powered by the alternator, not by the battery. The battery is still required after engine start to help stabilize the electrical system voltage.

Mitch is spot on. I don't have an alternator field switch for my primary nor my backup B&C alternator. Both are protected instead by pullable breakers on the panel in case all electrons need to be manually shut off.

I have verified that turning off the master in flight, while disconnecting the electrical system from the battery, does not stop the alternator from producing power. Pulling the breaker stops it instantly.

[bret]

From my understanding, the ANL will blow at a particular % over rated current but will still carry high voltage to your components, this high voltage will fry some electronics.

[Bicyclops]

The ANL is like an extremely slow blow fuse. It is there to protect the B lead and the alternator from the battery. It won't protect anything from the alternator. Say you have a B&C 40 amp alternator and a 40A ANL. The alternator will happily put out much more than 40 amps for a while if the battery was run way down and the ANL won't blow. It will probably suffer twice its rating for quite a long while. If there is a dead short, it will blow immediately because the battery will try and shove a thousand amps through it. It also does not care what the voltage is. You need an overvoltage sensor and/or "crowbar" device to blow the field breaker if the alternator runs away. The crowbar acts in milliseconds and will probably save your avionics.

Ed Holyoke

Originally Posted by bret

From my understanding, the ANL will blow at a particular % over rated current but will still carry high voltage to your components, this high voltage will fry some electronics.

[Larry DeCamp]

My conclusion from all of this is that the CB works as well as a switch in a runaway scenario, and' IF the OV relay fails the Avionics will fry before you can react. Thanks much ....Larry

[FasGlas]

It has always been a common practice to start the engine before turning on the alternator, field switch. It wouldn't matter how you did this, a switch or a breaker.

[gasman]

37 years of master and alternator on during start with no problems.

I do shut off the alternator if I am sitting in the RV and playing with the panel.

[rv7charlie]

[edited out; bad info]

[az_gila]

Originally Posted by Bicyclops

The ANL is like an extremely slow blow fuse. It is there to protect the B lead and the alternator from the battery. It won't protect anything from the alternator. Say you have a B&C 40 amp alternator and a 40A ANL. The alternator will happily put out much more than 40 amps for a while if the battery was run way down and the ANL won't blow. It will probably suffer twice its rating for quite a long while. If there is a dead short, it will blow immediately because the battery will try and shove a thousand amps through it. It also does not care what the voltage is.

You need an overvoltage sensor and/or "crowbar" device to blow the field breaker if the alternator runs away. The crowbar acts in milliseconds and will probably save your avionics.

Ed Holyoke

If you have an external Voltage Regulator this crowbar function is often built into it. Just like this Plane Power one -

http://www.chiefaircraft.com/pp-r1224b.html