I have always put a relay (contractor) on the alternator output when working with internally regulated alternators. At best, they have failure modes that can cause uncontrolled output, and some self excite and intentionally output without an input on the control line.

My alternator switch then controls both the control line (maybe field? )of the alternator and the output. Further I put a OV module on the power for the relay coil which will crowbar the power via the Alternator Control CB and ensure that the alternator is offline faster than I could even think of reacting.

Contactors are mechanical, and in electrical terms very slow, maybe to slow to prevent some damage. But, practically the best that I can do, given the cost of other options.

After reading the responses as well as a similar thread from 2012, I concluded that I have a self-exciting automotive style alternator that cannot be shut down with the master switch in flight, and that others are set up this way too and are either OK with it, or unaware of it. As the frightening posts by Allan and Joel show, it is unsafe if you can?t shut off your main bus in flight. The master switch should work just like it works on a certified airplane. I have ordered an alternator kit from Vans for $495 that I expect to fix my issue. PS: I wanted to post a photo of my alternator connector but couldn?t figure out how. My pics are on my iPad and in Google Photos, but where do I get a URL? I am new to online forums

Also I didn?t understand Allan?s explanation of how disconnecting the battery via the master switch caused his alternator output to skyrocket. Why didn?t the regulator just try to maintain the proper voltage on the main bus? I had no such problem as I was discovering that my master switch wasn?t shutting off the bus

,...The regulator that is in this particular alternator senses battery via the battery terminal on the alternator. Thus taking away the battery voltage signal to the terminal makes the alternator think the battery is dead so it runs wild.
Also I didn't build this aircraft, so many of the things I would have implemented were not in place. Thanks, Allan

stop the prop
 

Hi Allan - I would have never thought to stop the prop in this scenario, makes perfect sense now that I hear it.

Runaway alternators do seem to be a failure mode that comes up repeatedly with various impact - mainly smoke and fried electronics. I really like that I feel like I have another tool in the toolbox to deal with that unlikely scenario.

In my post I wanted only to familiarize you with the type you have. Glad you are changing over. I do believe that many RVs that have this type are totally unaware because unless a person turns off the master and the panel stays lit, how would you know? PP or B&C seem to be a better solution IMHO

Depending on how the electrical system is setup, that can still happen and everything is fine - my airplane is an example. I can isolate the battery and still keep everything powered from either of my two alternators, but I can also have a hard shutdown of each alternator if I need to. In the case of this particular fried airplane, it was the ability to do a hard-shutdown without stopping the engine that was lacking.

Glad it ended safely, albeit costly. I would add that luck was a small part. Your quick thinking to kill the engine and restart to disable the alternator was the main contributing factor IMO. A lesson to be learned for all of us. Keep thinking and keep trying.

Lary

I have seen several cases, in the auto world, where a system with a disconnected battery hums along at 14 volts. This is not that uncommon. A somewhat less common cause of battery failure is breakage of the plates, which creates symptoms similar to opening your master contactor. I would speculate that the current surge from the shorted B lead caused some damage to the VR, causing the voltage runaway. Just speculation.

Larry

Problem solved
 

Just replaced my car alternator with a Plane-Power unit from Vans for about $500 total, and now I can shut it off with the master switch, giving me piece of mind, plus I saved almost two lb with lighter mounting brackets and lighter alternator, making the trouble and expense less painful. Anyone want a used, working automotive type unit cheap?