Hi Bill; hope you don't mind me quoting you to expand on that.
I hope this pic link works; I tried to copy it from a post by Ross F in another thread.
http://www.vansairforce.com/community/showpost.php?p=1215946&postcount=55
The line labeled 'S' is the control line to the regulator (what almost everyone here is calling the 'field terminal').
Look down & right, at the coil symbol at the bottom of the image (below the star-shaped 3-coil system). This is the field winding in the alternator. Note that the wire from the left end of the coil is tied directly to 3 diodes, fed by that 3-coil network. This is DC going to the B-lead (output) of the alternator. Note also that this B-lead voltage is fed directly into the 'IC regulator'.
Now follow the wire from the other end of the coil, into the blank box labeled 'IC regulator'. In order for the field winding to generate a magnetic field (which, along with rotation, is what makes the alternator make power), that DC path must be completed to ground. It's not shown in the drawing, but inside that blank box there's a transistor that is a 'gate keeper', and controls the current flowing through the field winding and out of the blank box, on the wire just below the coil, to the ground symbol in the drawing.
Here's the point of all this:
If that (undrawn) 'gate keeper' transistor shorts out, ask yourself what happens to the field current. Then ask yourself how you shut down the alternator, with that transistor shorted.
Now, there may be some internally regulated alternators on the market that aren't wired this way. But I hope it's obvious that with this particular design, if the regulator fails, there is no certainty that it can be shut down by removing power from the 'field' terminal. It should also be obvious that being able to shut it down using the 'field' terminal *while it's working properly* is no guarantee that you'll be able to do it after a regulator failure (which is what causes an overvoltage event).