Thanks for the comments folks.  I'm not entirely sure but I dont think this type of switch will work for me.  The system I am wanting to switch is a 240v grid tie system.  The think is the voltage at the controller never drops below 240v dc when the inverter is connected to the grid, even if the turbine is stopped.  I dont know why this is but I'm guessing if the voltage stays the same then maybe the frequency does too.  Please tell me if I'm wrong.

I was thinking of something that could read rpm the way an annometer works with a magnetic reed switch.  I found a device yesterday from Inspeed called a wind switch.  I'm guessing it could be used to read rpm.  Its a tad expensive though.  Would anyone out there know how to make something similer with an adjustable range, but with a lot less $ signs.

Thanks for all your help