Gas turbine speed monitoring

I noticed there are two separate speed probes on our GE PG25 Gas Turbine - one for gas generator rotor speed, and one for power turbine rotor speed. Both probes given high velocity trip; gas generator rotor speed trip is set higher than power turbine rotor speed.

Why do we need 2 velocity measurements with 2 different trip set points?


Manuals available online for the PG25s state they have a “free power turbine”, implying a separate shaft downstream of the gas generator (compressor and turbine) which would operate on their own shaft.

Above is a snippet.