System and method for detecting abnormal movement in a gas turbine shaft

A sensor for detecting abnormal movement of a gas turbine shaft (4), comprising:

• a frangible or severable sensor element (11) having a first end proximal to a sensor connector or output (16) and a second end distal from the sensor connector or output (16), the sensor element (11) having a cutting plane (15) towards the distal end of the sensor element and being for cutting by a cutter or separator (8) coupled to or moveable with the gas turbine shaft (4),
• the sensor including a first oscillator circuit comprising an electronic amplifier and a first tuned resonance circuit including a first inductance and a first capacitance, the first oscillator circuit being oscillatable at a first frequency determined by the first inductance and the first capacitance,
• and wherein cutting of the frangible sensor element along the cutting plane (15) cuts a conductor (33) to form a second tuned or resonance circuit connected to the amplifier and having a second inductance and/or a second capacitance, the second inductance and/or the second capacitance being different to the first capacitance and/or first inductance such that the electronic amplifier and second tuned or resonance circuit form a second oscillator circuit oscillatable at a different frequency to the first oscillator circuit.