A sensor for detecting abnormal movement of a gas turbine shaft, comprising: a frangible or severable sensor element having a first end proximal to a sensor connector or output and a second end distal from the sensor connector or output, the sensor element having a cutting plane towards the distal end of the sensor element and being for cutting by a cutter or separator coupled to or moveable with the gas turbine shaft, characterised in that the sensor comprises: 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 cuts a conductor to form a second tuned or resonance circuit connected to the amplifier and having a second inductance and/or a second capacitance, the second inductuance 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 second frequency, the second frequency being different from the first frequency. a frangible or severable sensor element having a first end proximal to a sensor connector or output and a second end distal from the sensor connector or output, the sensor element having a cutting plane towards the distal end of the sensor element and being for cutting by a cutter or separator coupled to or moveable with the gas turbine shaft, characterised in that the sensor comprises: 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 cuts a conductor to form a second tuned or resonance circuit connected to the amplifier and having a second inductance and/or a second capacitance, the second inductuance 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 second frequency, the second frequency being different from the first frequency. 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 cuts a conductor to form a second tuned or resonance circuit connected to the amplifier and having a second inductance and/or a second capacitance, the second inductuance 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 second frequency, the second frequency being different from the first frequency.