Sensor for detecting abnormal movement of a gas turbine shaft

A sensor element for use in a backing sensor to monitor a gas turbine shaft. The sensor includes an oscillator circuit (30) having an oscillator and a resonance circuit (42), and a frequency detector (31) for monitoring the frequency of the ossilation circuit. Abnormal movement of the shaft cuts the circuit in the sensor along a cutting line (15). The cutting of the sensor changes the capacitance and/or inductance of the resonance circuit and thereby changes the frequency of oscillation.