Abstract: A sensor detecting incipient fault in oil-filled electrical apparatuses (transformer) includes a sensor head (1) directly mounted on an air vent port of a Buchholz relay (3) and a display/control unit (2) operably connected to the sensor head (1). The sensor head (1) senses hydrogen gas and transmits to the display/control unit (2) an electrical signal equivalent to the concentration of the free hydrogen accumulated in the Buchholz relay (3). The display/control unit (2) compares the value of the electrical signal equivalent with a stored value and outputs a signal when the received signal value exceeds a preset value. The sensor head (1) comprises a membrane (10,11) formed of a first (ceramic) material (10) to isolate free gases from the liquid phase to the gaseous phase, and a second (polymeric) material (11) to isolate hydrogen from the mixed gaseous phase.