The invention relates to an improved online incipient fault sensor device for detection of incipient fault in oil-filled electrical apparatuses such as a transformer, the device comprising a sensor head (1) directly mounted on an air vent port of a Buchholz relay (3); and a display cum control unit (2) operably connected to the sensor head (1) via a screened cable (12), the sensor head (1) sensing the hydrogen gas and transmitting to the display cum control unit (2) an electrical signal equivalent to the concentration of the free hydrogen accumulated in the buchholz relay (3), the display cum control unit (2) comparing the signal value with a stored preset value, and sends a first visual alarm when the received signal value exceeds a preset value, an audio alarm being further generated by the unit (2) in case the fault continues and/or the operator failed to notice the first visual alarm. The sensor head (1) comprises a membrane (10, 11) formed of a combination of two types of materials, the first type being a ceramic material (10) to isolate free gases from the liquid phase to the gaseous phase, and the second type being a polymeric material (11) to isolate hydrogen from the mixed gaseous phase.
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