During the ageing of in service medium voltage equipments, electrical losses are mainly due to partial discharges. These partial discharges can lead to the electrical failure of industrial equipments. Partial discharges include (i) bulk discharges occurring in insulating material voids, (ii) corona discharges affecting or not the insulator and (iii) surface discharges developing on an insulator surface from a triple point (conductor/insulating material/air). Bulk and surface discharges can respectively lead to a final volume or surface breakdown whereas corona discharges may only induce in a long term chemical surface ageing. Surface discharges and corona discharges produce chemical species in atmospheric air, i.e. ozone and nitrogen oxides. These products can be identified and quantified by a periodic sampling of the ambient air in the medium voltage equipment enclosure during accelerated ageing cycles. In previous studies, it was demonstrated that the nature and relative distribution of concentration of these stable gaseous products was closely correlated to the discharge regime (glow, streamer, spark regimes). Measurements have been carried out on a distribution cell and transformer using low cost electrochemical and semi conductive sensors; acoustic measurements were simultaneously performed. It is shown that gas discharges can be detected within a few minutes from their onset. The discharge regime can also be characterized thanks to the specific detection of nitrogen oxides while low regime corona discharges are simultaneously occurring. The change in the surface discharge regime is evidence of accelerated insulator surface ageing (risks of surface breakdown) ; equipment enclosure ambient air sampling can then be an accurate method for determination of the state of health of this type of equipment.