The paper presents the results of ageing experiments under combined electrical, mechanical (vibration) and thermal stresses on epoxy bonded mica insulation. Actual 11 KV machine coils designed for use in 1 MW rotating machines are used as the specimens. Both diagnostic and destructive tests were carried out to assess the amount of ageing. It was observed that the incremental loss tangent and partial discharge energy per cycle increase monotonically with time of ageing and that the dielectric strength fell monotonically. Thus indicating a high degree of correlation between these properties.