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This study presents phase-resolved observations of partial discharge (PD) formation in air in non-uniform electric field at various pressure conditions. The needle-plane equivalent electrode system refers both to external components of high-voltage insulating systems and to micro-sharpening in the internal structure of those systems. The ionisation zone, accumulation of space charge and formation of corresponding current pulses depend on electrode configuration, voltage level, pressure, temperature and humidity of air. The influence of pressure on the discharge mechanism in a non-homogenous electric field has been assessed on the basis of PD patterns at different voltage levels, electrode distance, curvature of high-voltage electrode and taking into account solid dielectric barrier in various configurations. The simultaneous occurrence of two discharge forms has been registered and separated on the phase-resolved patterns. Both corona forms, which are overlapped at normal pressure, are revealed at higher pressure and at double PD inception voltage. The obtained relationships could be used in diagnostic applications based on gaseous insulation at higher pressure. [ABSTRACT FROM AUTHOR]