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The behaviour of pressboard plates of 1 mm and 2 mm thicknesses immersed in mineral oil and stressed in the highly inhomogeneous electric field of a rod-plane gap under alternating voltages of the industrial frequency of 50 Hz is described. First, the partial discharges inception and breakdown voltages of the plates, measured under continuously increasing applied voltages, are given. Then the characteristic phases of the discharge development under applied voltages of each time-constant effective value are described in detail and conclusions are drawn as to the position in the test object, and the kind of partial discharges which appear with time. Also, the measured values of the discharge quantities are given. Finally, the measured relationships between life duration of the insulating plates and the relevant applied voltages of constant value are presented and some conclusions are drawn on the overall behaviour of the plates.