A seris of experimentel tests bad been carried out under the conditions at which the temperature and humidity were kept constant a-nd the pressure was changed.The results of tests show that the exponent m specified by IEC publication 60-1 for the test-objecas under alternating voltage and lightning impulse at flashover distance d≤1 m does not conform with the actual discharge regularities for the most of testbjects.So we suggest the correction formula with which the flashover voltage of external insulation of H.V.electrical apparatus at flashover distance d≤1m can be converted into the equivalent values under the standard stmospheric pressure