Discharge characteristics of external insulation of power system are significantly influenced by atmospheric humidity. However

the existing researches seldom concern high humidity condition. In this study

air gap discharge tests were carried out in artificial climate chamber to investigate the relation between humidity（especially high humidity） and discharge voltage. The results show that:1)the discharge voltage increases with increasing humidity when relative humidity is less than 80%; 2)when relative humidity is above 80% the discharge voltage decreases slightly with the increase of humidity in the condition without dew

while it reduces to half of normal value in the condition with dew. The electric field in the air gap was calculated by the software ANSYS

and the result indicates that dew seriously distorts the electric field

making the maximum field strength doubled and resulting in a severe reduction in discharge voltage

which is consistent with the conclusions drawn from discharge tests in the artificial climate chamber.