The electric safety distance of a switch cabinet with air as insulation medium is obviously affected by altitude

electrode shape

and so on. But there is no relative standard for the selection of air gap of switch cabinet at low pressures.To study the lightning impulse discharge performance of switchgear air gaps under high altitude condition

4 types of air gap existing in 40.5 kV switchgear and typical rod-plane air gap were chosen as research subjects

and the effects of electrode shape

air pressure p

humidity h and air gap distance d on discharge characteristic were investigated. Tests results show that the discharge characteristic difference between rod-plane air gap and switchgear air gaps is noticeable. The lightning discharge voltage U50 can be expressed as U50=U50

0（p/p0+ch）n

in which U50

0 is discharge voltage at the sea level altitude p0. The variation of humidity will cause dispersion of U50 and the relationship between U50 and d（<400 mm）is non-linear. The research results can provide guidance for the design

selection and installation of switchgear in high altitude areas.