There are few interesting remarks applicable to electrical equipment operating at high altitude:
1- Dielectric strength rating (BIL, power frequency voltage, etc.) of electrical components non exposed in direct contact to air are not affected by the altitude. Example of this are transformer winding submersed in oil, vacuum or SF6 interrupter devices
2- The correction factor for dielectric strength (CFd), continuous current(CFc) and temperature (CFt)are affected in different way by the altitude (H) 1000 m above sea level as specified per ANSI/IEEE Std C37.40 (similar to IEC Std 282-1.2.)
A close linear analytical approximation is proposed as follow:
a- Dielectric: CFd = 1.077 – 2.646x10-5 (H) (1% per 100 meters
b- Cont. Current: CFc = 1.018 –5.994x10-6 (H)
c- Temperature: CFt = 1.04 –1.199x10-5 (H)
3- Correction factor start to at different altitude accordance with ANSI/IEEE Std for different equipment as follow:
a- Metal Enclosed DC LV.bkr, Pwr circ. bkr, flex insulated bus, LV AC pwr.,........ >2000 m
b- Control switchboards .................... >1500 m
c- Relays and relay systems ........................ >1500 m
d- Surge Arrester.............................. >3000 m
4- Maximum altitude rating is as follow:
1- Transformers (oil, cast or resin), Instrument transf., regulator, shunt reactors <4500 m
2- HV power circ. bkr, low V pwr circ. bkr, Metal enclosed rigid bus <3900 m
3- HV air switches, insulators and bus supports: <6000 m
4- HV fuses, insulators and bus supports: <6100 m
5- Metal Enclosed DC LV bkr, Pwr bkr, flex insulated bus, LV AC pwr., <3900 m
6- Control switchboards,( C37.14-79.) <3550 m
7- Reclosers, sectinalizers load interrupting sw. & pad mtd swgr for AC <4900 m
8- Relays and relay systems <6000m
5- Creepage distance derating is different than the dielectric correction factor. Several investigators suggest start apply correction factor few meters above sea level.