IEEE C57.13 Clause 4.2 states that the dielectric strength of air decreases as altitude decreases. The dielectric strength on air should be multiplied by the altitude correction factor provided in Table 1.
Dieletric strength is the property of a medium, in this case air, to exhibit negligible or no electrical conductivity thus acting as an insulator.
|Altitude meters||Altitude correction factor for dielectric strength|
|NOTE - An altitude of 4500 m is considered a maximum for instrument transformers conforming to this standard (IEEE C57.13).|
Table 1 indicates that at an altitude of 1000 m or below, the dielectric strength correction factor is 1.0. This means that even the tallest building in the world, the Burj Khalifa which is 828 m tall, there will still be no reduction in the dielectric strength of air. It will be different however, if you install an electrical equipment on highlands such as Leadville, Colorado with an elevation of 10,430 feet (3179 m) where the multiplying factor will be 0.75.
Reduced dielectric strength of air means that its insulating property is reduced requiring increased clearances between phases and/or ground (earth) leading to larger electrical equipment size. Larger equipment physical size will comparatively increase its cost.