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What is the altitude correction factor for insulation level for metal enclosed medium voltage switchgear? What must be considered for equipment selection?

For installations at an altitude higher than 1,000 am, the insulation withstand level of external insulation at the service location shall be determined by multiplying the rated insulation levels
by a factor "Ka" in accordance with below formula:


Also can be illustrated by the following graph:

For purpose of equipment selection, the "corrected" insulation level in terms of "power frequency withstand" and "lightning impulse withstand" voltages must be equal or higher than the desired rated values at the given installation altitude.

Additional tip:
In some cases as altitude is relatively high to a limit that will change the selected equipment (especially if Air-Insulated technology is being used), you may also consider gas-insulated or solid-insulated switchgear systems to minimize the foot print/size utilized by the equipment on-site; this is due to the fact that these technologies are normally offering higher immunity to environmental conditions even when it comes to high altitudes.
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