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What is the effect of "Altitude" on equipment insulation level? What is the "Altitude correction factor"?

In general, the equipment rated insulation level is being specified at normal service conditions, among which the normal altitude is 1000m or below.
For installations at an altitude higher than 1000 m, 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 figure 1 of IEC 62271-1 standard (see figure attached).
Important remarks:
  1. The attached graph is valid only for external insulation (insulation distances are in atmospheric air).
  2. For internal insulation (e.g. in SF6), the dielectric characteristics shall be identical at any altitude with no special precautions; however, the manufacturer consultation might be necessary according to product documentation. 
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