For installations at an altitude higher than 1000 m from sea level, the required insulation withstand level of external insulation at the desired height must be calculated.
The basis of rating for switchgear is the standard reference known as sea level conditions.
For internal insulation, the dielectric characteristics are identical at any altitude and no special precautions need to be taken.
For external and internal insulation, refer to IEC 60071-2.
ka calculated in two standards as below:
- IEC 60071-2: Ka=e^[m × (H/8150)]
- IEC 62271-1:2011 : Ka=e^[m× ((H-1000)/8150 )]
m is taken as a fixed value in each case for simplification as follows:
•m= 1 for power-frequency, lightning impulse and phase-to-phase switching impulse voltages;
•m= 0.9 for longitudinal switching impulse voltage;
•m= 0.75 for phase-to-earth switching impulse voltage.
Please see an example in IEC 62271-1:2011 standard
Further the following graph if H=2000m and m=1:
Ka=e^[(2000- 1000 )/8150]=e^[1000/8150]=1.13
This factor must apply in the calculations determine the amount of switchgear BIL.