The switchgear complies with the relevant standards and specifications applicable at the time of type tests.
Dielectric strength – The dielectric strength is verified by testing the switchgear with rated values of short duration power- frequency withstand voltage and lightning impulse withstand voltage according to IEC 62271-1/VDE 0671-1 (see table“Dielectric strength”).
The rated values are referred to sea level and to normal atmospheric conditions (1013 hPa, 20 °C, 11 g/m3 humidity in accordance with IEC 60071/ VDE 0111).
The dielectric strength decreases with increasing altitude. For site altitudes above 1000 m (above sea level) the standards do not provide any guidelines for the insulation rating. Instead, special arrangements apply to these altitudes.
Site altitude – As the altitude increases, the dielectric strength in air decreases due to the decreasing air density. This reduction is permitted up to a site altitude of 1000 m according to IEC and VDE.
– For site altitudes above 1000 m, a higher insulation level must be selected. It results from the multiplication of the rated insulation level for 0 to 1000 m with the correction factor Ka.
Example: 1800msite altitude above sea level 12 kV switchgear rated voltage 75 kV rated lightning impulse withstand voltage Rated lightning impulse withstand voltage to be selected 75 kV · 1.10 = 82.5 kV