Description
36/121 Revision 05 • INSTALLATION AND OPERATING INSTRUCTIONS • 8DJH • 500-8067.9
Altitude correction factor For site altitudes above 1000 m, the altitude correction factor K
a
must be considered.
Curve m=1 for rated short-duration power-frequency withstand voltage and rated lightning
impulse withstand voltage according to IEC 62271-1 / VDE 0671-1.
Fig. 21: Correction factor K
a
as a function of the site altitude in m above sea level
Dielectric strength
Calculation example
Rated voltage (r.m.s. value) kV 7,2 12 15 17,5 24
Rated short-duration power-frequency withstand voltage (r.m.s. value)
– Across isolating distances kV 23 32 39 45 60
– Between phases and to earth kV 20 28 36 38 50
Rated lightning impulse withstand voltage (peak value)
– Across isolating distances kV 70 85 105 110 145
– Between phases and to earth kV 60 75 95 95 125
3000 m site altitude above sea level (K
a
= 1.28)
17.5 rated voltage of the grid
95 kV rated lightning impulse withstand voltage (see "Dielectric strength" table, rated voltage 17.5 kV)
Rated lightning impulse withstand voltage required =
95 kV * 1.28 = 122 kV
Result:
According to the above "Dielectric strength" table, switchgear for a rated voltage of 24 kV with a rated lightning
impulse withstand voltage of 125 kV is to be selected.
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