Annex D
(informative)
Altitude correction for clearances

Clearances in air are a function of the atmospheric pressure according to Paschen's Law.
Clearance distances provided in Table 9 are valid up to 2000 m above sea level. Clearances
above 2000 m must be multiplied by the factor provided in Table D.1.

Table D.1 – Correction factor for clearances at altitudes between 2 000 m and 20 000 m

(see 4.3.6.4.1)

Impulse tests performed below 2000 m altitude for the purpose of verifying air clearances must
use test voltages which have been corrected for air pressure (altitude). Test voltages which
have been corrected for three altitudes are provided in Table D.2. Altitude correction of test
voltage is not required for impulse testing of solid insulation. The voltage values of Table D.2
apply for the verification of clearances only.

Table D.2 – Test voltages for verifying clearances at different altitudes