4.7 List of standard impulse withstand voltages
The following peak values, expressed in kv, are standardized:
4.8 Ranges for highest voltage for equipment
The standard highest voltages for equipment are divided in to two ranges:
Range I: Above 1 kV to 245 kV included. This range covers both transmission and
distribution systems. The different operational aspects, therefore, shall be
taken into account in the selection of the rated insulation level of the
equipment.
Range II: Above 245 kv. This range covers mainly transmission systems.
4.9 Selection of rhe standard insulation levels
The association of standard withstand voltages with the highest voltage for equipment has
been standardized to benefit from the experience gained from the operation of systems
designed according to IEC standards and to enhance standardization.
The standard withstand voltages are associated with the highest voltage for equipment
according to table 2 for range I and table 3 for range I I . The associations obtained by
connecting standard withstand voltages of all columns without crossing horizontal marked
lines are defined as standard insulation levels.
NOTES
1 In some countries other values of U, and of rated withstand voltages are still in use for range I.
Table A.l of annex A reports these values as well as the relevant associations. which, however, do not
constitute standard insulation levels.
2 If. for the switching impulse withstand voltage test, the apparatus committees specify a positive
component lower than the negative. the required withstand voltage of the external insulation is not proven
unless a suitable test conversion factor is introduced.
Furthermore, the following associations are standardized for phase-to-phase and
longitudinal insulation:
- For phase-to-phase insulation, range I, the standard short-duration power-frequency
and lightning impulse phase-to-phase withstand voltages are equal to the relevant
phase-to-earth withstand voltages (table 2). The values in brackets, however, may be
insufficiento prove thathe required withstand voltages are met and additional
phase-to-phase withstand tests may be needed.
- For phase-to-phase insulation, range II, the standard lightning impulse withstand
voltage phase-to-phase is equal to the lightning impulse phase-to-earth.