The association of standard rated 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 rated withstand voltages are associated with the highest voltage for equipment according to Table 2 for range I and Table 3 for range II. These standard rated withstand voltages are valid for the normal environmental conditions and are adjusted to the standard reference atmospheric conditions.
The associations obtained by connecting standard rated withstand voltages of all columns without crossing horizontal marked lines are defined as standard insulation levels.
Furthermore, the following associations are standardized for phase-to-phase and longitudinal insulation: – For phase-to-phase insulation, range I, the standard rated short-duration power-frequency and lightning impulse phase-to-phase withstand voltages are equal to the relevant phaseto-earth withstand voltages (Table 2). The values in brackets, however, may be insufficient to prove that the required withstand voltages are met and additional phase-to-phase withstand voltage tests may be needed.
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