The phase-to-phase (or longitudinal) insulation may require a lightning impulse withstand
voltage higher than the standard phase-to-earth withstand voltage as shown in Table 2. In
such cases, the relevant tests shall be performed immediately after the phase-to-earth
insulation tests increasing the voltage without changing the test arrangement. In evaluating
the test results, the impulses leading to disruptive discharge to earth are considered as nonevents.
When the number of discharges to earth does not allow the test to be performed, a combined
test shall be adopted with an impulse component equal to the phase-to-earth lightning
impulse withstand voltage and a power-frequency component with the peak value of opposite
polarity equal to the difference between the phase-to-phase (or longitudinal) and the phaseto-
earth lightning impulse withstand voltages. Alternatively, for external insulation, the
relevant apparatus committees may specify that the phase-to-earth insulation be increased