5.4 Alternative test situation
When it is too expensive or too difficult or even impossible, to perform the withstand tests
in standard test situations, the apparatus committees, or IEC technical committee 42, shall
specify the best solution to prove the relevant standard withstand voltages. One possibility
is to perform the test in an alternative test situation.
An alternative test situation consists of one or more different test conditions (test
arrangements, values or types of test voltages, etc.). It is necessary, therefore, to
demonstrate that the physical conditions for the disruptive discharge development,
relevant to the standard situation, are not changed.
5.5 Phase-to-phase and longitudinal insulation standard withstand voltage tests
for equipment in range I
a) Power-frequency tests
For some equipment with 123 kV 5 U, 2 245 kv, the phase-to-phase (or longitudinal)
insulation may require a power-frequency withstand voltage higher than the phase-toearth
power-frequency withstand voltage as shown in table 2. In such cases the test
shall preferably be performed with two voltage sources. One terminal shall be energized
with the phase-to-earth power-frequency withstand voltage and the other with the
difference between the phase-to-phase (or longitudinal) and the phase-to-earth
power-frequency withstand voltages. The earth terminal shall be earthed.