- Three-impulse withstand test in which one disruptive discharge on the self-restoring
insulation is tolerated. If this occurs, nine additional impulses are applied during which
no disruptive discharge is tolerated.
- The up-and-down withstand test with seven impulses per level in which disruptive
discharges on self-restoring insulation are tolerated.
- The up-and-down test with one impulse per level, which is recommended only if the
conventional deviation, z, defined in IEC 60-1 is known. The values suggested there,
z = 6 Yo for switching and z = 3 Oh for lightning impulses, shall be used if, and only if, it
is known that ZI 6 % and z 5 3 OX, respectively. Otherwise other methods shall be used.
In all the test procedures described above no disruptive discharge is tolerated on the
non-self-restoring insulation.
No statistical meaning can be given to the three-impulse withstand test in which no
disruptive discharge is tolerated (P, is assumed to be 100 O/.). Its use is limited to cases in
which the non-self-restoring insulation may be damaged by a large number of voltage
applications.
When selecting a test for equipment in which non-self-restoring insulation is in parallel
with self-restoring insulation, serious consideration should be given to the fact that in
some test procedures voltages higher than the rated withstand voltage may be applied and
many disruptive discharges may occur.
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.
NOTE - A typical example is the use of a single voltage source for the tests of longitudinal insulation,
while insulating the base, instead of a combined voltage test. In this case, the demonstration mentioned
above concerning the disruptive discharge development is a very stringent condition for the acceptance of
the alternative.
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.