During wet tests, the rain shall be applied simultaneously on all air and surface insulation under voltage.
If the atmospheric conditions in the test laboratory differ from standard conditions, the test
voltages shall be corrected according to IEC 60-1.
All impulse withstand voltages shall be verified for both polarities, unless the relevant
apparatus committees specify one polarity only.
When it has been demonstrated that one condition (dry or wet) or one polarity or a
combination of these produces the lowest withstand voltage, then it is sufficient to verify
the withstand voltage for this particular condition.
The insulation failures that occur during the test are the basis for the acceptance or
rejection of the test specimen. The relevant apparatus committees or IEC technical
committee 42 shall define the occurrence of a failure and the method to detect it.
When the standard withstand voltage of phase-to-phase (or longitudinal) insulation is
equal to that of phase-to-earth insulation, it is recommended that phase-to-phase (or
longitudinal) insulation tests and phase-to-earth tests be performed together by
connecting one of the two phase terminals to earth.
5.2 Standard short-duration power-frequency withstand voltage tests
A standard short-duration power-frequency withstand voltage test consists of one
application of the relevan! standard withstand voltage to the terminals of the insulation
configuration.
Unless otherwise specified by the relevant apparatus committees, the insulation is
considered to have passed the test if no disruptive discharge occurs. However, if one
disruptive discharge occurs on the self-restoring insulation during a wet test, the test may
be repeated once and the equipment is considered to have passed the test if no further
disruptive discharge occurs.
When the test cannot be performed (such as for transformers with non-uniform insulation),
the relevant apparatus committees may specify frequencies up to a few hundred hertz and
durations shorter than one minute. Unless otherwise justified, the test voltages shall be
the same.
5.3 Standard impulse withstand voltage tests
A standard impulse withstand voltage test consists of a specified number of applications of
the relevant standard withstand voltage to the terminals of the insulation configuration.
Different test procedures may be selected to demonstrate that the withstand voltages are
met with a degree of confidence that experience has shown to be acceptable.
The test procedure shall be selected by the apparatus committees from the following test
procedures which are standardized and fully described in IEC 60-1 :
- Three-impulse withstand test in which no disruptive discharge is tolerated.
- Fifteen-impulse withstand test in which up to two disruptive discharges on the
self-restoring insulation are tolerated.