The acceptance criteria is a withstanding one meaning no breakdown. For example these are excerpts from a draft version of India’s equivalent standard for your insight only and they follow a freedom of information policy.
A-10 DRY LIGHTNING IMPULSE WITHSTAND VOLTAGE TEST: The insulator shall be tested dry under the conditions described in Appendix “C”. The impulse generator shall be adjusted to a standard 1.2/50 impulse at the specified impulse withstand voltage adjusted for the atmospheric conditions at the time of test. Fifteen such voltage waves shall be applied to the insulator. If there is no flashover or puncture, the insulator shall be deemed to have passed the test. However, two flashovers on the external insulation are permissible. The insulator shall pass the impulse withstand voltage test with voltages of both positive and negative polarity. The insulators shall not be damaged by these tests, but slight marks on the surface of the metal fitting shall be permissible. The original oscillograms recorded during the test shall form part of the type test report.
A-11 WET POWER FREQUENCY WITHSTAND VOLTAGE TEST AND WET POWER FREQUENCY MAXIMUM VOLTAGE TEST: The insulator shall be arranged as described in Appendix “C” and exposed for at least 1 minute before application of voltage and throughout the test the artificial rain produced in accordance with Clause 3.3 of IS:2071 (Part- I). The test voltage to be applied to the insulator shall be the specified wet power conditions at the time of test. A voltage of about 75% of the test voltage so determined, shall be applied and then increased gradually to reach the test voltage, it shall be maintained at this value for 1 minute. The insulator shall not flashover or punctures while the test voltage is applied.
The test shall then be repeated at successively higher voltage in step of 2.5 kV or 5 kV until the insulator is unable to withstand the applied voltage for a period of 1 minute. The voltage, corrected as described in Appendix “B”, of the test immediately preceding the last test shall be taken as the maximum wet power frequency withstand voltage.