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Note that references are to IEC 60950 Third Edition 1999-04. Where the references in the Second Edition differ, they appear in parentheses.

Insulating materials are testing using either a 50 Hz or 60 Hz AC voltage or a DC voltage equivalent to the peak voltage of the AC test voltages given in Table 5B (Table 18) of the standard. The required test voltage depends on the type of insulation (e.g. basic, supplementary, reinforced or operational) and the working voltage across the tested insulation.

You can find a full discussion of the need for the various types of insulation in the circuit separation section.

Examples of test voltages  are:

Basic Insulation - (e.g. between mains supply leads connected together and the earthed body) - 1000 Vac or 1414 Vdc for 120 Vac working voltage; 1500 Vac or 2121 Vdc for 230 Vac working voltage.

Double Insulation (e.g. between primary circuitry and SELV circuitry ) - 2000 Vac or 2828 Vdc for 120 Vac working voltage; 3000 Vac or 4243 Vdc for 230 Vac working voltage.

The voltage is raised gradually till the test voltage is reached and is held at that value for 60 seconds.

Note that the test voltage used depends on the actual working voltage to which the insulation is subjected. As an example, take a 230V ac input switching power supply, where the actual voltage developed between primary and secondary circuitry within the unit is 700 V peak. In this case, the test voltage for basic insulation would be 2034 Vdc or 2877 Vdc; and for double insulation 3000 Vac or 4243 Vdc.

There shall be no insulation breakdown during the test. Insulation breakdown is considered to have occurred when the current which flows as a result of the application of the test voltage rapidly increases in an uncontrolled manner, i.e. insulation does not restrict the flow of the current. Corona discharge or a single momentary flashover is not regarded as insulation breakdown.

This test is carried out after humidity treatment to ensure that the insulation is not hygroscopic. It is also carried out after abnormal tests , impact tests and heating tests to ensure the integrity of the insulation after these tests.