Dielectric Properties clause is split into 2 sub-clauses:
- 10.9.2 Power-Frequency Withstand Voltage
- 10.9.3 Impulse Withstand Voltage
10.9.2 Power Frequency Withstand Voltage
Power Frequency Withstand Voltage is the rms value of sinusoidal power frequency voltage that the equipment can withstand during tests made under specified conditions and for a specified time.
The test voltage is determined by the rated insulation voltage (Ui) of the assembly as detailed in Table 8 (main circuits) and Table 9 (auxiliary and control circuits) of IEC 61439-1. For example, a Ui of 800V will have a test voltage of 2000V rms.
There is an additional test on operating handles made of insulating material to which a 1.5 test voltage is applied, Clause 10.9.5.
10.9.3 Impulse Withstand Voltage
The 1.2/50µs (rise/decay) impulse withstand voltage is applied to the assembly five times for each polarity onto the main circuits as defined in Clause 10.9.3.2.
The test voltage is determined by the rated impulse withstand voltage (Uimp) of the assembly as detailed in Table 10 of IEC 61439-1. For example, a Uimp of 8kV will have a test voltage of 9.8kV. As kA Testing Facility is located at sea level, the stated sea level voltages are applied during testing.
There is an additional test on withdrawable units across open contacts. The test voltage will depend on the Uimp of the assembly and can be found in Table 102 of IEC 61439-2.
Equipment must be selected to match or be higher than the tested values to prevent damage to the assembly when testing.
For the Ui rating, material group IIIa is applied unless there is documentation of a higher material group or CTI ratings of the insulators.
Where control devices and instruments are fitted i.e. auxiliary circuits, these should to be electrically disconnected from the mains and not subject to testing. If they are connected to the main circuit, they shall be able to withstand the impulse and dielectric tests to be performed on the main circuit.
Auxiliary circuits shall either be:
• Not capable of connecting to the main circuit, in which case the Ui and Uimp capability of the auxiliary circuit(s) shall be supplied
• Connected to the main circuit and therefore able to withstand main circuit Ui and Uimp