IEC 60243-2-2013 pdf Electric strength of insulating materials – Test methods – Part 2: Additional requirements for tests using direct voltage

8 Electrical apparatus 8.1 Voltage source The test voltage applied to the electrodes shall be provided by a power supply having the following characteristics and components. A choice of voltage of either positive or negative polarity shall be provided, one of the connections to the electrodes being earthed. The voltage ripple on the test voltage shall not exceed 2 % of the voltage at all values greater than 50 % of the breakdown voltage. The test voltage shall also be free from transients or other fluctuations exceeding 1 % of the applied voltage. When testing test specimens of low capacitance, it may be necessary to add a suitable capacitor (for example, 1 000 pF) in parallel with the electrodes in order to reduce the influence of transients in initiating premature breakdowns. The controls on voltage shall be capable of varying the test voltage smoothly and uniformly between zero and the maximum test voltage and with the requested rate of voltage rise. The rate of voltage rise shall be controllable within ±20 % of the specified rate. Steps in voltage rise shall not exceed 2 % of the expected breakdown voltage value. Controls which automatically increase the voltage at a selectable rate are preferable. A current-sensitive interrupting device shall be used for switching off the direct voltage source. For many materials, a dangerous voltage may persist across the test specimen for a considerable time after the d.c. test voltage is removed. Disconnection of the power supply to the direct voltage source does not necessarily result in the output voltage or the voltage at the electrodes being reduced to zero. For this reason, it is essential that the electrodes be short- circuited and connected to earth for a period equal to minimum twice the total charging time to ensure that the charge is dissipated. For some large test specimens it will be necessary to maintain the shorted condition for 1 h or even longer.