Testing of breakdown voltage of transformer oil - Test procedure
IEC 156 oil tester
9.1 Sample preparation
Immediately before filling the test cell, the sample container is gently agitated and turned over several times in such a way as to ensure as far as possible a homogeneous distribution of the impurities contained in the liquid without causing the formation of air bubbles.

Unnecessary exposure to the ambient air of the sample shall be avoided.

9.2 Filling of the cell
Immediately before commencing the test, drain the test cell and rinse the walls, electrodes and other component parts, with the test sample. Drain and slowly fill with the test sample avoiding the formation of air bubbles.

Measure and record the temperature of the liquid.

Position the cell in the test equipment and start the stirrer if used.

9.3 Application of voltage
The first application of voltage is started approximately 5 min after completion of filling and checking that no air bubbles are visible in the electrode gap.

Apply voltage to the electrodes and uniformly increase voltage from zero at the rate of 2,0kV s-1 ± 0,2kV s-1 until breakdown occurs. The breakdown voltage is the maximum voltage reached at the time the circuit is opened either automatically (established arc) or manually (visible or audible discharge detected).

Record the value.

Carry out six breakdowns on the same cell filling allowing a pause of at least 2 min after each breakdown before re-application of voltage. Check that no gas bubbles are present within the electrode gap. If a stirrer is used, it shall run continuously throughout the test.

Calculate the mean value of the six breakdowns in kilovolts.