Electric Strength Test (Breakdown Voltage Test):
This is the most generally known test applicable to mineral insulating oils, and it was originally established to test the breakdown voltage of the oil. We elaborates the method of its measurement in Box 2.
Box 2: Method of Measurement of Breakdown Voltage
An oil test cell is used in that an alternating voltage is applied among two metal spheres 12.5 mm in diameter along with a gap of 2.5 mm among them. The voltage is increased until breakdown occurs. The flashover must be fast stopped to permit six successive measurements of the rupture voltage on the same sample. The value of the electric strength of the sample tested is the average of the six measurements.