The design efficacy of the insulation system is ascertained through the one minute Power Frequency Withstand test. The withstand test voltages are higher than the rated voltages. Typically, a dry type transformer of 8.7 kV class must withstand 19 kV for one minute at power frequency. IEC 60060 standard specifies the definitions and requirements for High Voltage Testing of various equipments.

High Voltage Testing Transformers are used as the test source to generate these high voltages in the laboratory. A single unit of High Voltage testing transformer can generate at best up to a voltage of 350 kV. Beyond this rating the transformer tends to become bulky and unwieldy. One of the most commonly used methods of obtaining a High AC Voltage is by combining two or more units of HV testing transformers in Cascade.

High Voltage testing transformers are essentially low in power rating and are not used continuously. The voltage ratings are high and thecurrent ratings are low , not in excess of 1 A. The output voltage (especially for Capacitance & loss factor measurement) must be truly sinusoidal with a good form factor.

Cascade connection of one or more units of High Voltage Testing Transformers is one of the most common ways of generating High Voltages at power frequency in the laboratory.