The transformer is subjected to a test voltage equal to the total losses of
the transformer. The test is continued until a steady-state oil temperature rise is
established or oil temperature rise has fallen below 1 °K per hour and has
remained there for a period of 3 hours. The test current is reduced immediately to
rated current and maintained for 1 hour, then disconnected the test power supply.
The resistance of HV and LV winding are measured for a sufficient time.