Transformer Winding Resistance Testing: Temperature, Tap and Settling
A winding resistance test measures milliohms, and at that level the reading is decided by two corrections and one comparison, not by the instrument.
tap changer
Transformer Turns Ratio Testing: Finding Tap-Changer Problems
A turns ratio test is only as good as the tap position it is compared against. The pattern of deviations across the tap range separates a real changer fault from a measurement error.