The DC winding resistance measurement is an important measurement in accessing the true condition, state and accuracy of a CT. Winding resistance in a CT will change over a period of time depending on the specimen age, use, external conditions and loading effect.

It is recommended to measure DC winding resistance periodically on a single tap or multi tap CT and trend the values. A high precision low resistance measurement circuit is required to obtain this small winding resistance.

The winding resistance of a current transformer is found by dividing the voltage drop across the winding (measured from dc milli voltmeter) with the applied dc current through the winding. The CT should be demagnetized after the completion of winding resistance test.

Tip: Run a saturation test to demagnetize the CT at the completion of all winding resistance tests.