Winding resistance measurements are an important diagnostic tool for assessing possible damage to transformers resulting from poor design, assembly, handling, unfavorable environments, overloading or poor maintenance.

The main purpose of this test is to check for gross differences between windings and for opens in the connections. Measuring the resistance of transformer windings assures that each circuit is wired properly and that all connections are tight.

Winding resistance in transformers will change due to shorted turns, loose connections, or deteriorating contacts in tap changers. Regardless of the configuration, the resistance measurements are normally made phase-to-phase and the readings are compared with each other to determine if they are acceptable.

Transformer winding resistance measurements are obtained by passing a known DC current through the winding under test and measuring the voltage drop across each terminal (Ohm's Law). Modern test equipment for this purposes utilizes a Kelvin bridge to achieve results; you might think of a winding resistance test set as a very large low-resistance ohmmeter (DLRO).