This guide describes diagnostic tests and measurements that are performed in the Þeld on oil-immersed power transformers and regulators. Whenever possible, shunt reactors are treated in a similar manner to transformers. The tests are presented systematically in categories depending on the subsystem of the unit being examined. A diagnostic chart is included as an aid to identifying the various subsystems. Additional information is provided regarding specialized test and measuring techniques. Interpretive discussions are also included in several areas to provide additional insight on the particular test, or to provide guidance on acceptance criteria. These discussions are based on the authorsÕ judgement of accepted practice. It should be noted that sometimes the results of several types of tests should be interpreted together to diagnose a problem. ManufacturerÕs acceptance criteria should also be consulted as it may take precedence over the criteria in this guide.