This book was created to provide guidance in maintenance, diagnostics, and testing of transformers and associated equipment. This book applies primarily to the maintenance and diagnostics of oil-filled power transformers (500 kilovolt-amperes [kVA] and larger), although routine maintenance of other transformer types is addressed as well. Specific technical details are included in other documents and are referenced in this document. Guidance and recommendations herein are based on industry standards and experience gained. However, equipment and situations vary greatly, and sound engineering and management judgment must be exercised when applying these diagnostics. All available information must be considered (e.g., manufacturer=s and transformer experts’ recommendations, unusual operating conditions, personal experience with the equipment, etc.) in conjunction with this document.