This guide applies to mineral-oil-immersed transformers and addresses: The theory of combustible gas generation in a transformer The interpretation of gas analysis Suggested operating procedures Various diagnostic techniques@ such as key gases@ Rogers ratios@ Duval triangle@ and other methods Case studies and examples Evaluation criteria and guidelines A bibliography of related literature Purpose The purpose of this document is to provide a guide for evaluating the condition of a transformer using analytical tools and methods involving transformer mineral oil and associated developed gases