Applies to mineral oils, both uninhibited and inhibited against oxidation, in service, under normal operating conditions, in transformers, switchgear and similar electrical equipment where oil sampling is permissible. This publication is intended to give a common basis on which more complete codes may be prepared when these are necessary. In this guide, the causes of oil deterioration are discussed, and recommendations are made for the various tests. It also gives indications regarding the type of treatment to which the oil should be subsequently subjected, if applicable.