The scope of this guide includes general philosophy, practical applications, and economic considerations involved in power transformer protection. Emphasis is placed on practical applications. General philosophy and economic considerations in protecting transformers are reviewed. Types of faults in transformers are described. Technical problems with the protection systems, including the behavior of current transformers (CTs) during system faults, are discussed. Associated problems, such as fault clearing and reenergization, are discussed as well. The purpose of this guide is to provide protection engineers with information that helps them to properly apply relays and other devices to protect transformers used in transmission and distribution systems. Revision Standard - Superseded. Revision of IEEE Std C37.91-2000. This guide is intended to provide protection engineers and other readers with guidelines for protecting three-phase power transformers of more than 5 MVA rated capacity and operating at voltages exceeding 10 kV. In some cases, a user may apply the techniques described in this guide for protecting transformers of less than 5 MVA ratings or operating at voltages less than 10 kV. The guide provides information to assist protection engineers in applying properly relays and other devices to protect transformers used in transmission and distribution systems. General philosophy, practical applications, and economic considerations involved in power transformer protection are discussed. Emphasis is placed on practical applications. Types of faults in transformers are described. Technical problems with the protection systems, including the behavior of current transformers (CTs) during system faults, are discussed. Associated problems, such as fault clearing and re-energization, are discussed as well.