The purpose of this guide is to aid in the effective application of relays and other devices for the protection of power transformers. Emphasis is placed on practical applications, the general philosophy and economic considerations involved in transformer protection are reviewed, the types of faults experienced are described, and technical problems with such protection, including current transformer behavior during fault conditions are discussed. Various types of electrical, mechanical, and thermal protective devices are also described and associated problems such as fault clearing and re-energizing considerations are discussed.