This guide covers practical applications, general philosophy, and economic considerations of power transformer protection. 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 economics of transformer protection are reviewed. The types of faults experienced are described, and technical problems with such protection, including current transformer (CT) 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 reenergizing are discussed. Revision Standard - Superseded. The protection of power transformers is covered;various electrical protection schemes are explored;and guidelines are given for the application of these schemes to transformers. Alternative detection methods including mechanical,thermal,and gas analysis are discussed.