Methods of testing and evaluating dissolved gases in mineral based transformer oils found in Load Tap Changers (LTCs) are discussed and recommended in this guide. General types of LTC mechanisms, breathing configurations and electrical design are included as evaluation criteria for determining when mechanical damage or failure has occurred. Dissolved Gas of the LTC is required. This guide is not manufacturer specific, rather category specific. Additional files for download can be found at http://standards.ieee.org/downloads/C57/C57.139-2010/