The scope of this guide is to describe field test methods that assure current transformers (CTs) are connected properly, are of marked ratio and polarity, and are in a condition to perform as designed both initially and after being in service for a period of time. Annex A describes wiring integrity checks, uses of test jacks and current shorting switch, and relay test equipment. Annex B illustrates excitation voltage measurement differences between rms responding voltmeters, commonly used under field conditions, and average responding voltmeters commonly used in laboratory tests and also discusses the effect of the source impedance. Annex C describes the characteristics, and other pertinent information, for optical current sensor systems used with protective relaying. It provides an overview of the components used in an optical sensor system, discusses the differences from conventional CTs, and provides testing information. Annex D is the bibliography for this guide.