C57.13.1 WG - Field Testing of Relaying Current Transformers

In the application of protective relays, the most widely used input quantity is current. A multiplicity of different protective relays either utilizes current directly, combines it with other currents as in differential schemes, or combines it with voltage to make impedance or power measurements. The source of relay input current is from current transformers which may be located on the bushings of power circuit breakers and power transformers, on the bus bars of metal clad switchgear, or installed as separate items of equipment located as required. The purpose of this guide is to describe field test methods that will assure that the current transformers are connected properly, are of marked ratio and polarity, and are in condition to perform as designed both initially and after a period of service.