ANSI/IEEE C57.13.1-198X Guide for Field Testing of Relaying Current Transformers

The most widely used input quantity in the application of protective relays is current. The source of this quantity is from current transformers (CT's) which are located on or near the protected equipment. CT's are used to provide isolation from the power system and reduction in magnitude to a level usable by relays and meters. Protective relays use this quantity in numerous ways to determine if the protected system is abnormal and requires isolation. Failure of CT's to perform as designed has resulted in failure to trip, failure to trip promptly and tripping when not required.