This guide describes the characteristics and classification of current transformers (CTs) used for protective relaying. It also describes the conditions that cause the CT output to be distorted and the effects on relaying systems of this distortion. The selection and application of CTs for the more common protection schemes are also addressed. The purpose of this guide is to present a comprehensive treatment of the theory and application of CTs to assist the relay application engineer in the correct selection and application of CTs for protective relaying purposes. Revision Standard - Inactive-Reserved. Revision of IEEE Std C37.110-1996. The characteristics and classification of current transformers (CTs) used for protective relaying are described. This guide also describes the conditions that cause the CT output to be distorted and the effects on relaying systems of this distortion. The selection and application of CTs for the more common protection schemes are also addressed.