Current transformers for protection relays, as opposed to those use strictly for metering purposes, have an IEEE standard classification. There are two classifications, Class T CTs and Class C CTs. The ‘T’ stands for “tested” and the ‘C’ stands for “computed”.
Class T CTs generally have a high level of flux leakage (due to the way the primary is configured as multiple windings around the core) which requires the performance of the CT to be tested. Class T CTs are rarely used for commercial power system protection relays, and they will not be discussed further here.