This article focuses on practical deployment: how CTs feed protective relays, how to select and size CTs for different protection schemes, common installation and testing practices, and how modern sensor technologies change protection design. The purpose of this study is to learn more about CT operation in association with protection relays and to lay down a few rules for sizing them properly. Occasionally, errors in CT and VT connections can occur, such as missing or broken neutral wires, multiple or. Why the power system needs to be protected? All current and voltage vectors have 120 degrees phase shifts and a sum of 0. SIA-B can be used with an auxiliary.