Abstract: There is disclosed a protective coupler for use with key telephone line units (KTU) to provide isolation between the terminal equipment and the central office line. The protective coupler plugs onto the KTU option socket and establishes a dielectric barrier to protect the system from possible overvoltages, guarantees a balanced termination, and meets leakage requirements while remaining operable to pass control and voice signals between the network and station equipment. The central office side of the barrier contains a current sink for control purposes as well as a gain compensation circuit to adjust for varying distances from the central office.