The physical means by which electric energy is received from the electric utility is known as an electric interconnection. The primary elements include transmission or distribution circuits, transformers, and switching devices in the form of circuit breakers, fuses, and isolating disconnecting switches. The secondary elements include sensing devices to obtain information to operate the primary equipment intelligently, and relays to initiate removal of equipment from service automatically and quickly when an electric fault or disturbance occurs. It is important to remember that the physical laws of nature which govern the operating behavior of an electric system do not recognize defined lines of electric facility ownership. Thus for a well-engineered interconnection it is mandatory that problems in electric-system protection be studied and analyzed critically without regard to ownership. From the viewpoint of service reliability and service continuity, it is basic to understand that the best conceived, best implemented relaying system is no substitute for an adequately designed power system; and, indeed, inadequately applied protective relaying will contribute to inadequate and unsatisfactory performance of an otherwise well-designed electric system. In considering a new installation, or changes to an existing arrangement, it is very important that protective relaying and safety be given careful attention in the early stages of planning.