Relay protective systems

Reviewing American practise in the use of relay protective devices for electrical generating, transmission and distributing systems, there appears to have been no material change in the construction or commercial application of this class of apparatus in a considerable period of years. Such improvements as have been made relate principally to mechanical details for increasing the reliability of operation and maintenance of adjustment of relay apparatus. Recently, in response to demands for modifications of the electrical characteristics of time limit overload relays, certain changes have been made in existing types of relays, and in addition a new type of relay has been introduced. These developments have tended to greatly increase the facilities for obtaining selective action between different relays in stations where such operation of relays is desired.