ABSTRACT
During the early years of power system design, protective relays were electromechanical devices that were reliable and easy to understand. In many cases, each relay was a small, special duty motor or solenoid. Each relay consisted of (1) stationary assemblies (coils, core steel, and supports) that were connected to current and voltage transformers and (2) moving assemblies (disks, restraint springs, and plungers) to which output contacts were attached. Some early relays included servo-motors that could be used to change relay settings remotely. Specific relays were mounted in steel drawout cases with removable glass cover plates that facilitated visual inspection and periodic testing requirements. Electromechanical relays were packaged either on a function or a phase basis. For example, time and instantaneous overcurrent relays were packaged together on a phase basis. This means that four relays, each contained in a separate case, were required if three-phase and one-ground overcurrent relays were needed. Impedance relays were packaged on a function basis. This means that a Zone-1 impedance relay that protects all three phases of a transmission line would be contained in one relay case and the complementary Zone 2 impedance relay would be contained in a separate relay case.
