INTRODUCTION AND DEFINITIONSWhat is a relay; more specifically, what is a protective relay? The Institute ofElectrical and Electronic Engineers (IEEE) defines a relay as an electricdevice that is designed to respond to input conditions in a prescribed mannerand, after specified conditions are met, to cause contact operation or similarabrupt change in associated electric control circuits. A note adds: Inputsare usually electric, but may be mechanical, thermal, or other quantities or acombination of quantities. Limit switches and similar simple devices are notrelays (IEEE C37.90).Relays are used in all aspects of activity: home, communication, transportation,commerce, and industry, to name a few. Wherever electricity isused, there is a high probability that relays are involved. They are used inheating, air conditioning, stoves, dishwashers, clothes washers and dryers,elevators, telephone networks, traffic controls, transportation vehicles, automaticprocess systems, robotics, space activities, and many other applications.The IEEEdefines a protective relay as a relay whose function is to detect defectivelines or apparatus or other power system conditions of an abnormal or dangerousnature and to initiate appropriate control circuit action (IEEE 100).Fuses are also used in protection. IEEE defines a fuse as an over-currentprotective device with a circuit-opening fusible pat that is heated and severedby the passage of the overcurrent through it (IEEE 100).Thus, protective relays and their associated equipment are compact unitsof analog, discrete solid-state components, operational amplifiers, and digitalmicroprocessor networks connected to the power system to sense problems.These are frequently abbreviated simply as relays and relay systems. They areused in all parts of the power system, together with fuses, for the detection ofintolerable conditions, most often faults.Protective relaying, commonly abbreviated as relaying, is a nonprofit,nonrevenue-producing item that is not necessary in the normal operationof an electrical power system until a faultan abnormal, intolerablesituationoccurs._ 2006 by Taylor & Francis Group, LLC.A primary objective of all power systems is to maintain a very high levelof continuity of service, and when intolerable conditions occur, to minimizethe extent and time of the outage. Loss of power, voltage dips, and overvoltageswill occur, however, because it is impossible, as well as impractical,to avoid the consequences of natural events, physical accidents, equipmentfailure, or misoperation owing to human error. Many of these result in faults:inadvertent, accidental connections, and flashovers between the phase wiresor from the phase wires to ground.Natural events that can cause short circuits (faults) are lightning (inducedvoltage or direct strikes), wind, ice, earthquake, fire, explosions, falling trees,flying objects, physical contact by animals, and contamination. Accidentsinclude faults resulting from vehicles hitting poles or contacting live equipment,unfortunate people contacting live equipment, digging into undergroundcables, human errors, and so on. Considerable effort is made tominimize damage possibilities, but the elimination of all such problems isnot yet achievable.A dramatic illustration of the need and importance of power systemprotection is shown in Figure 1.1. This spectacular lightning strike occurredover Seattle during a storm on July 31, 1984, and in a region where lightning