Simplicity in protective relaying

SIMPLICITY is of paramount importance when designing and laying out protective relay circuits. These circuits are called upon to operate so infrequently that they should be more reliable than the apparatus or lines they protect. Certain limiting conditions and requirements requested by circuit designers generally cause false tripping and make the circuits less reliable. A simple protective system with all the extra devices left out operates so satisfactorily that the reasons for its good operation are often forgotten. The simple rules concerning protective systems in use on the Southern California Edison System that have made it possible to attain such a high standard service to its customers are reviewed in this paper.