Design of Relays - Introduction

Design of Relays Introduction (Manuscript received September 28, 1953) The electromechanical relay is the basic building block of modern dial switching systems and also of various automatic control systems and computers. All of these systems depend on the action of the relay which is simple in its functions, but has to meet complex requirements placed on it the systems in which it is used. Perhaps the best illustration of this apparent conflict is to note that a relay has simply to close or open electrical contacts when its coil is energized or deenergized. However, these simple functions must be performed equally well by millions of relays, and each of these must continue to perform reliably for millions and in some cases for more than a billion operations during its lifetime. Furthermore, in many cases the relay must function in a few thousandths of a second, or use little electrical power. The reliability required in telephone switching systems would be considered unreasonable in many other types of equipment and can be judged from the fact that a single failure in 5,000,000 operations is considered to be poor performance. Stated another way, satisfactory operation for the average relay in modern telephone switching systems is less than one failure in forty years of operation. The measurement of such low trouble rates is, in itself, a difficult and challenging problem. The need for such a high degree of reliability and for the associated requirement of high speed is evident from a few figures relating to l 2 THE BELL SYSTEM TECHNICAL JOURNAL, JANUARY 1 9 5 4 modern cross-bar switching systems.