A critical study of reliability of metal to carbon relays used in railway signaling with special reference to high contact resistance

Abstract

The research study is an effort to critically study the factors responsible for poor reliability of metal-to-carbon relays used in railway signaling, with a special reference to High Contact Resistance phenomenon, and to propose measures for enhancing the reliability by conducting a detailed analysis of the occurrence of HCRand by suggesting measures for alleviating the failures due to it.The railway signaling relies on interlocking of various signaling functions such as signals, points, track circuits and LC gates, by means of electromechanical relays, for safe working of trains. Metal-to-carbon relays are one of the two types of relayspredominantly used on Indian Railways, and consist of armature, heelpiece, core, relay coil, contact springs and adjustment card. The salient feature of this relay is that one of the contacts is of metal, that is silver, while the other contact is made of carbon,that is Silver Impregnated Graphite or SIG (silver impregnation for enhancing the conductivity of graphite). These relays are manufactured and used in accordance with Indian Railways and British Railways specifications; there are many variants to suit the varied applications such as QN1 (line relay), QNA1(line relay for electrified areas), QT2 (track relay), and QSPA1 (slow to release relay). Relays are wired as per the signaling circuits, and the relay contacts make or break as per the operation command from the operating panel and the concerned circuit logic. When the contacts of various relays are available in a predetermined sequence, the signaling function operates. Thus, reliability of these relays plays a crucial role in the overall availability of signaling systems.From the data studied from Indian Railways, the average failures of metal-to-carbon relays amount to approximately 7% per annum, causing train detentions of more than 300 hours per annum. Poor performance of these relays results into increased numbers of signal failures, causing disruptionsin train operation and adversely affecting the punctuality and safe