Fail Safe Operation of Audio Frequency Track Circuits for Railway Signalling

An Audio Frequency Track Circuit (AFTC), a recent addition to railway signalling, confirms the absence of a train in a section of track. While the basic concept is established, several issues relating to proper operation of the AFTC need theoretical study. Installation practices like structure bonds and cross bonds, could lead to failure of AFTC operation, especially in the presence of breaks. An analysis is carried out, to study these influences on AFTC performance. Conjectured fault scenarios are identified which could lead to failure of the system, on the side of safety or otherwise. A three-wire model of the track with PSpice as a simulation tool is used for analysis and the reduction in margin due to structure and cross bonds is also established.