The impact of capacitive voltage, due to concentric cable, on Signalling circuits

Railway signalling line circuits are critical to the safe operation of the railway. This paper discusses the electromagnetic compatibility aspects of these circuits at the fundamental operating frequency. At low frequencies, we are [generally] only concerned with the voltage and current rating of the conductors and ensuring that the volt drop at rated current remains within acceptable values. In the case of high frequency signals attenuation and propagation delay become more significant and a transmission line approach is necessary. Here we consider the middle ground; long AC line circuits where the effect of cable capacitance needs to be carefully considered in order to prevent malfunction of safety critical circuits and may be affected by difference in cable geometries, selected for other equally worthy reasons.