A high integrity monitor for signalling interference prevention at 50 Hz

The design of a high integrity monitor for the prevention of signalling interference is described. It belongs to the family known as Interference Current Monitor Units, or ICMUs, and has to be extremely reliable since a fault would otherwise compromise railway safety; a calculated value of Mean Time Between Wrong Side Failure (MTBWSF) of five thousand million hours was specified and achieved. High levels of out of band currents had to be rejected, in particular up to 2 kA DC The basic design was kept as simple as possible with only one frequency band being detected. A hardware only solution, i.e. no software, was adopted and triplication of the circuit was used to achieve the MTBWSF specification.