Abstract: A system for detecting a jam condition in a series of sequentially moving members which are passing a plurality of stations. The system finds particular application in the production of corrugated board or the like in which at the first station there may be slitting or slotting or printing drums or the like, which are performing an operation on a series of moving corrugated board members. There are detection means at each station and two successive signals from one station, indicating the presence of a member at that station without a similar signal from the next sequential station, will cause a jam signal to be provided by the circuit.