SIGNAL PROCESSING SYSTEM FOR OVERLOAD RELAY OR THE LIKE

An improved system for sensing current flow in a single phase or polyphase circuit to be protected and deriving a single signal representing composite current flow. A ramp-like reference signal is applied to one input of a comparator at sampling intervals, and the outputs of a plurality of current amplifiers coupled to the other comparator input so that the duration of a change in signal output level of the comparator is an inverse function of composite current. An accumulator stage stores portions of incoming current signals less a predetermined fraction of previous accumulator contents. In the event of circuit interruption the accumulator is decremented in a manner which reflects the inferred thermal history of the system.