Abstract: A T-carrier digital test system which actively monitors each T-carrier wire pair on a time division multiplexing basis while either live communications or test signals are on the line, and determines from the monitored data whether an incipient or hard failure of the line exists. It achieves this result by utilization of interacting devices called Scanners and Scanner Controllers. Each Scanner is housed in a repeater case and is connected to and monitors the output of each regenerative repeater in that case. The Scanner also connects to a fault line which runs in parallel to the T-Carrier wire pair and acts as the communications media and power path between several Scanners and the telecommunications facilities at each end of the cable pair.