In a time-allocation telephone system with 96 local lines served by a common switching channel via four two-wire branch channels each allotted to a respective group of 24 lines, an integrating network counts the number of phases in a 100-phase clock cycle during which either wire of a particular branch channel is energized. If the number of unit charges stored in the integrating network during a cycle exceeds the maximum number (48) possible during normal operation, a comparator also receiving a fixed reference voltage trips a flip-flop to inhibit the transmission of switching signals to the main channel from the branch channel in the following cycle. At the beginning of each new cycle, the flip-flop is reset with immediately following discharge of the integrating network.