A cable TV system, which is arranged to permit a transmission of signals in the return direction from local networks (10), included in the system, to a head end (11) of the system within a predetermined carrier frequency band, serving for the transmission of signals in said direction, comprises means (11', 23) for monitoring the system in respect of disturbances occurring within said carrier frequency band. Said monitoring means (11', 23) comprise detector means (32, 33, 34, 35) within each local network (10) which are arranged to compare the energy level of signals occurring within said carrier frequency band with at least one predetermined reference level and to generate logic signals, depending upon the result of said comparison, and memory means (36) within each local network (10) which are arranged to store information about time intervals during which said energy level exceeds said reference level, derived from the values of said logic signals at different times.
