Abstract: A protective device includes a transmission line comprising two elongate co-extensive electrical conductors capable of sensing vibrations, converting these vibrations into electrical signals and transmitting the electrical signals. It also includes signal-processing means electrically connected to one pair of ends of the conductors. The signal-processing means includes normalizing means for normalizing the electrical signals, and a comparator for comparing the normalized signals with previously normalized known signals, thereby to identify the source of the signals transmitted along the cable, and hence an alarm/non-alarm condition.