Abstract: An electronic security device for monitoring equipment is disclosed, with each of the items of equipment being linked to a computer network by a plug and a cable at the level of a network interface, the device being arranged between an apparatus of the cabled network and the network interface of the item of computer equipment, the device comprising at the level of each linking cable between the management apparatus of the cabled network and the item of equipment to be monitored: a detection circuit able to detect on said cable, without perturbation, the presence of electrical energy originating from the item of equipment and revealing normal network activity and an isolating circuit able to be activated in the absence of such detection by the detection circuit, so as to isolate the cable in question of said cabled network, and an evaluating circuit able to be activated after the isolation performed by the isolating circuit, so as to apply to the cable a signal having a predetermined waveform, so as to detect