A safety device (10) comprises cutout means (23, 24) for automatically disconnecting an electrical appliance (11) from the power line (12, 13) supplying power to it. An electric signal generator (19) and a detector (21) of said electric signal are connected in series with the interposition of coupling transformers (17 and 18). The series is connected, with condensers (15, 16), between the conductors (12, 13) of the power line and the earth connection (14) of the appliance (11), so that the path of the signal between the generator (19) and the detector (21) is substantially closed by the electrical impedance between the earth connection (14) and the earth reference (26) of the source of power (25).
When the incoming signal to the detector is of an amplitude smaller than a predetermined value, the signal detector (21) commands the cutout means (23, 24) to disconnect the appliance (11) from the power line (12, 13).