Abstract: In the method of the invention, a signal is injected into the power cord of an appliance and a reference voltage is set in response corresponding to a reference impedance at the point of signal injection. The reference impedance is present with the appliance connected to the power source. The reference voltage is continuously compared against a present voltage corresponding to present impedance at the point of signal injection. An alarm sounds if the present voltage changes to exceed the reference voltage, indicating a higher impedance because the appliance has been disconnected. In one embodiment, the apparatus includes an impedance bridge having an adjustable reference arm for producing a reference voltage corresponding to the impedance of the cord and appliance and a sensing arm for producing the present voltage. In a second embodiment, the apparatus injects an adjustable current into the power cord and appliance for a predetermined time to provide a present voltage.