Abstract: The supervised conductive loop comprises a plurality of switch elements connected in series with a DC current source. Encoder modules are located in parallel with each of the switch elements. The encoder module comprises an oscillator and an impedance element. When a switch element is opened, the oscillator associated with that switch is energized and places an AC signal on the loop. The frequency of each oscillator uniquely identifies the associated switch element. The impedance element, formed by a resistor and a capacitor, is used both as a filter for the power supply voltage to the oscillator as well as a bandpass filter to permit oscillator signals present on the loop to bypass open switch elements and be detected at any location in the loop. Frequency identifying means are coupled to the loop to permit an indication of which oscillators have been operated.