A data transmission system for power line carriers includes a transmitter which generates a stable carrier frequency at a bandwidth of less than one hertz. The transmitter is connected to the power line to transmit the generated frequency over the power line to a receiver. The receiver implements filters of a bandwidth of one hertz or less to attenuate any signals that do not correlate with the generated frequency. Information on the phase of the power line to which the transmitter is connected is represented in the form of a generated signal which modulates the stable frequency generated by the transmitter on the power line. The receiver is located on another portion of the power line and compares the phase information from the transmitter with the phase of the power line to which the receiver is connected and displays the resulting comparison for interpretation.