Abstract: The system locates breaks and short circuits in a two-lead line with respect to a test device inserted in the line. At one remote end of the line is an apparatus for selectively applying a voltage and a current. The test device comprises a circuit in parallel with the line leads for transmitting a first frequency signal along the line which modulates the current in response to the applied voltage when no break is existing between the remote apparatus and the test device, and a circuit in series on one of the line leads for transmitting a second frequency signal along the line which modulates the line current in response to the applied current when no short circuit exists between the remote apparatus and the test device. The remote end of the line is selectively connected to first and second frequency detectors. The line can include a subscriber loop including a telephone subscriber installation at the other end.