A device, method and system for testing in situ the wiring between a data terminal equipment and a ring or similar type local area network having a loop conduction path with a plurality of data terminal equipment (DTE) coupled to said loop conduction path. The DTE is provided with a mechanism for generating d.c. current. The d.c. currents flow from the DTE through a length of interconnecting conductors and self-shorting connectors towards the loop conduction path. Current measuring devices monitor the current at the interconnecting conductors for imbalance current flow and/or over current. The presence of either a current imbalance and/or over current indicates a malfunctioning (short circuit, broken wire, etc.) in the wiring.