Using the Varley-loop configuration to localize insulation faults in multipaired pulp-insulated cables has been practiced for quite some time. The standard configurations require precision potentiometers for manual nulling and calibration.
However, modifying the Varley loop, and including a 3.5-digit digital panel meter chip (ICL 7106), eliminates the need for bridge balancing (see the figure). As a result, it will directly display the wire resistance or distance to the insulation fault accurately.
Looking at the circuit, B and C represent two wires of a faulty pair in a multipair telecommunications cable. A fault in the insulation, of resistance Rj, lies at a distance of Lf from the near end. The corresponding conductor resistance to the fault is Rf. At the far end, conductor B is strapped to a good wire (A), which may be available within the cable.
The constant-current source drives approximately 1.2 mA through the wires A and B and the reference resistance (Rr). The voltage across Rf is applied to In Hi and In Lo via the insulation fault resistance (Rj). The voltage across Rr is applied to the inputs Ref Hi and Ref Lo. The number displayed by the dpm is equal to (Rf/Rr) × 1000. By properly selecting Rr, Rf or Lf can be displayed directly.
When Rr = 100 Ω, Rf is measured in the 0-to-199.9-Ω range. Lf is displayed directly in meters by suitable choice of Rr, taking into account the wire gauge, temperature, and the temperature coefficient of the conductor's resistance. For example, Lf is measured in the 0-to-1999-meter range when Rr = 86.0 Ω for a copper conductor of 0.51 mm in diameter at 20°C.
The circuit is particularly useful in cable factories where accuracies better than ±0.2% are required for Rj less than 0.5 MΩ. For field use, it must be modified to include additional ranges and compensation for externally induced potentials.