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 (). 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.
