Underground Cable Fault Detection and Stringing Methods

Cable Stringing and Tensioning Techniques

• By Nivelet: Choosing the span for more favorable regulation, placing two on each end of the span at a vertical distance from the fulcrum. Proceed to the tension of the conductor and, with reference to the lowest point of the cable stringing, align this to the mark visually using two levels.
• By Dynamometers: Installed at the point of attachment on the hoist or tension element. Proceed to tension the conductor until the dynamometer reaches the daN (decanewton) mark; once achieved, secure the cable to the anchor points.

Damage and Repair in Underground Networks

Faults are detected by the performance of the installed protection, usually a fuse in the LV (Low Voltage) box located in the Transformer Station (CT) or the distribution box inserted in the line, which results in a power outage for a number of users.

Common Causes of Network Abnormalities

These abnormalities can result from several causes: broken pipes by excavation, insulation failures, landslides, short circuits, overloads, etc. Before replacing the protections, we must determine why the trip occurred and identify the cause of the breakdown. The various types of damage generally fall into two categories:

• Interruption of the continuity of the line.
• Deterioration of the insulation.

Fault Localization and Detection Methods

For the location of different faults, specific equipment is required to provide an approximate point where the fault occurred. Localization methods depend on the kind of breakdown, and these can be detected by radar:

1. The Conductor is Not Interrupted

• a) Short circuit or earth fault through a small resistance (less than 50 Ω).
• b) Referral to ground through a very large resistance (exceeding 50 Ω).

2. The Conductor is Broken at the Fault Spot

• a) There is an earth fault on one or more conductors.
• b) Conductors have good insulation values (no earth fault or short circuit).

Modern methods are known by the generic name of trackers. This consists of traversing the path of the line equipped with special sensors to locate the fault. These include the acoustic method and the method of induction.

The most effective techniques for detecting faults in underground cables are based on the principle of reflection, where a short electrical pulse is sent along the cable. This is known as radar, which measures the time between the moment of transmission and the arrival of the reflected pulse.