Locating Underground Cable Faults

November 2, 2017


High-voltage cables run across pylons and overhead structures, passing over highways and the outskirts of cities. When one comes down, it's a quick call to the electrical authority, and the power is back on before the day is over. Underground faults present an obvious extra layer of difficulty. The cable is under several feet of soil, maybe passing under a road or through the fringes of a park. It's neither logistically desirable nor cost-effective to call in the diggers and pull up the ground at random. A better way is called for, and this is where the mature science involved in locating cable faults comes into play.

Thankfully, one property we can still count on in electrical cabling is the presence of metal. Copper and aluminium construction is common within stranded and solid cables. Other metals are found in the armoring of underground cabling, the protective layers that sheath the conductors from the effects of the soil. The entire cross-section of the typical underground cable is an amalgam of insulators and conductors, with protective layers including paper and plastic, but it's the metal that makes the key difference in the field of radiolocation. RADAR penetration technology traces the path and depth of the cable using tuned frequencies to map every little peak and furrow of the route, but this only serves as a first step in locating the cable fault. The cable has been found, pinpointed to the exact twist and turn, but the next stage is to determine the spot where the electrical fault has occurred.

When electrical cabling is damaged below ground, the result is a sudden grounding of the current, this is short-circuit. This grounding phenomenon, the sending of electrical energy to earth, can be detected with specialized equipment. Employ the A-Frame test, using the recorded route data to pinpoint the fault. The test uses the depth and location data to orient the tester, meaning the route should be rendered somehow on the ground. Utility workers typically stain the ground with a colored spray marker to help the engineer locate the fault. Walk the sprayed path of the cable with the A-Frame. It's little more than a simple antenna equipped with a ground fault locator device, a basic example of electrical principles at work. Still, the test makes a pleasant change from the complexity of ground-based RADAR location, and it's time-proven to accurately locate faults.

There are other ways to locate a fault, but the critical part of the work has been completed. The location and depth of the cable has been plotted with some form of ground penetrating RADAR echoing frequency-tuned waves from the metal of the wire. The A-Frame test does the rest, a task suited for any qualified electrician. One disadvantage of the technique is the need to disconnect the cable at both ends, and the test is designed for short-circuits. A time-domain reflectometer, TDR, tester is another viable option. This specialized device is used for open-circuit cable faults, though it has its own drawbacks.

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