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Locating That Underground Cable Fault

What method or combination of methods is best for locating underground cable faults?

Finding the location of an underground cable fault doesn't have to be like finding a needle in a haystack. There are many locating methods, coupled with new detection technologies, that make this task much easier and less time consuming. However, you should understand that there is no single method or combination of methods that is "best." Your selection of the appropriate method for the situation and your skill in employing that method are the keys to safely and efficiently locating cable faults without damaging the cable. Let's see what's involved.

Basic cable fault locating methods. There are two basic methods of locating an underground cable fault.

Sectionalizing This procedure, as shown in Fig. 1, risks reducing cable reliability, because it depends on physically cutting and splicing the cable. Dividing the cable into successively smaller sections will enable you to narrow down the search for a fault.

For example, on a 500-ft length, you would cut the cable into two 250-ft sections and measure both ways with an ohmmeter or high-voltage insulation resistance (IR) tester. The defective section shows a lower IR than the good section. You would repeat this "divide and conquer" procedure until reaching a short enough section of cable to allow repair of the fault. This laborious procedure normally involves repeated cable excavation.

Thumping When you supply a high voltage to a faulted cable, the resulting high-current arc makes a noise loud enough for you to hear above ground. While this method eliminates the sectionalizing method's cutting and splicing, it has its own drawback. Thumping requires a current on the order of tens of thousands of amps at voltages as high as 25kV to make an underground noise loud enough for you to hear above ground.

The heating from this high current often causes some degradation of the cable insulation. If you're proficient in the thumping method, you can limit damage by reducing the power sent through the cable to the minimum required to conduct the test. While moderate testing may produce no noticeable effects, sustained or frequent testing can cause the cable insulation to degrade to an unacceptable condition. Many cable fault locating experts accept some insulation damage for two reasons: First, when thumping time is minimal, so is the cable insulation damage; secondly, there is no existing technology (or combination of technologies) that can entirely replace thumping.

Newer fault locating technologies. There are some relatively new methods of locating cable faults that use rather sophisticated technology.

Time Domain Reflectometry (TDR) The TDR sends a low-energy signal through the cable, causing no insulation degradation. A theoretically perfect cable returns that signal in a known time and in a known profile. Impedance variations in a "real-world" cable alter both the time and profile, which the TDR screen or printout graphically represents. This graph (called a "trace") gives the user approximate distances to "landmarks" such as opens, splices, Y-taps, transformers, and water ingression.

One weakness of TDR is that it does not pinpoint faults. TDR is accurate to within about 1% of testing range. Sometimes, this information alone is sufficient. Other times, it only serves to allow more precise thumping. Nevertheless, this increased precision can produce substantial savings in cost and time. A typical result is "438 ft 5 10 ft." If the fault is located at 440 ft, you only need to thump the 20-ft distance from 428 ft to 448 ft, instead of the entire 440 ft.

Another weakness of TDR is that reflectometers cannot see faults-to-ground with resistances much greater than 200 ohms. So, in the case of a "bleeding fault" rather than a short or near-short, TDR is blind.

High-voltage radar methods There are three basic methods for high-voltage radar, ranked here in order of popularity, with the most popular described first: arc reflection, surge pulse reflection, and voltage pulse reflection. The arc reflection method, as shown in Fig. 2 (on page 64N), uses a TDR with a filter and thumper. The filter limits both the surge current and voltage that can reach the cable under test, thus allowing minimal stress to the cable. Arc reflection provides an approximate distance to the fault (when there is an ionizing, clean arc produced at the fault and the TDR in use is powerful enough to sense and display a reflected pulse).

The surge pulse reflection method, as shown in Fig. 3, uses a current coupler and a storage oscilloscope with a thumper. The advantage of this method is its superior ability to ionize difficult and distant faults. Its disadvantages are that its high output surge can damage the cable, and interpreting the trace requires more skill than with the other methods.

The voltage pulse reflection method, as shown in Fig. 4 (on page 64P), uses a voltage coupler and an analyzer with a dielectric test set or proof tester. This method provides a way to find faults that occur at voltages above the maximum thumper voltage of 25kV.

The open neutral and cable fault locating Bare neutrals corrode quickly in contaminated soil that holds corrosive chemicals or excessive moisture. Open neutrals often thwart the effectiveness of high-voltage radar. Beware: In the existence of an open neutral, nearby telephone or CATV cables will complete the circuit.

One test to detect an open neutral requires shorting a known good conductor to a suspect neutral, as shown in Fig. 5 (on page 64P), then measuring the resistance with an ohmmeter. If the reading is 10 ohms or higher, you can suspect an open neutral. Remember, other objects can complete the circuit.

Another test uses a TDR. The trace on an open neutral will show a much flatter positive pulse than it will for an open conductor. On lower-end TDRs, this pulse may not be visible. When the conductor is completely open, the trace will almost never include a reflected pulse indicating the end of the cable.

If the TDR displays an open neutral, then an AC-voltage gradient test set can locate the break in a direct-buried unjacked cable. The test set's transmitter forces AC current to flow through the neutral, and the conducting earth surrounding the damaged section acts as an electrical jumper. An A-frame , as shown in Fig. 6 (on page 64P), then detects the resulting voltage gradient in the soil.

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