Introduction to Cable Fault Locating. Thumping a Cable

Cable fault locating can be quite challenging even though there is a number of fault locating techniques and quite a few tools to help out with underground cable faults. Most of these issues are due to technicians not properly interpreting the test results and then selecting the wrong tool for the task at hand, and ultimately a lot of wasted time is due to taking a shortcut in the process.

The challenges of underground fault locating may be minimized significantly by understanding the equipment and techniques available. Education and experience in cable fault locating will help correct and make improvements in interpreting of the results and definitely will help correct and make improvements in the selection of which equipment and technique is most appropriate for a particular task. However, only a high level of awareness is going to correct wasted time that is afforded through taking shortcuts.

In the following series of articles we are going to discuss various cable fault locating techniques along with their advantages and the drawbacks.

The Basics of Capacitor Discharge Technique

One of the oldest and most popular techniques out there is a technique called the capacitor discharge technique or thumping a cable. In application this technique is actually very straightforward and simple. In essence, a device called the thumper, or surge wave generator, creates a high voltage surge which is launched out onto the faulted cable. This surge of energy will travel along the cable conductor. If it reaches an area of dielectric breakdown or a failure in the insulating material, the transmitted energy will discharge through the gap at the fault and discharge all of the energy through the faulted insulation. All the fault current will then come back through the neutral or shield on the cable, back into the surge wave generator, and then will be dissipated safely to earth potential.

If a several thousand volt surge is sent into the cable, and all that energy is discharged through the gap or fault, it is going to cause a small explosion. Buried in the ground, this small explosion will cause a percussion and sound wave to travel up through the layers of earth. As a result, a thumping sound can be heard on the surface of the ground. To locate the defect in the underground cable, a repair crew has to walk along the surface of the ground listening for this thumping sound. Once the fault is pinpointed, the crew digs a hole and repairs the faulted cable.

Advantages and Disadvantages of Thumping a Cable

The advantages of the capacitor discharge technique are that it is very accurate to do pinpointing and is easy to learn. This technique requires fairly minimum training, in particular, some safety training on how to properly handle the instrument, how to properly make connections, and how to set the various controls.

The weak point of this technique is that it is extremely time consuming and potentially harmful if misused. In some cases it may take hours or even days to walk the cable and definitely locate the fault. During that time the cable is exposed to high voltage surges, leading to a higher rate of repeat failures on the service age polyethylene cables. Repetitive thumping could be accelerating the
vented tree
Water trees
channel growth of other water trees that under natural aging may have lasted several more years of service life. In essence, the cable is being set up for a future fault in the process of trying to find the current fault. Thus, this technique, if used all by itself, can be potentially harmful. The same is not necessarily true for the paper insulated lead-covered cables, where typically higher voltages and more energy is required to locate faults, with no noticeable damage to the cable.

Also this technique will not find faults that do not arc over. So, for instance, if there is a dead short, what is called a bolted fault, where basically the conductor and neutral have come together and bonded, if there is no gap, there is no sound no matter how much energy and voltage is discharged into the circuit. There is no energy being discharged or jumped across, there is just basically a constant current loop. So, since there will not be an acoustic event, it will not be possible to pinpoint a bolted fault, or a dead short. Again, in order to have sound, there needs to be a gap, and there needs to be air.

Another challenge for the capacitor discharge technique is a gap that is too large. For instance, the cable blows apart, and in the process of blowing apart the conductor had burned back into the dielectric material, or the neutral had burned back, causing a sizable distance in that gap. No matter how much voltage is applied to it, the gap is physically too large to actually arc across. The choice here may be the burn down technique, with the insulation resistance burnt away to reduce the gap size in order to get the thumper to discharge an arc across properly. But, again, this is going to come with stress to the cable.

To reduce the amount of stress applied to the cables under test, the surge or thumping technique is combined with pulse echo. In the next article we will describe the pulse echo technique and will explain how it can be utilized as a compliment to the thumping technique.

KharkovEnergoPribor Ltd. - is the leading Ukrainian company specialising in designing and manufacturing high voltage field-testing equipment, insulating liquids testing and cable and calibration test vans for national power grids, oil and gas, automotive, railway and aircraft industries.

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This release was published on openPR.