Cable Fault Locating (CFL) ( like a challenging sport ) requires a good working knowledge of the game's basics and an understanding of your opponent's shortcomings, as well as your team's. Here's a tip to help play the game.
Thumpers come in many shapes and sizes. Some manufacturers call them Surge Generators or Capacitive Discharge Sets. In parts of the country they're known as bangers or pulsers. Nevertheless, all operate on the principle of charging a large capacitor from a high voltage supply, then discharging the stored energy into the weakened cable under test.
At the fault point in the underground cable, the rush of energy flashes over this weak spot, resulting in a tremendous acoustic and seismic jolt ( the effects of which travel up through the ground, causing a significant bang or thump). The noise usually sounds like a rap with your knuckles on the side of a desk. Loose or newly filled soil, sand and gravel tend to reduce the sharpness and loudness of the thump. Ducts, cement and asphalt tend to spread out the noise over a larger area. The loudest thumps are heard with the unaided ear - also often felt through the soles of your shoes when standing directly overtop. Of course, the best thumpers are those that can produce the loudest noise at the lowest possible voltage.
Basic operation involves a high voltage DC supply which charges up capacitor C (or several in combination), resulting in the build-up of a significant stored charge. This charge is allowed to discharge periodically (about once every 5-7 seconds) across the spark gap S to the output, then into the cable under test. The supply regularly recharges the capacitor after its energy is discharged into the cable. When the test is finished or aborted, the discharge switch D is closed (usually automatic on powering down.) This allows the remaining capacitor charge, as well as the residual charge held by the cable, to be safely discharged through R.
The inescapable laws of physics dictate that the higher the voltage charge in C, the louder the thump and the larger the capacitance, the louder the thump. In the past, to get a respectable energy output, many manufacturers chose to build a hefty high voltage end. This was the easiest way to get good thumping energy. For many years, the effects of this high voltage weren't a big concern. But studies have shown how this seriously decreases cable life when applied for extended periods and at excessively high voltages. The relationship between these important components is based on the equation in Figure 2.
If we were to plot the relationship on a graph using as an example a typical 30kV thumper with a single capacitor of 2.22 microfarads, we would have quite a respectable energy output of 1000 Joules. But this energy would only be available at the maximum output voltage of 30kV. If we reduced the output voltage to 1/2 or 15kV (say this is because our cable is old, or we were told not to exceed 15kV, and the cable rating is 15kV maximum), then our energy would drop to one quarter, or only 250 Joules. This is because voltage is squared in the foregoing equation. Many fault locating specialists agree, based on extensive experience, that 200 Joules is the minimum acceptable thump energy for reliable fault detection. This means that if we dropped the voltage of this hefty 30kV thumper much below half voltage, we could have considerable difficulty in locating a fault. Say we took the voltage down further, to 1/4 or 7.5 kV. We would then only have 1/16th the original energy, or a mere 62.5 Joules - inadequate for fault locating.
This is the main reason why so many thumpers get cranked up to excessive voltage levels (relative to the cable under test) during fault location. But fortunately, there is a solution!
Capacitor Switching and Constant Energy Thumpers
Thumpers are now available with multiple voltage ranges and switchable banks of capacitors - typically reconfigured as the voltage range switch is moved, say from 15kV down to 7.5kV. In the example, as this switch is changed to half the voltage, the internal capacitor bank quadruples its capacitance. Although this is often accomplished by combination series/parallel switching, a simplified version is shown in Figure 4.
Although not illustrated here, still other thumpers are available with 3 voltage ranges, providing also 3 capacitor bank configurations - each quadrupling the capacitance of the previous higher voltage range. This provides for a much more useful, constant output capability for more efficient thumping. The curves for the two range thumper described above, are shown in Figure 5.
It can be seen from the graph that there is far superior voltage coverage at the lower voltage extremity, which is precisely where most primary voltage cables fail the majority of the time. When 15 or 25kV rated cables fail, they will generally show voltage weakness at 7.5 or 15kV. This is because they are usually designed to carry system voltage of about half their rating. Hence they will usually fail during cable testing at these lower voltages. Should they require some persuasion about 15kV at the outset, the cable usually weakened by a short application of up to 40kV from the DC Hipot function, which the 2 range thumper is also equipped with. Although the capacitor energy curves do have considerable (natural) drop-off in energy at their lower voltage ends, they still manage to give extremely good output - at above 200 Joules, right down to about 5kV. This covers a considerable number of older PE and PILC cables still around today. If these are to be fault located effectively, a capacitor switched thumper with good low voltage thumping capability is essential.
Summary
Capacitor switching provides a far more useful thumper output, making fault location on all cables - but especially 5kV or older service aged cables, much less risky. The temptation to drive voltage to dangerous levels is reduced dramatically with this technology, while at the same time improving safety to personnel.
Walter Wittig is a cable testing specialist. He has been involved with cable testing and fault locating for the past 19 years. He heads Cable 3000, a company which specializes in training, consulting in cable testing and sale of test equipment.