Without cable fault indicators the utility must rely on more manual techniques for finding the location of a fault. There are a large number of different types of fault-locating techniques and a detailed description of each is beyond the scope of this report. Some of the general classes of methods follow.
Thumping
This is a common practice with numerous minor variations. The basic technique is to place a dc voltage on the cable that is sufficient to cause the fault to be re-established and then try to detect by sight, sound, or feel the physical display from the fault. One common way to do this is with a capacitor bank that can store enough energy to generate a sufficiently loud noise. Those standing on the ground on top of the fault can feel and hear the “thump” from the discharge. Some combine this with cable radar techniques to confirm estimates of distance. Many are concerned with the potential damage to the sound portion of the cable due to thumping techniques.
Cable radar and other pulse methods
These techniques make use of traveling-wave theory to produce estimates of the distance to the fault. The wave velocity on the cable is known. Therefore, if an impulse is injected into the cable, the time for the reflection to return will be proportional to the length of the cable to the fault. An open circuit will reflect the voltage wave back positively while a short circuit will reflect it back negatively. The impulse current will do the opposite. If the routing of the cable is known, the fault location can be found simply by measuring along the route. It can be confirmed and fine-tuned by thumping the cable. On some systems, there are several taps off the cable. The distance to the fault is only part of the story; one has to determine which branch it is on. This can be a very difficult problem that is still a major obstacle to rapidly locating a cable fault.
Tone
Atone system injects a high-frequency signal on the cable, and the route of the cable can be followed by a special receiver. This technique is sometimes used to trace the cable route while it is energized, but is also useful for fault location because the tone will disappear beyond the fault location.
Fault chasing with a fuse
The cable is manually sectionalized, and then each section is reenergized until a fuse blows. The faulted section is determined by the process of elimination or by observing the physical display from the fault. Because of the element of danger and the possibility of damaging cable components, some utilities strongly discourage this practice. Others require the use of small current-limiting fuses, which minimize the amount of energy permitted into the fault. This can be an expensive and time-consuming procedure that some consider to be the least effective of fault-locating methods and one that should be used only as a last resort. This also subject send users to nuisance voltage sags.
