Cable Fault Locating-like a challenging, strategic sport, requires a good working knowledge of the game's basics and an understanding of the opponent's shortcomings-as well as your own team's. Here's a tip to help play the game.
Many utilities have a thumper in their arsenal of primary cable fault locating equipment. This is because it is usually considered to be the most important first test set after a Megohmmeter to acquire in dealing with primary cable faults.
Although some organizations consider a ground gradient (or pool of potential) tracer as a good all-round fault locator - which it is, it may fall short in some applications. Although they shine in most direct buried secondary fault locating tasks, they will be severely limited with non jacketed concentric neutral primary cables - which is the true domain of the thumper.
This is because most primary cable faults - especially on modern XLPE cables tend to be high resistance faults. This makes it hard for significant ground gradient current to find it's way out to soil - in fact, it prefers to travel along the bare concentric. Also, the plastic dielectric is extremely resilient. Once a fault has flashed over, generally causing a fuse or breaker to open, the plastic immediately begins to heal itself, almost always resulting in a high resistance shunt fault from phase to concentric. With resistances usually well in the megohm range, this often becomes an insurmountable challenge for the typical low voltage ground gradient detector. The application of a thumper's higher voltage output usually causes the fault to break down, resulting in a discharge or explosion underground of such a magnitude that it can be sensed at the surface - many times as a dramatic vibration felt through the feet while standing above. The physics of this explosion are discussed in detail in the last issue of CFL TEK TALK.
Those who have used a thumper would be quick to tell you that it's easy to find all thumps. Many are quieter, because of the nature of the fault, cable depth, soil makeup, encasing conduit, asphalt or concrete surfaces - or just being on the wrong cable path ... it happens often ! The cable may also be so long that the tedious task of listening gives way to longer and longer distances between pauses for listening to the thump, so that it becomes likely you'll miss hearing the fault entirely. Some faults can only be heard 1 or 2 meters away, so if the detector only gets put onto the ground every 3 or 5 meters, you'll walk right by the fault. In these challenging situations it is extremely useful to have the helping electronic hand of a good seismic detector or microphone.
In years gone by, shovel sticks and traffic cone listening "devices" gave way to passive stethoscope type units, then electronic units were perfected. Sound intensity or loudness was one of the main ways that you could discern where the thump was. However, this could be misleading if the sound were channelled and sounded louder several meters away from the true fault point. This is likely to happen when cable goes under a roadway, coming out of duct on either side, as is common practice in many areas. The sound is usually louder at the ends than directly over the fault somewhere in the middle of the roadway, due to channelling. Unless the roadway is extremely wide, the ducts collapsed or filled, etc. this is not always a huge problem. However, it illustrates the channelling problem well. Concrete, asphalt or frost on the surface have a similar effect. Hence, it's easy to see that sound loudness may not be the best judge of the fault's true location.
The Answer? Thunder & Lightning Principles
Most of us have probably witnessed a distant lightning flash, then heard the sound (or vibration)
of the thunder clap some seconds later. This happens because light travels much faster than sound.
Counting the seconds between the two events allows us to approximate the distance to the
lightning strike. This same principle is used by the Digiphone Pinpointer to measure the dynamic
energy existing around an underground cable fault. It is the most precise unit of it's kind available
today.
Essentially there are two forms of energy of interest to us for underground thump location. The first is electromagnetic, due to the instantaneous rush of underground current produced by the thumper when it's capacitive output pulse (or discharge) flashes over at the fault point. This travels about as fast as the light from the lightning bolt. The tremendous surge of energy across the fault produces a second useful release of energy - the seismic or mechanical jolt caused by so much energy. This travels more slowly, finally reaching the surface fractions of a second later. Any energy travelling some distance away from the fault - either back toward the thumper or in the other direction, will have an even longer delay than the energy travelling directly up to the surface. Hence it is extremely easy to see where you are relative to the fault, based on the distance to fault display of the receiver.
The advantage of this Pinpointer is that it is extremely easy to see when you are approaching the fault - the display shows successively lower readings. Over the fault, the display reaches a minimum (representing cable depth) and then as you move away, shows successively higher readings. Many of the readings can be taken when there is little or no thump detectable by other means. Cable path can be verified simply by observing when the electromagnetic pulse strength displayed gets stronger or weaker. In conduit or other channelling situations, it is no longer sound loudness that leads you to a fault- but rather, the shortest delay as displayed by the distance to fault meter. It's foolproof, fast and efficient.
Summary
Thump fault location has come a long way since the early days of seismic and other detection
devices. Modern technology now allows precise location of thump faults that often could not be
found any other way. Distance readout relative the fault point help tremendously in pointing the
worker in the right direction.
Walter Wittig has been involved with cable testing and fault locating for the past 19 years. He currently heads Cable 3000, located in Ajax, Ontario, specialising in training, consulting in cable testing and sale of test equipment.