Electronic locators and fault finders are essential tools for personnel who install and repair underground electrical and communications cable.

Correctly used, locators accurately identify the location and depth of underground cable, even when it is inside conduit, and fault finders pinpoint breaks or faults in underground cable. In some situations, equipment may be able to perform both functions; in other conditions, separate tools are required.

Electronic locators and fault finders are essential tools for personnel who install and repair underground electrical and communications cable.

Correctly used, locators accurately identify the location and depth of underground cable, even when it is inside conduit, and fault finders pinpoint breaks or faults in underground cable. In some situations, equipment may be able to perform both functions; in other conditions, separate tools are required.

Locators can also be used to find or confirm locations of water and sewer service lines, natural gas pipe and other underground utilities before making excavations or installing new utilities by trenchless construction methods.

While the technology is not new, evolution and improvements make today’s equipment more versatile and much easier to use than early models. Current models do a much better job filtering out extraneous signals on job sites cluttered with utilities.

“The basic principles used to locate cables and pipes have not changed for many years,” observed Guillermo Warley, vice president for product development, Schonstedt Instrument Co. “But locators today use the latest technologies to increase battery life, decrease weight and make them more user friendly. The increased use of digital signal processing (DSP) produces incremental improvements in accuracy, but of course there are many factors beyond the control of the instrument that can affect accuracy.”

Information provided and ease of use are a big difference today, said John Archambeault, locator product manager at McLaughlin Mfg. Co.

“Some models,” he said, “can automatically scan frequencies on a line and passively locate it. Today, on-the-job programming must be simple, and some units are so well made that calibration is almost unnecessary. Durability, ease of use and service are all better.”

“Combination locators and fault finders are an advancement that has been significant to the industry,” added John Bieberdorf, Ditch Witch electronics product manager.

Locators

A cable locator has two basic components: a hand-held receiver and compact transformer. Both are battery-powered, relatively light and housed in weather-resistant cases. The receiver locates underground lines by detecting magnetic fields created by electrical current passing through cable or tracer wires. Information is displayed in a window at the top of the receiving unit.

Receivers use different frequencies and modes to identify different types of utilities. The receiver unit is all that is needed to locate live electrical cable and in some instances, the receiver alone may be able to locate passive signals of communications cable, although the transmitter unit may be connected to induce a stronger signal.

“Electromagnetic locators can easily find the cable in conduit and sometimes find opens and shorts,” said Jim Walton, Radiodetection’s vice president of sales. “They detect an electromagnetic field radiating around the cable that either already exists from 60Hz power current or reradiated VLF radio signals or are created by a specific signal generator tuned to a known frequency which has been applied to the electrical cable by direct connection, signal clamp, or induction.”

In addition to location, information provided may include strength of the signal, direction of cable, current amperage of the signal at the cable, depth of the cable and current direction of the signal, depending on make and model.

Some transmitters have built-in ohm and voltage meters to measure resistance and voltage on a given utility, showing the integrity of the utility, added Archambeault.

With a variety of features available, buyers should select locators tailored to their specific needs, because extra features add to the cost, advised Jim Carefoot, product manager at Tempo, a Textron company.

“Locators,” he continued, “must reliably follow a path down to six feet in depth, have both a clamp-on and direct-connected transmitter, and be durable.”

“If most of the work is to locate the same type of utility under very similar ground and site conditions with the same levels of congestion, a single frequency instrument may be the way to go,” said Schonstedt’s Warley. “At the other extreme, if a variety of utilities are being located under many different circumstances and conditions, then a multifrequency instrument with various ways to connect to cable or pipe would be the way to go. In either case simplicity is a timesaver and a plus. At equal performance levels, the instrument that is easier to use will increase productivity and accelerate the learning curve of personnel inexperienced in locating.”

Fault finders

Fault finders are designed specifically to locate faults or breaks in underground electrical and communications cable.

Said Carefoot: “A fault finder is needed when a buried electric cable trips breakers or there are other signs that there is a nick in the insulation or a cut wire. A locator will not pinpoint such a fault. When necessary, the cable path can be found with a tracing locator, then the fault is tracked down the path with the fault locator.”

Two of the most common tools are TDR cable fault locators used to pinpoint major faults or breaks in power cables inside conduit, and A-frame sheath fault detectors for ground faults in direct-buried cables, said Ditch Witch’s Bieberdorf.

“A TDR,” he continued, “can be useful when a ground fault is not present, but the cable is broken. Sheath fault locators work only when the conductor is in direct contact with the earth. TDRs are single tools and find faults only. Sheath fault detectors can locate lines as well as faults.”

Electromagnetic locators can easily find the cable in a conduit and sometimes find opens and shorts, said Radiodetection’s Walton.

“For opens and shorts,” he said, “a TDR may be most effective and will give a distance to the fault. The combination of the two tools is recommended for faults inside conduit. For larger cables, a high-impulse fault locator—often called a ‘thumper’—may be required in conjunction with a TDR. For direct-buried secondary electrical cable faults, both electromagnetic and voltage gradient methods can be found in the same tool. A TDR would be in a separate instrument. For primary electrical cable faults, an electromagnetic locator normally is used to find the cable path and then a high-impulse fault locator finds the fault.

“A TDR,” he continued, “applies a signal on two conductors through direct connection methods and is looking for a reflection created by a change in impedance. That will give cable footage distance to the fault. However, remember that cable footage is not the same as above-ground footage and, therefore, accurate locating of the cable is also essential.

“A voltage gradient method applies a signal from a signal generator to the cable through direct connection. It then has two electrode probes that are placed in the ground along the cable path every two or three steps. The fault locator gives a directional indication of where the fault is located along with a dB signal strength of the fault. Once the fault has been passed, the directional arrow will change direction. You then can pinpoint the fault normally within a few inches.”

Locators for damage prevention

The focus of this report has concentrated on the use of electronic locators and fault finders to identify problems. While these may be the primary reasons that many electrical contractors use such equipment, the same type of locators are the basic tools used by utility companies and contractors to find and mark buried utilities to avoid damage during projects requiring excavation.

The nation’s one-call system is responsible for notifying member utility companies that jobsite locates must be made, and construction personnel are not supposed to dig or conduct boring operations until utilities have been marked.

Despite growing emphasis on preventing damage to buried utilities, hits of buried utilities continue at an alarming pace, and statistics show that many are caused when utility owners fail to make locates, mismark locations, and when unmapped utilities are struck. Therefore, many contractors confirm locations with their own crews, recognizing that utility strikes are costly and potentially dangerous.

Because electricians often work on privately owned power and communications networks where one-call systems have no jurisdiction, contractors may need to make the locates.

Finding live power lines is perhaps the easiest locating task because it can be accomplished using only the receiver of the locating equipment package. Cable and pipe that do not carry electrical current are more challenging and require use of the locator’s transmitter.

“There are different means to impose the current on the pipe or cable, with varying degrees of effectiveness,” said Warley. “It is also possible to detect certain pipes and cable without using a transmitter. In this case the receiver tries to detect fields created by currents already flowing on those cables or pipes, like 60Hz or certain radio signals that reradiate from underground pipes and cables acting as elementary antennas.”

A careful visual inspection of a job site is the first step in making locates or confirming accuracy of those made through the one-call system.

“Physically identify the presence of other utilities by noting boxes, pedestals, vaults, meters, etc.,” said Carefoot. “Then use locating equipment to trace the path of each line as is done for electrical cable.” EC

GRIFFIN, a construction and tools writer from Oklahoma City, can be reached at 405.748.5256 or up-front@cox.net.