Whether a cable needs to be located before excavation to prevent damage or a damaged cable needs to be located for repair, tracing buried cables is an art. Arming yourself with an in-depth understanding of the principles of locating will make trouble spots in the field child’s play.

Challenges of Private Utilities

Buried utilities are located by the utility company when a dig notice is called in, but this only covers buried plant owned by the utility. All utilities after the meter are private property and the property owner’s responsibility to locate. These private power lines and communication cables pose special maintenance challenges for facility owners.

While the utility companies keep accurate maps, standardize uniformed methods of installment and install easy access points, private utilities are often unmapped, installed by different contractors in a variety of ways and may be virtually hidden.

Becoming the Locator

You are the locator - the most complex instrument on the market. The instrument is just a transmitter and receiver. Become familiar with your choice of cable locator. Read the instruction manual and practice on a known utility regularly. Obtain any information before starting a locate. Maps, types of installation, access points and types of cable are useful in facilitating a quick and accurate locate.

At the site, look for clues indicating buried plant like junction boxes, utility cabinets, conduits dipping underground and old trench lines. Identifying all buried and overhead utilities, including non-target utility lines, will aid in pinpointing the target cable.

Passive and Active Locating

The type of buried utility and your needs of the locate will determine the method of cable locating, the method of signal application, and the frequency choice.

The two methods of locating are Passive locating and Active locating. Passive locating is locating lines without the use of a transmitter. The receiver is utilized by detecting ambient frequencies emitted from the utility.

Active locating is using a transmitter. The transmitter applies a frequency, or tracing tone, to the cable. The injected signal is then detected by the receiver.

Passive 60 Hz locating detects the 60 Hz emitted from power distribution lines in the United States, 50 Hz cycle in Europe. By simply setting a receiver to the 60Hz function the user can sweep an area detecting power lines with current flowing. A load must be on the line for this mode to work and 60 Hz may not be detected even with a load. Also, due to electric lines being grounded to water lines or common grounding shared by telecommunications and cable television the 60 Hz signal may be present on many non-target utilities. Since the signal may be present on all lines it is not possible to differentiate between lines.

Passive Locating is a fast method of locating lines and sweeping an unknown section, but its limitations are great enough that a locator should never rely solely on a passive locate.

Active locating is locating with the use of a transmitter and a receiver. The transmitter applies a frequency to a conductor; a receiver then detects the signal. Transmitter use is the most important tool for locating. Method of connection, placement, frequency use and grounding are all components of successful transmitter use.

Understanding Circuits

 The key to successful transmitter use is understanding that a circuit must be created to apply a tracing tone.

A circuit is the complete path of an electric current including the source, the metallic conductor, the grounding of that conductor, and the return path to the independent ground of the transmitter.

The source, the transmitter, sends the signal down the cable. This current must go to ground at the far end of the utility and return to the source via the earth and finally to the independent ground of the transmitter. Even during induction when the transmitter is not independently grounded, a circuit is created. The better the circuit created the easier and more accurate the locate.

Methods of Signal Application

The three methods of applying a tracing signal are conduction, coupler induction and transmitter induction. This simply translates to using the clips, the clamp or the box.

Direct Connection

Direct connection (clips) is directly connecting to a line with one of the tests cords from the transmitter going to the cable and the other test cord going to ground stake.

Direct connection is the ideal method of putting signal on the line. This method applies the most amount of signal and allows the user to use all available frequencies.

When possible, it is best to un-bond the line at the end of which you are connecting; connect the positive clip to the wire; insert the ground rod for your transmitter and make sure the other end of the line is grounded or bonded. If possible check the continuity of the line; check the signal from the line; and you are ready to go.

With Direct Connection you may connect to a ground rod on a dip, utility box, un-bonded ground cables at a pedestal, gas meters, water meters, water or gas valve, neutral wire, or tracer wire. Do Not Directly Connect To An Energized Line.

The transmitter’s ground is crucial in creating a good circuit. The independent ground creates a return path for the signal, thus completing the circuit.

More Ground = More Signal

Less Ground = Less Signal

The grounding rod should be made of highly conductive material and have sufficient surface area. The ground rod can overcome less ideal soil conditions that inhibit the signal’s return path.

Soil conditions affect a good ground. Moist soil means better ground and return path. Rocky soil will provide a poor ground and return path while black earth provides a better return path.

If a line is energized and there is not access to a common ground shared by that cable than a coupler may be used to apply a tracing tone.

Coupler Induction

When services cannot be interrupted (i.e. power lines) a coupler may be used to induce the tracing signal. Couplers work well with higher frequencies, but a lower frequency also may induce.

Both ends of the line being coupled must be grounded or bonded in order to create a circuit with a return path. Clamp the coupler around the cable and set the transmitter to the specified coupler frequency. The signal strength of a coupled line will not be as strong as a line directly connected to, nor will the signal carry as far, but this method works well when attempting to apply a tracing tone to power utilities.

If a three-phase power cable is wrapped together, the signal will cancel out due to the circular spacing of the three phases; couple individually to the ground or neutral.

Transmitter Induction

Transmitter induction allows the user to induce a signal through the soil directly to a buried line simply by setting the transmitter on the ground over the cable.

Inside the transmitter there is an inductive antenna. By placing the box (transmitter) over the cable and setting the transmitter to the highest frequency the signal will induce on the line below. A high frequency must be used in order for the signal to penetrate the earth and induce on the line.

Induction shotguns the signal into the ground inducing all the buried lines below it and the transmitter may even induce overhead lines. It may be hard to identify lines using the induction method. It is best to start where a line surfaces. This will help verify the line.

In each of these methods there is a current transmitted to the line, a path down the line to a ground, and a return path to the start of the circuit or signal.

Both Induction methods are circuits even though the transmitter is not grounded.

If there is a poor return path (i.e. dry rocky soil) then you have a poor circuit.

If there is not grounding or insufficient grounding at the end of the line then there is no place for the current to go to ground. This means no current (tracing signal) on the line.

If the direct connection is bonded at the utility box then the signal will travel down all lines bonded, thus decreasing the signal strength on your line and increasing interference from other lines.

Frequency

Frequency selection has been mentioned in each of the methods of signal application. Frequency selection is a major tool of cable locating and identification.

Frequencies used by locators fall into three classifications: low frequency, mid-range frequency and high frequency. Depending on locator brand, lower frequencies are below 10kHz, mid-range frequencies fall between 10Khz and 40kHz and high frequencies range from 40kHz to 500kHz

No one frequency is perfect for all locating scenarios. Going out armed with multiple frequencies is the best bet.

High frequencies are great at getting over “hurdles” on the line. A “hurdle” may be a sheath fault, rubber gaskets on water pipes, eroded in a trace wire, poorly conductive cast iron pipes, rust on a pipe, poor grounding conditions, or any other problem in getting a signal on the line. High frequencies are also used for induction or coupling. Te higher energy allows the signal to induce onto a line.

High frequencies are not ideal for congested areas (multiple lines next to each other), long distances, or any area where bleed over occurs. The high frequency’s ability to jump onto a line also is a hindrance. High frequencies will self-induce or bleed over onto nearby lines, while low frequencies stay true to their lines.

Low frequencies work well on lines that conduct easily (i.e. cable TV, telephone, and power lines). Low frequencies are more likely to stay true to their line because they are less likely to bleed over. This makes them ideal for congested areas.

Low frequencies also work well for long distances. But, unlike high frequencies, a low frequency will not pass through hurdles, transmit down a poor conductor, or induce.

Power Selection

Locators are available with power output ranging from one tenth of a watt to 10 watts. More power is not the key. Frequency choice creates the best signal.

The right frequency at 200 milliwatts may serve you better than the wrong frequency at 3 watts.

Now that the transmitter is hooked up by one of three methods and the frequency is set, you are ready to trace the signal with the receiver.

First, set the receiver to the transmitted frequency and select the desire signal reception mode. Most receivers come equipped with both peak and null modes for locating.

An antenna in a horizontal position will peak when over the target line, with the signal fading away on each side of the line. If the antenna is vertical, the signal over the line will null while toning sharply on each side of the line.

Peak is more accurate than null. With interference, null may be drawn off slightly to one side or another. While working in high interference situations (i.e. near power lines), use peak mode.

Adjusting the receivers gain increases or decreases the amount of signal that is being filtered. By manually adjusting the gain, a locator can gain out or tune out false signal, ghost lines or other interference. Typically the gain is tuned high for null and tuned low for peak. The gain in peak must be turned down so the signal is not peaking off the scale. It is best if the tone is only heard directly over the line or only about one foot or less side-to-side of the line. The gain in null mode should be turned up to the point where the quiet space over the line is only about three inches.

During the initial sweep it is best to have the gain set high and continue to adjust it down until the target line is detected, then adjust the gain to your desired setting.

Now that the receiver is set to the correct frequency, a locate mode has been selected, and the gain is tuned in, walk 10 to 15 feet away from the transmitter to start the initial sweep. Walk a half circle or complete circle around the transmitter’s connection point, tracing your path with the receiver. Continue to sweep the entire area even after a line has been detected. This will ensure only the target cable is carrying the tracing tone.

Follow the tracing tone back to the transmitter to again ensure the signal is on the target conductor. Now walk the path of the utility letting the receiver lead the way.

As a conductor runs deeper under ground or rises to a shallow depth the signal strength will vary accordingly. If the conductor is a uniform depth then the signal strength should remain fairly constant over the length of the locate only decreasing slightly as the signal weakens over the distance.

Determining Depth

Depth may be determined by two separate methods: triangulation and push-button digital depth.

Both methods of measurement have a margin of error. No depth measurement is 100 percent accurate. All depth measurements are estimates and may be erroneous. The only way to determine the exact location and depth of a utility is to expose the line and verify visually.

Push-button digital depth works by utilizing two antennas at a fixed distance within the receiver. The receiver takes a signal strength reading from the two antennas, calculates the difference in strength, and then calculates the depth of the cable. For good measure, 45-degree depth or triangulation should be used to help verify a digital depth reading.

Triangulation is achieved with the receiver in null mode. Mark the point you wish to measure directly over the line. From directly over the line the receiver is turned 90 degrees from the line. The receiver is then brought to a 45-degree angle. A tone will be heard. Drag the receiver away from the line until the tone nulls out. The distance between the receiver and the mark is the depth of the line. A false depth reading may be caused by nearby buried metallic objects, such as a second cable or pipe, sewer, fence, or railroad track. Confirm your depth measurement by repeating the above steps on the opposite side of the pipe or cable.

Understanding the principles of locating and how locating equipment operates will aid you in quickly and accurately finding utilities. The best way to stay confident in your locates is to be familiar with your equipment and the utility system you are locating.

RYCOM Instruments, Inc. headquartered near the Greater Kansas City, Mo., area, designs and delivers locating equipment for the world's largest and smallest utility companies. Backed by years of successful research and development, RYCOM offers product lines specialized to meet any locating need. For more information on RYCOM, visit us at www.rycominstruments.com.