Damage prevention to buried utilities is more important today than ever been before. Utilities for communication and power are commonly buried to the benefit of our municipal skylines. But the fallout is a tangle of lines and pipes below the surface without any common way of managing the infrastructure. The resulting method of management has been to actively seek out buried utilities each and every time a dig or repair is required. Until the day comes when accurate maps are maintained, utility locating will be an important part of the repair and damage prevention process. The question becomes, ‘How can accurate locates be performed each and every time?’

You are the locator, and you are the most complex locating device available. It is important to remember that the locating device you carry is nothing more than a tool. The first steps to accurate locating do not include using the locating device. Take a moment to survey the location. Where are the risers and boxes? Are there lines visibly going into the ground? Is the ground sandy and rocky or dark soil? Paying attention to these clues will help to determine the proper frequency, power setting and best location to begin tracing. It is up to the locator to take the beginning steps that create fast and accurate locates.

Field Testing for Consistency

Accurate locating without a properly calibrated utility locator is nearly impossible. Field testing is a simple way to ensure your locator is working its best. Both the receiver and transmitter should be regularly tested at a known location. Field testing the receiver checks for accuracy over the line, consistent signal strength and depth. When testing the transmitter, check for correct loading, that the batteries are charged, and that there are no shorts in the leads of the connectors and induction clamps.

Begin the field test by setting up where a known utility has been previously marked. Using a direct connection, verify a centerline and measure depth. For locating devices with push-button depth, verify the depth finding using the 45º method. It is very important to note that the only way to completely verify a depth reading is to expose the utility. When your results vary more than 5% from previous field tests it is time to have the locator calibrated by a professional test and repair group. Most manufacturers recommend sending in the receiver once a year for proper calibration. Consider letting a professional calibrate the locator more often when it is shared, has been mishandled or been exposed to extreme moisture.

The Advantages of High and Low Frequencies

Locating transmitters and receivers work together to apply a tracing frequency for the receiver to pick up. Frequencies vary by manufacturer and there are a number of frequencies to choose from. Each frequency, from low to high, has its specific benefits. The problem is that improper frequency selection can create gross marking errors and costly mistakes. Having a multiple frequency locator available is important and helps ensure you have the best frequency for the conditions at the site. Low frequencies start around 512 hz and high frequencies top out at over 100 kHz. To get the most of your frequency, make sure you have a good ground for the signal to return.

Start your locate with the lowest frequency. Low frequencies work best on good conductors like cable, telephone and power lines, and will go farther down the line than high frequencies. They also have the least tendency to jump, or couple, to nearby utilities making them best for areas congested with other utilities. Marking the wrong utility because of coupling is one of the most common locating errors. Low frequencies do not work well with lines that have sharp turns, are corroded or have sheath faults. Because of their tendency to not couple, low frequencies are not recommend for using an induction clamp to apply signal to a line.

As your locate progresses, switch to higher frequencies. High frequencies are best for getting over hurdles on the line. A hurdle may be a sheath fault, poor grounding conditions, or sharp turns in the line. High frequencies are also used with induction clamps to purposely couple the signal onto lines that cannot be directly reached. Induction clamps come in a variety of sizes and even include transmitters that induce onto lines by placing them on the ground. Because of their ability to couple, high frequencies are advised to use in congested areas. It is possible to provide some protection from mis-marks due to coupling. Locators that provide a “current measure” feature can verify the line with the actual connection from a line that the signal has coupled to. Most important to remember, is that high frequencies have a shorter straight line tracing distance than low frequencies.

Most locating transmitters provide multiple power settings. Frequency choice is always more important than power choice. The right frequency at 200 mw may serve you better than the wrong frequency at 3 watts. FCC regulations also limit the amount of power that can be put behind higher frequencies. Even though your transmitter can produce a high power output, you will likely not be using all the power available. Transmitters that automatically set the impedance match the power output to the frequency to ensure that the correct amount of power is placed on the line.

Strong Circuits Finish Depend on Grounding

In order to get a tracing tone on a line the current from the transmitter must be able to go to ground and the frequency must be able to conduct down the line. To create this, connect the transmitter to one end of the line, ensure the line goes to ground at its end, and the properly place the transmitter ground rod to create the physical circuit required to locate. Isolate the target utility by unbonding it from other lines to create maximum signal placement on the line. In all cases, the more and larger ground you can create, the more signal available for the receiver.

Problems can arise from improper transmitter ground rod placement as well as soil conditions at the ground rod. Rocky and sandy soil conditions limit the ability of the signal to return to the transmitter ground rod. It is possible to increase the grounding conditions in dry rocky soil by pouring water at the point of the transmitter ground. Proper ground placement is also essential. The best ground rod placement is 90º to the transmitter, further than 10 feet from the transmitter and away from other lines. Placing the ground too close to the transmitter can create destructive interference and lessen the signal on the line.

Good grounding rods are made of highly conductive materials and have sufficient surface area in contact with the soil. It is possible to use other grounds of opportunity such as fence posts, stop signs, and parking meters. If the ground of opportunity is too large to hook to, use a c-clamp to wrap around the ground post and connect the transmitter ground clip to the c-clamp.

Final Thoughts

An accurate and fast locate depends on many factors. Training for the user and using devices with the proper capabilities will always be the best way to profitably and safely locate buried utilities. It should be remembered that locating devices are delicate pieces of equipment and require proper handling to extend the life of the device. When in doubt it is always best to verify the results of the initial locate by an experienced user.