A Guide to Pipe and Cable Locating

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Not all utilities are this easy to locate! But there is a way to learn for certain where the utility is located.

The simple procedure of tracing buried pipes and cables comes in handy in outside-plant environments, where you need to know the location of a cable before the backhoe rips up the earth near the buried cable. Using a good pipe and cable locator and soft excavation reduces damage to our infrastructure!

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When first introduced approximately 40 years ago, underground locators needed to do little more than find buried water, gas, or sewer lines. Today, locating has become more complex as telecommunications cables join utility lines in the underground environment. Surprisingly, though, today`s underground cable locators rely on the same basic technology found in their early counterparts - injecting an electrical signal onto the located cable. Locators can also find problem-stricken telecommunications cables.

What sets these tools apart from their inside-plant counterparts (tone generators and probes) is that they need to be able to differentiate the target cable from other nearby cables and underground utilities, as well as provide an estimate of depth.

Professional and reliable Pipe and Cable Locators are utilized to locate fiber optic cables, CATV cables, gas lines, power lines, and sewer and water lines. The need to locate the exact location and depth of a pipe or utility cable is important information for the safety of the end-users line or the protection of damage prevention while digging in or around the work area.

Pipe and cable locators come in a variety of types and kHz’s, and some even come with w/fault locator capabilities that allow you to find the exact break in a cable line.

Pipe and Cable Locators Variety

Composition of an underground cable locator

Since its development, the cable locator has generally consisted of two parts - a transmitter and a receiver. The transmitter puts an electrical signal onto the traced cable or pipe, while the receiver picks up that signal, allowing the locator operator to trace the signal`s path and follow the located cable.

The electromagnetic field created by the transmitter can usually be set to a specific frequency, depending on the type of conductor in the cable. Frequency choices can range from less than 1 kilohertz to about 480 kHz.

While the basic technology for locating hasn`t changed much, the ways of getting the signal onto the cable have. In addition to passive signals, which do not require the use of a transmitter, the three most common techniques of sending signals are direct connection, general induction, and inductive coupling.

Types of Signals and Locating Techniques

A passive signal is one that is naturally occurring around a conductor, or in this case, an underground utility. Some examples of passive signals include the following:

  1. Current flowing along an electric supply cable.
  2. Earth return current from power systems that use metal pipes or cable sheaths as a convenient conductor.
  3. Radiofrequency currents from very low frequency (VLF) radio transmissions that have penetrated the ground and flow along with a buried utility.

A passive sweep is performed to search for inaccessible, abandoned, or unknown utilities using only a receiver. To perform a passive sweep, a survey grid is traversed in “power” mode, with the receiver blade in line with the direction of movement and at right angles to any utilities that may be crossed. When the receiver indicates the presence of a utility, it is pinpointed, traced, and marked. The sweep is then continued until all detected utilities have been marked and the entire grid has been traversed in both directions. After completing the sweep, the entire process is repeated in the “radio” mode to search for utilities that radiate VLF radio signals.

Passive signals enable utilities to be located but not identified, as the same signal may appear on multiple utilities within the grid. To solve this problem, an active signal must be applied to each individual utility line.

An active signal is one that is intentionally generated by a transmitter. In this mode, the signal can be applied directly to the utility via direct connection or induction. This enables utilities to be identified and tracked, and their depth to be determined with a receiver.

Direct connection involves plugging a connection cable into a transmitter output socket and connecting directly to the target line. This can be accomplished with connection clips (and leads) or with a transmitter clamp. Connection clips are generally used to apply a signal to metallic conduits, site lighting structures, and metallic pipes. This is the preferred method for locating secondary electricity, water, and gas.

Many electric, telephone, and cable lines are housed within plastic conduits or buried into the ground without protection. In addition, directly connecting to these lines is usually too risky or forbidden. In such cases, a transmitter clamp is used to apply a signal to the cable without interrupting service to the line. The clamp is easy to apply, but the signal may not travel as far as it does with connection clips and works best if the target line is grounded at each end. This is the method of choice for locating primary electric, telephone, and cable lines.

Pipe and Cable Locators Clips and Leads
Pipe and Cable Locator's Clamp

If an active signal cannot be applied to a line because it is inaccessible, a general induction sweep must be performed. The transmitter contains an antenna, that when placed on the ground directly on top of a utility line, can induce a signal into it. The advantage of using induction is that a signal can be applied without access to the line, and it is very quick and easy to use. The disadvantages are that induction efficiency is poor on deep targets, it is only useful at depths down to 6 feet, and the signal can induce into lines other than the target. In addition, signal strength is often lost in the surrounding soil, the signal is shielded by reinforced concrete, and a signal will not apply to a well-insulated line unless it is effectively grounded at each end. Despite its shortcomings, an induction sweep can sometimes successfully locate unknown or abandoned utilities when GPR results are inconclusive.

An active signal cannot be applied to non-conductive (non-metallic) utility lines. To combat this, a detectable duct rod or self-contained transmitting sonde must be inserted into the line via a manhole, handhole, cleanout, or catch basin. The disadvantages of this method are that some non-metallic utility lines do not have access points or might be obstructed by detritus. Nonetheless, this is the best method for locating fiber optics, future use lines, sanitary and storm sewer.

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