How to Choose Buried Cable Detection Systems?

Instrument used for detecting underground cables

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A cable locator or cable avoidance tool (CAT) is an instrument used for detecting the presence and approximate location of buried services in advance of undertaking excavation works. It aims to avoid accidents while excavating.[1] A number of types of detecting technology can be employed by such instruments, including use of magnetic fields, radio frequencies, signal generation, metal detectors, ground-penetrating radar and RFID.[2]

Description

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Underground cables are one of the things that enable telecommunication and power transmission. They are especially beneficial to densely populated areas, particularly those locations where overhead cable posts are unavailable or are not ideal.[3]

Locating underground cables—as well as other underground facilities—is an integral pre-excavation process mandated by laws and guided by a number of industry standards.[4] During excavations, underground cables become at risk of getting damaged. In fact, utility strikes are a common occurrence on construction sites, resulting in compromised worker and public safety, repair costs, and work delay, among others.[5]

Based on the 2018 Damage Information Reporting Tool (DIRT) Report of the Common Ground Alliance (CGA), some of the most common root causes of utility strikes include failure to call 811 before digging, improper excavation practices, and inaccurate marking of underground facilities.[6]

Furthermore, the CGA also noted that locator errors are one of the common causes of strike incidents. This emphasizes the need for understanding how cable locators work as well as how different cable types can be most accurately detected.

Detection Methods

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Different cable locators employ different methods to function. The two known methods, Passive Utility Detection locating naturally present or naturally produced signals known as a passive signal. The other is to locate a signal that is applied AKA an Active signal ,[7] are called the Passive Utility Detection and Active Utility Detection.

It’s important to note, however, that while both methods can help locators distinguish an underground utility, they do not confirm its presence nor its absolute location. Furthermore, an unmarked ground is not a guarantee that there are no utilities underground.[8]

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by James Caviness

Nothing stops a job faster than breaking ground only to find out you have some underground cables that were not accounted for. Not only is this a dangerous situation, but damaging an underground utility can be an expensive mistake. Fortunately, there are tools available that can prevent this from becoming a more serious issue and prevent it from happening again in the future. Cable locators come equipped with the tools needed to locate just about any underground cables depending on the specific application- even still, some cable locator systems are able to locate just about any type of pipe. Knowing how these units operate and using them effectively can ultimately prevent injury, as well as high costs from damaged utilities.

Cable locators are used for locating underground utilities such as telephone lines, fiber optic cable, an individual power cable utility line, water pipes, electrical wire, gas pipelines, power, or sewer lines. These cables are buried deep below ground and may not always be visible. When digging up a lawn or driveway, a cable locator is used to detect the location of buried utility cables before digging begins. A cable locator uses electromagnetic fields to determine the depth measurement and direction of buried a buried utility. A pipe locator system can use various communication technologies for output, including meters, displays, and audible indicators.

The cable's course must be known before attempting to locate a problem in an underground cable. If you don't know the location of the cable, especially if it is hidden under a buried wire or in a metal pipe, how can you find a fault? It is not unusual to find maps of the utility cable routes included in the documentation for a project. It is for this reason that good documentation is important for every project you are working on. In the absence of a chart of the path of the cable, you will have to find and trace it on your own. There are several reasons why it is necessary to locate the underground cable, not just to detect faults but also to be able to excavate near utility paths when there is an excavation planned. As the title suggests, this article will talk about the various kinds of underground cable locators and how they work.

What Is a Cable Locator?

A cable locator is a specialized tool used to detect the presence of an underground cable, underground pipe or utility. These pipe locator units are used to prevent injury or damage caused by striking an underground cable while excavating. Homeowners and property managers may benefit from cable locators as well, as many older homes or lots could still have active underground utilities to consider when starting a new project.

A cable locator consists of two parts: transmitter and receiver. The transmitter sends out a signal into the soil. As the signal travels through the soil, it encounters an electrical current flowing along the path of the buried cable. The current creates an alternating magnetic field around the cable. The receiver detects the presence of the magnetic field created by the cable and displays the distance and direction of the cable. In some cases, you may use An electrical power quality analyzer (PQA) to measure voltage, current, frequency, phase angle, harmonics, and waveforms to determine if there are any problems.

Underground cable locators are able to locate any pipe or cable underground as long as there is current passing through the target. Some of the best underground cable locators will be able to energize targets with multiple signals depending on the application; lower frequencies can offer a better depth of detection, while higher frequencies are able to detect breaks in the line. Selecting the right frequency for an underground cable locator can be the most important decision in finding the target. Many models will have a standard operating frequency power mode of 50/60 Hz which may be noted as 5060 hz as well.

Underground cable locators are a unique tool for a specialized job. While we all may want a system that can do it all, it is important to use the correct tool for the correct job. Other underground utility location devices, like a magnetic locator used for iron or steel pipes, will NOT be effective in tracing underground cables. With that said, many of the more popular underground cable locators could be used to energize an iron/steel target and locate it, so in some cases, it can be a benefit to just have an underground cable locating system.

Cable Locator Detection Principles

With many different types of underground utilities out there, there are just as many options for locating them. Generally, cable locators are broken down into two major categories: passive power and active. With passive detection, the cable locator is using the utility's own current to pick up on its electromagnetic field. Active locators will have their own power source to energize the targeted cable. Then, a specific frequency of the signal is sent through the line that the locator can then trace out. Comparing the two, passive locators can be a benefit if the cable has current already but is essentially blind to any targets without current. It is worth mentioning that with a passive locator, energized targets in close proximity may cause issues if the locator can not tell them apart. Active locators will use their own signal frequency, so extraneous targets are typically not an issue. Active locators are best used when there are a number of targets on the ground, and accuracy is important.

Detecting Cables Using Magnetic Fields

When using an industrial metal detector or cable locator, the transmitter emits a radio frequency (RF) signal that penetrates the earth. The RF signal interacts with the electric currents running along the buried cable or buried pipes. The interaction produces a magnetic field around the cable that is detected by the receiver. The strength of the magnetic field decreases exponentially with increasing depth. The receiver measures the amplitude of the magnetic field and determines the distance to the cable based on the exponential decay rate of the magnetic field.

How to Use a Cable Locator

If the underground cable detector already has a current running through, a passive ground cable locator would work to receive the signal put out by that cable. If searching for a deadline or want to ensure precision, an active cable locator would be best. If using a multi-frequency cable locator, select the proper frequency. Energizing signal clamps are used to induce current onto the cable. The energized cable transmits the signal that is then received by the locator itself and can be traced out. If depth is an important factor, using a system with a depth indicator would be beneficial, but it is still possible to determine the target depth without it. By positioning the locator at a 45-degree angle alongside the traced-out line, it would be possible to determine the depth of the cable.

The infographic below illustrates how to use a cable locator to locate underground cables and utilities.

Step 1: Set Up Your Transmitter

Set up your transmitter according to the manufacturer's instructions. Make sure the antenna is pointed toward the area where you plan to dig.

Step 2: Turn On Your Receiver

Turn on your receiver and wait until it starts displaying the distance to the nearest cable.

Step 3: Dig!

Dig a hole large enough to accommodate the size of the cable locator. Place the cable locator near the edge of the hole so that the antenna faces outward. Press the button on the transmitter to begin locating the cable.

Types of Cable Locators

There are two options when it comes to using a cable locator: since frequency and multi-frequency locators. Single frequency locators offer one signal frequency for the target location. This setup is best for homeowners or property owners who need to locate underground utilities. Single frequency units can be a bit easier to use and can act as a fast, effective way to locate an underground cable. Multi-frequency cable locators offer more options both in terms of the types of cables that can be detected but also in specialized applications. Multi-frequency cable locators can use different frequencies in order to detect or compensate for breaks in the line or other environmental factors.

Categories of Cable Locators

There are three categories of cable locators: handheld, vehicle mounted, and pole mount. Handheld cable locators are portable devices that measure the distance to buried cables. Vehicle-mounted cable locators are permanently installed on vehicles and use the same principles as handheld cable locators. Pole mount cable locators are fixed to poles and use the same principles as handheld cable locators.

How Cable Locators Work

Cable locators function by detecting the electromagnetic signal put out by an energized line; passive detectors would locate based on the cable's own current, and active units would use their own power source to energize the cable. The locator itself is a receiver that picks up on the transmitted signal and can be used to then trace the location of the cable. Many locators will also have a depth indicator to let you know the depth of the target, as well as other features depending on the specific application and detector accessory used. An underground locator known as a wire tracker is made up of an inductive antenna, a locating cable, and an inductive antenna. With this type of device, it is possible to pinpoint cable faults in buried wires with precise pinpointing and make accurate depth measurements so that cable faults can be easily identified. The models can not locate plastic pipes underground.

• Handheld cable locators work by measuring the time required for a signal traveling along a wire to reach a detector. This time is impacted by the signal strength. By knowing the speed of light in air and the length of the wire, the distance to the cable can be calculated.

• Vehicle-mounted cable locators use the same principle, but instead of a wire, the cable itself acts as the transmission medium. The cable has a metal core wrapped in insulation. The cable is placed inside a coil that generates a strong magnetic field. The magnetic field causes the cable to act like a conductor carrying electricity. The voltage induced in the cable is measured by the coil and converted into a distance reading.

• Pole-mounted cable locators use similar technology to handheld cable locators. However, rather than placing the coils directly over the cable, the coils are attached to a pole. The pole serves as a base for the coil and allows the coil to rotate freely. The rotation of the coil changes the orientation of the magnetic field generated by the coil. The change in the magnetic field induces a voltage in the cable. The voltage is then measured by the coil and translated into a distance reading. The advantage of pole-mounted cable locators is that they do not require direct contact with the cable. Therefore, they are less likely to damage the cable during installation.

Signal Tracing Methods

The transmitter and receiver are the two main components of a cable finder. A transmitter is essentially an alternating current generator that delivers signal current to the traced subterranean line. Typically, the transmitter may tune the signal to a specified frequency. This signal generates an electromagnetic field of the same frequency everywhere around the wire, which a receiver may detect.

There are two ways in which the signal can be transmitted into the cable: either directly or through induction. The direct connection method of transmitting a signal over a cable is the most efficient method of transmitting a signal over a cable and should be utilized whenever possible. In order to complete the circuit, one end of the cable has a direct connection to the transmitter, and the other end has a ground connection to complete the circuit through an inductive clamp. Using the induction technique, there is no need for a physical connection to be made between the cable and the amplifier. By generating an electromagnetic field from above, the device sends the signal by electromagnetically generating it. A receiver picks up on the electromagnetic field formed by the broadcast signal as it travels through the cable. Above ground, at the top of the wire, the electromagnetic field may be observed. Additionally, a receiver may be configured to detect the electromagnetic field produced by already-energized wires (as in passive locating).

Single Versus Multi Frequency Locators

There are two primary kinds of cable utility locators based on the frequency of the signal they detect: single frequency locators and multiple frequency locators. Copper, aluminum, and iron all have a preferred frequency range. A multi-frequency finder enables you to fine-tune the transmitter frequency to the line's material. This makes it simple to determine what is necessary and what is not. Additionally, a multi-frequency finder aids in simplifying the procedure. The higher the frequency, the more signals are picked up, but the lower the frequency, the simpler it is to follow a specific line. Numerous tracers and locators are available that detect frequencies of 60 Hz or 50 Hz. An underground wire locator enables the tracking of live wires.

Active Versus Passive Fault Locator Technology

Cable fault locators are used in electrical power distribution systems to locate faults on cables that are connected to the distribution system. They can be either active or passive devices. An active cable fault locator uses an electromagnetic field that is generated by a current source at the transmitter end of the line. Unlike active CFLs, passive CFLs do not require any external energy supply to operate. An underground utility locator (UULs) locates buried utilities, such as pipes, cables, wires, conduits, sewers, and storm drains. Different technologies are used by UULs, including magnetic induction, ground penetrating radar, electromagnetic induction, electric field detection, and acoustic emission. A ground resistance tester (GRT) measures the electrical conductivity of soils in order to determine their suitability for construction projects. Before excavation, during construction, after construction, and when the building foundations have been laid, they can be used to test the soil.

© 2021 Detector Electronics Corp - Revised August 2022