Methods of Detection of Buried Cable

Methods of Detection of Buried Cable : In this article, we will try to know that how to detect a buried cable. Equipment provided for the detection and location of buried cables and any others metal services are known as locators. While not infallible, locators, if intelligently used, can provide a powerful support to local knowledge, plans and records.

Except when using a rodding oscillator, none of these locators can detect or locate non-metallic services. It also describes the Location Connection Point, which gives the cable locator operator access to the metallic elements of optical fiber cable and blown fiber tube.

How cable locators work – Principles of buried utility detection

Methods of Detection of Buried Cable
Detection of Buried Cable

Undertaking any excavation will inevitably bring site workers into close proximity to underground utilities (electricity, gas, telecommunications, water). Consideration should always be given to knowing the exact location of all buried utilities before and during the excavation process.

To safeguard against utility strikes, a cable locator is used for detecting the presence and proximity of buried utilities.

When an alternating current (AC) travels along a cable, an electromagnetic field is generated. The alternating current not only creates a magnetic field but also the oscillation of the current between positive and negative creates a frequency known as Hertz (Hz). The electromagnetic field generated by an AC current can be detected by a cable locator, such as the Leica Digicat 550i.

There are two main detection principles:

  • Passive location – Used to locate an electromagnetic field already present on a utility
  • Active location – Adds a specific signal using a signal transmitter onto a located utility

Passive Location

Some signals may already be present on a buried utility, either through signals radiating off a power cable or the re-radiation of a radio signals induced onto the utility from a radio transmitter.

Principles behind Power Mode

Principles behind Power Mode

Principles behind Power Mode

when an AC current travels along with the utility it generates an electromagnetic signal,Using a cable locator (Leica Digicat 550i), a surveyor can detect the buried cables’ position by searching for the electromagnetic field.
Locating electrical cables using a cable locator on its own will only allow the operator to detect utilities with a live current, like a street light cable during the night. However, when lights are turned off, no current flows and the buried electrical cable is not detectable using a cable locator.

Principles of Radio Mode

Low frequency long wave radio signals transmitted from a radio mast can pass into the ground, inducing a signal onto metallic utilities. The utilities re-emit these signals and they can be located and traced using a cable locator in RADIO mode.

Principles behind Auto Mode

Leica Digicat cable locators feature Auto mode, combining the benefit of simultaneous detection in Power and Radio modes. Auto mode helps confirm the presence of utilities on first site visits.
Principles behind Auto Mode

Active Location

Up to 60% of buried utilities could be missed when detecting in passive modes alone. Just because they are not detected in a simple sweep, does not mean they are not there and it is safe to excavate.

A signal cable locator and signal transmitter (Leica Digitex t100) will significantly improve the detection process. This small portable unit induces a signal to a cable or pipe, which can be traced by the cable locator. This is called active locating.

Applying an active signal

A majority of buried utilities may not be detected by searching for passive signals using the locator on its own. These hidden utilities may not carry a live current or radiate radio signals, requiring a signal to be induced directly onto the utility to locate them. To detect these additional utilities, an electrical current or signal will need to be applied onto the buried metallic utility, which enables the utility to be traced and identified by the locator.

Induction mode

Induction is a quick and simple way to apply a signal to a utility without the need to make any physical connection. An internal aerial generates a magnetic field into the ground. Any buried metallic utilities routed within close proximity to the signal transmitter will be induced with the signal, allowing the utility to be located and traced with a cable locator.

Connection mode

This is the most efficient way of applying a signal to a utility and should be used whenever possible (especially when taking a depth reading). The output from the signal transmitter can be directly connected to a cable or pipe. A circuit is completed by a connection to an earth stake or ground connection point.
Applying a signal directly to the utility allows the operator to positively identify and trace its path.

Locating the signal

To detect the magnetic fields emitted from a buried utility, the locator uses aerials built up of wire wrapped around ferrite rods. The aerials are used to amplify the small electromagnetic signals emitted by the utility and to provide an input to the locators’ circuitry.
Leica Digicat locators feature an enhanced Signal Strength Indicator (SSI), a graphical display detailing the signal strength as a bar graph and a numerical SSI reading. The highest signal reading (peak response) is obtained when the utility is directly below the locator.

Methods of detection

Especially in the telecommunications sector, we talk about the OSP project, before digging, we should have full knowledge of whether there are other utilities in this route. Detection of other utilities will have to be done to get the correct information. 

There are a number services which may not or cannot be detected by a receiver in its power or radio mode alone, amongst which are power cables that are dead ended, evenly balanced or non current carrying, also, traffic light control cables (evenly balanced low current), street light power cables, gas pipes, water pipes and some telecom cables.

The use of both the Transmitter and Receiver to make a thorough search of the work area greatly enhances the ability to detect these. There are several methods by which locators detect buried services.Which I will describe in detail below-

Conducted signal : The receiver detects an electromagnetic field radiated from a signal generated by a transmitter, which is directly connected to the service being detected. For telecommunications cables, the transmitter may be connected to bunched copper pairs, cable screens, metal conduit or lead sheathing. For optical fiber cables and blown fiber tubing the aluminium foil barrier or metal strength member may be used.

This is the preferred method of detection, but it has a number of limitations as follows: -

This method requires direct connection to the service and must never be used on any services. It must never be used on electricity power cables. The transmitter must never be connected across any pair of wires on a cable or to a single wire of a pair.

When locating Cable by direct connection, the transmitter is connected so that the signal current flows along the cable and returns via the earth. This causes induction into other nearby cables or underground services, as well as into telecommunications pairs within the cable to which the transmitter is connected.

If the mutual impedance of the wires of a telecommunications pair is balanced, no interference takes place, but if they are unbalanced, interference results. Most telecommunications circuits are seriously unbalanced only whilst calls are being set up and overhearing may then be quite loud. Once the calls are established and conversation commenced, the circuits are then only slightly unbalanced and the amount of overhearing is greatly reduced.

The amount of overhearing caused also depends on the manner in which the transmitter is connected to the cable. Most overhearing occurs when a group of bunched pairs or a cable screen is used for the metallic path and where no earth is connected at the distant end. Least overhearing occurs when an unprotected lead sheath is used for the metallic path.

Interference with data transmission circuits. Data circuits, telemetry control circuits are often more susceptible to interference than audio circuits, and when such circuits are known to exist in a cable being located, special efforts must be made to keep interference to a minimum.

This can be done as follows: -
➤Where there is a choice, the cable sheath should be used as the metallic path in preference to pairs in the cable.
➤Keep the tone connected for as short a time and over as short a distance a possible.
➤Provide as good an earth as possible at the distant end.

For optical fiber cables and blown fiber tubing it is necessary to make direct contact to the metal strength member or foil barrier. This can be done by connecting to Location Connection Point (LCP) which will be identified by a disc attached to it.

Induced signal (close couple) : The receiver detects an electromagnetic field radiated from a signal generated by a transmitter. A toroidal coil in the form of a clamp, excited by the transmitter, is placed around an exposed section of the service being located.

The limitations of this method are: -
➤It requires access to an exposed part of the target service.
➤There is a risk of causing interference to working circuits,details already mentioned above.

For optical fiber cables and blown fiber tubing it is essential to connect the metal strength member or foil barrier to earth, at the near end, and preferably at both ends.

Induced signal (loose couple) : The receiver detects an electromagnetic field radiated from a signal generated by the transmitter. The transmitter is placed above a known position of the service being located and induces a signal into the service, through the ground.

This is the most flexible of the methods described here and it does not suffer from the limitations of the preceding methods. Its main limitation is that it cannot discriminate between the target and adjacent services and will sometimes give misleading results.

For optical fiber cables and blown fiber tubing it is essential to connect the metal strength member or foil barrier to earth, at the near end, and preferably at both ends.

Hum detection : The receiver detects the electromagnetic field radiated from 50 Hz AC Mains electricity. This method is useful for detecting electricity cables but it suffers from the Limitations described below: -

➤A cable may be live but produces no hum because it is carrying no current.
➤A well-balanced 3-phase cable may not produce a detectable hum.
➤The masking effect of nearby pipes or cables may give misleading results.

Radio frequency : The receiver detects re-radiated low frequency radio signals, which are absorbed by buried cables and long metallic pipes.

This method suffers from two main limitations: -

➤It depends upon the strength of the prevailing radio signals, which are often very variable.
➤The transmitted signals do not discriminate and will radiate from any buried metallic services in the vicinity.

Metal detection : The receiver detects transmitted signals reflected from buried metal objects.These are generally unsuitable for any operations because the items they detect are too small, and would cause confusion.

Rodding oscillator detection : The receiver detects a signal generated by a small oscillator. The oscillator is inserted into a non-metallic pipe or duct and is passed along it, usually attached to a duct rod. The receiver is then passed over the ground above the likely position of the cable until the signal is detected.

This method is ideal for detecting blockages in empty cable duct or other non-metallic pipe such as drainpipe. It will not work with metallic pipe, which will screen the oscillator’s signal from the detector.

How to Detect the Buried Cable Depth

Depth may be measured by one of two methods, directly or by triangulation. This is dependent upon the particular design of the receiver.Both methods require the vertical position of the target service to be found-

Location by peak

In this mode, the target service is identified by a rise in the response from the receiver. When the receiver is over the service and in the correct orientation the output from the receiver will be at a maximum. This is usually used for coarse location and tracking.

Location by null

In this mode, the target service is identified by a break in the response from the receiver. As the receiver approaches the position of the service the response rises until the receiver is above the target service when the response drops suddenly to a minimum. This is usually used for precise location and depth measurement.

Direct depth measurement is then achieved by holding the receiver on the ground over the service and switching to depth mode. The depth is then read directly from the visual output.

Triangulation is done by rotating the receiver to 45 degrees and relocating the target service. The depth is then obtained by measuring the distance between the vertical and 45 degree position, the distance being the same as the depth.

Note: The depth measured using a locator, by any method, should be treated as an indication only. It should never be quoted as a precise figure for digging purposes.