Locating cables and pipesFollow
This article introduces the principals and techniques of locating buried cable and pipe utilities with the RD7200 system. For more information on the theory of cable and pipe location, refer to The theory of buried cable and pipe location which is available to download from www.radiodetection.com
The RD7200 locator supports a range of active and passive frequencies.
For a complete list of supported frequencies, refer to the RD7200 Locator Specification datasheet.
Passive frequency detection takes advantage of signals that may be present on buried metallic conductors. Dependent on the model, RD7200 locators support up to three types of passive frequencies: Power, Radio and CPS signals.
You can detect these frequencies without the aid of a transmitter if they are present on the utility you are surveying.
The RD7200 locator allow you to take advantage of the harmonic signals found on power networks.
When strong or interfering power signals are present, accurate tracing of a target cable can be challenging. Power Filters™ allow you to establish if a single large power signal comes from one source or from the presence of multiple cables. The different harmonic characteristics of the detected lines can then be used to trace and mark their route.
Once in Power Mode, press the key to switch out of Radiodetection’s sensitive Power Mode and scroll through the five individual Power Filters.
The use of an individual Power Filter harmonic can also allow you to locate power lines in situations where the total signal would otherwise be too large.
Active frequencies are applied to a buried conductor using the transmitter. The transmitter can apply a signal using three methods:
In Direct Connection, you connect the transmitter output directly to the utility. The transmitter will then apply a discrete signal which you can locate using the locator. This is the preferred method of applying a transmitter signal to a utility and in the majority of applications will apply a stronger signal to the utility, which may increase the locate distance.
To directly connect to a non-energized conductive utility:
- Switch the transmitter off.
- Connect the Direct Connection lead into the transmitter accessory socket.
- Clip the red connection lead to the utility ensuring that the area around the connection is clean and that a positive connection is achieved.
- Clip the black connection lead as far away as possible and at 90° to the ground stake or suitable ground point nearby ensuring that a positive connection is achieved.
- Switch the transmitter On.
- The display will show the Direct Connection lead connected icon.
Direct Connection lead icon
WARNING! Direct Connection to live conductors is POTENTIALLY LETHAL. Direct connections to live conductors should be attempted by fully qualified personnel only using the relevant products that allow connections to energized lines.
WARNING! The transmitter is capable of outputting potentially lethal voltages. Take care when handling the terminals, connection leads and ground stake, notify other technicians working on the line of the hazard and guard exposed conductors to prevent accidental contact.
WARNING! Ensure the TX transmitter is switched OFF before making a connection and before disconnection of the direct connection lead to a service.
In this mode of operation the transmitter is placed on the ground over or near the survey area. If a Direct Connection lead or signal clamp is not plugged into the transmitter, it will automatically go into induction mode.
In this mode, only frequencies applicable for induction mode will be made available as the key is pressed.
Once activated the transmitter will induce the signal indiscriminately to any nearby buried conductors.
Please note that these signals will also be airborne and it is advisable to keep the distance between the transmitter and locator at least 10m / 30’ – this distance may need to be increased, particularly if depth measurements are taken.
An optional signal clamp can be connected to the transmitter and clamped around a cable or pipe to apply the transmitter signal. This method of applying the transmitter signal is particularly useful on insulated live wires and removes the need to disconnect the supply to the cable. Clamps are available up to 8.5” / 215mm in diameter.
WARNING! Do not clamp around uninsulated live conductors
WARNING! Before applying or removing the clamp around a power cable, ensure that the clamp is connected to the transmitter at all times.
Choice of frequency for active location
The choice of signal frequency is an important factor for effective tracing and identification of buried lines, and there is no single frequency that covers all conditions. For simple instruments to be used by relatively non-technical personnel, there is no option but to make a compromise, and choose a single frequency high enough to give good performance in the induction mode, but not so high that it will couple too easily into unwanted lines. Active signals between 8kHz and 33kHz are commonly used for these applications. For more comprehensive equipment for problem-solving by technically competent technicians, a range of frequencies may be provided. Typical examples of these and reasons for their use are illustrated below.
512Hz active signal
This low frequency is most useful for line tracing and identification over long distances. It does not couple easily to unwanted lines however it is too low for induction, and it falls within the band of power frequency harmonic interference.
8kHz active signal
This medium frequency is the most useful general-purpose signal, high enough for induction, outside the power frequency interference band, and with limited coupling to wanted lines however it may not be high enough to impose a strong signal on small diameter line like telecom cables.
33kHz active signal
This higher frequency is easily applied by induction to most lines, so is very useful for initial search. It travels on small diameter line however it couples more easily to unwanted lines, and loses its strength over shorter distances than lower frequencies.
100kHz active signal
This very high frequency range deals with the difficult cases – induction onto small diameter lines in dry sandy soil, and short lengths of cable. It is very easy to apply by induction however it couples very easily to unwanted lines, and does not travel far.
It is important to select the correct or appropriate frequency for your particular application. For more information see “The theory of buried cable and pipe location”, which is available as a free download from www.radiodetection.com
To select a frequency on the locator:
- Press the key to cycle through available frequencies
- Alternatively, hold down the key and press the or keys to cycle up or down the range of frequencies
If locating using an active frequency you must also set your transmitter to output the matching frequency.
You can change your transmitter’s output frequency manually using your transmitter’s keypad.
To manually select a transmitter output frequency:
- Press the key to cycle through available frequencies
NOTE: Some frequencies require that you connect an accessory, for example an A-Frame, before the frequency is available.
The RD7200 system supports up to four antenna modes, exclusively dedicated to locating cable and pipes, and to suit your particular application or the local environment. These are:
- Peak mode
- Peak+ mode
- Guidance mode
- Null mode
Peak mode provides the most sensitive and accurate mode for location and depth measurement. It provides a sharp Peak response with a corresponding small decrease in sensitivity. Peak mode cannot be disabled using the menu.
In Peak mode the following indicators are displayed by the LCD:
- Signal strength
To select Peak mode:
- Press the key until the Peak mode icon is displayed on the LCD
NOTE: The depth and current values will display automatically, although these values should not be considered accurate until the locator is directly over the target line.
Peak+™ mode combines the accuracy of the Peak bargraph with a choice of Guidance or Null directional arrows.
Guidance arrows provide visual indication of the direction to the target utility, and are designed to get you close to the Peak position faster, before using the Peak bargraph to pinpoint the target utility,
Using Null arrows allow you to check for distortion before marking a point, and should be used when accurate locate measurements are required.
Switching between arrow types:
When in Peak+ mode, press and hold the key to toggle between Guidance and Null arrow types.
You can also select the default arrow type in the locator menu:
- Press the key to enter the menu.
- Scroll to the ARROW menu using the or keys.
- Press the key to enter the ARROW menu.
- Select NULL or GUIDE using the or keys.
- Press the key twice to return to the main locate menu.
When using Guidance arrows:
Use the proportional arrows to guide the locator along the path of the target cable or pipe. To accurately locate the center point of the target utility, for example to mark a point or take a survey measurement, use the Peak bargraph to pinpoint the Peak position.
When using Null arrows:
Use the arrow heads to place the locator above the NULL point. If the Peak response is not at a maximum then this is evidence of a distorted field. If the Peak response is at its maximum level where the Null point is located then there is no or very limited distortion present.
In Peak+ mode the following indicators are displayed by the LCD:
- Right and left arrows
- Signal strength
To select Peak+ mode:
- Press the key until the Peak+ mode icon is displayed on the LCD.
Guidance mode offers good performance in distorted fields and provides three indicators to guide the user towards the target line.
The Left and Right Proportional Arrows become shorter as the locator approaches the target, and the Target Position Needle will move towards its central position. The Signal Strength reading will also reach its maximum value when the locator is positioned above the target.
Any deviation from all three indicators showing the target position in the same location could signal the presence of a distorted field.
In Guidance mode the following indicators are displayed:
- Proportional left and right arrows
- Target Position Needle
- Signal strength
To select Guidance mode:
Press the key until the Guidance mode icon is displayed on the LCD
Null mode is used to verify a locate signal in environments with limited or no interference or distortion. Null mode gives a null response when it is directly over the line.
In Null mode the following indicators are displayed by the LCD:
- Signal strength.
- Right and left arrows.
To select null mode:
- Press the key until the null mode icon is displayed on the LCD
The sharp, null response can be easier to use than the Peak response but is vulnerable to interference and should not be used for locating, except in areas where there is no interference present.
Guidance Mode offers improved performance in such conditions while Peak+ mode can combine the Peak bargraph with Guidance arrows to combine fast and accurate locate tools.
The LCD compass provides a visual indication of the direction of the target cable, pipe or sonde. The compass is available for all frequencies apart from Power, Radio and Passive.
Line tracing can be accelerated by switching the locator to Guidance mode.
Move the locator left and right while walking along the path of the line to place the target position Needle directly over the line. As you move the locator over the line, the left and right arrows (and an accompanying tone) will indicate if the target line is to the left or right of the locator.
Locating a target line in Peak, or Peak+, mode accurately defines the position of a target line after it has been traced and its position is approximately known. Start with medium output power from the transmitter, medium frequency on the transmitter and locator, and Peak or Peak+ mode on the locator.
Set the locator sensitivity to approximately 50% by pressing the or key.
NOTE: it may be necessary to adjust the sensitivity level throughout the pinpointing to keep the bar graph on scale.
- With the antenna perpendicular to the line, make traverses across the line. Define the point of maximum response.
- Without moving the locator, turn it round as if it is on a pivot. Stop at the point of maximum response.
- Hold the locator vertical with the antenna just above the ground and move the locator from side to side across the line. Stop at the point of maximum response.
- With the end of the antenna close to the ground, repeat steps 2 and 3.
- Mark the position and direction of the line.
Repeat the steps of the procedure to increase pinpoint accuracy.
If using Peak+ mode, switch to null arrows by pressing and holding the key. Alternatively switch to Null antenna mode.
Move the locator to find the Null position. If the position of the Peak and the Null pinpoints correspond, it can be assumed that the pinpoint is accurate. The pinpoint is not precise if the marks do not correspond, but both marks will show an error to the same side. True line position will be close to the Peak position.
The line lies half the distance to the other side of the Peak position as the distance between the Peak and the Null positions.
Pinpointing a target line
Pinpointing with Peak and Null indicators
Sweep and search
There are a number of techniques available for locating unknown lines in an area. Using these techniques is particularly important before conducting any excavation work to help ensure that buried lines are not damaged.
A passive sweep is used to locate Power, Radio or CPS signals that may radiate from buried conductors.
To perform a passive sweep:
- Press the key to select the passive frequency you wish to locate. You can select from the following passive frequencies:
- CPS (Cathodic Protection System)
Stop when the locator response rises to indicate the presence of a line. Pinpoint the line and mark its position. Trace the line out of the area being searched. Resume grid search in the area.
In some areas there may be a confusing amount of 50 / 60Hz power signals. Lift the locator 50mm / 2” from the ground and continue the sweep, or use the key to switch from Power mode and use Power Filters to discriminate individual lines.
Switch the locator to Radio Mode. Increase sensitivity to maximum and repeat the above grid search procedure over the area. Pinpoint, mark, and trace out any lines that are located.
In most, but not all areas, radio mode will locate lines that do not radiate power signals and a grid search should be made in both power and radio modes.
An inductive search procedure is a more certain technique for locating unknown lines. This type of search requires a transmitter and locator and two people. This type of search is referred to as a ‘two person sweep’. Before starting the sweep, define the area of search and the probable direction of lines crossing the area. Ensure the transmitter is switched on in induction mode.
The first person operates the transmitter and the second person operates the locator. The transmitter induces a signal onto lines as it passes over them and the lines are then detected with the locator at a suitable distance away from the transmitter (around 15 meters / 50 feet – although this will depend upon the level of induction power used.)
Hold the transmitter with its length aligned with the assumed direction of any lines.
The second person holds the locator at the start of the area to be searched and with the locator antennas at right angles to the probable direction of the buried lines. Set the locator sensitivity level as high as possible without the locator picking up any airborne signals directly from the transmitter.
When the transmitter and locator are in line both operators start to move forward in parallel. The operator with the locator sweeps it backwards and forwards, keeping the locator vertical, as they proceed in parallel with the transmitter. This method allows for misalignment of the transmitter, locator and buried line.
The transmitter applies the strongest signal to the lines directly below it, which are then located with the locator. Move the transmitter from side to side to establish the highest signal which indicates that the transmitter is also directly above the line(s).
Mark the ground at the point of each Peak signal detected with the locator. Repeat the search along any other possible paths of lines. Once the positions of any lines have been marked, reverse positions, place the transmitter over and along each line in turn, and trace the line out of the search area.
Sometimes it is possible for some utilities to be masked by other utilities and this can happen when one or more utilities are in close proximity to each other or when stronger signals may radiate. In certain applications and congested areas the ‘Nulling’ technique allows operators to eliminate the induction signal directly beneath the transmitter but at the same time induces the transmitter signal onto other nearby utilities that previously have not been able to be located.
Nulling Out effect
Two persons Nulling Out technique:
- Place the transmitter close to the services you wish to trace (a parallel sweep may be used for this) and using the locator at a distance of approximately 10m/30’ from the transmitter, pinpoint the strongest signal.
- The sensitivity on the locator is adjusted so that the bar graph response is approximately 75%
- Release the side support arm.
- With the locator over the centre of the signal the second operator should hold the transmitter at waist level, with the transmitter on its side – release support arm pointing down.
- The transmitter is then moved from side to side to find the ‘Null’ spot between two detectable signals; the transmitter should then be lowered towards the ground keeping it in the ‘Null’ spot.
- At ground level we are ideally looking for a ‘Null’ no wider than 50mm/2” (Sensitivity on the locator may need to be adjusted to achieve this).
- With the transmitter left on the ground in the ‘Null’ spot. The locator is used to check for additional signals either side of the ‘Nulled’ signal.
- Should the ground be uneven the transmitter may be turned across the line of the service provided the release support arm remains facing downwards. (this will improve the stability of the unit)
One person Nulling Out technique:
Nulling Out - Single person sweep
- Lay the transmitter on its side using the side support arm.
- Sweep the area around the transmitter with the receiver at least 10m/30’ from it.
- Reposition the transmitter at 5m/15’ intervals around the area and repeat step 2
- Stop when there is a response
- Pinpoint and mark any lines
- Trace the lines out of the area