Locating TechniquesFollow Anthony Caplin
Updated : Created :
Identifying target utilities
If several conductors are running in parallel, and it is not possible to connect a transmitter, each line may be located separately. Proceed as follows:
- Perform a sweep of the area to find the position and number of conductors in the area.
- Map the direction in which the conductors are going.
To trace the lines:
- Select induction mode on the transmitter.
- Select the same frequency as on the locator.
- Place the transmitter on its side and in line with a conductor.
- Ensure the conductor is directly below the transmitter.
- This will ‘null out’ the signal directly below the transmitter, placing no signal on that conductor, and enable the other conductors to be located.
- Locate each conductor and mark its position.
- Move the transmitter down the route and continue locating and marking.
- Trace each conductor out of the search area until the target can be accurately located.
Unwanted signal coupling
Unwanted coupling of the signal from the target line to another nearby line is one of the most common location problems. It leads either to an error in the marked position and depth of the target line or to marking the wrong line. A certain degree of coupling is unavoidable in many situations but there are ways in which a careful user can reduce coupling and increase location reliability.
- Avoid applying the signal by induction. The signal may be coupling to more than one line directly from the transmitter. Use the signal clamp where possible.
- Identify points where lines may be bonded or in close proximity to each other. Work toward these points rather than away from them. For example, if gas and water pipes are bonded within a building, apply the signal at the valves or access points in the road rather than in the building.
- Reduce coupling to a parallel line by using a low signal frequency where available.
- Return signal flowing on another line. Use a double ended connection to by-pass the ground return if possible.
- Choose a signal application point where the line is furthest from other lines and not in a congested area.
- When using single-ended connection, site the ground stake as far from the target line as possible and away from other buried lines.
- Avoid using existing structures for ground connections; other buried lines may be bonded to them.
- A bad ground connection or just laying the ground lead on the surface at right angles to the line may result in less coupling than a good ground provided long distance tracing is not required.
|Nulling utilities & interference from services|
Signal & ground connection
Sometimes when locating, it is not possible to insert the ground stake into the earth, for example, when locating on hard ground such as roads. In this case, the ground return can be made by attaching the ground lead to the metal frame of a manhole.
Using lighting columns
Direct Connecting to a metal, street-lamp column is almost as effective as connecting to the cable sheath itself. Normally the cable sheath is bonded to the metal column, therefore a simple connection onto the column enables the user to locate the street lighting quickly and safely without having to call out a technician from the lighting company.
If the lighting column is made from concrete make the transmitter connection to the cable sheath unless the cable is earthed to the inspection door frame. Connection to the cable sheath applies the transmitter signal for a considerable distance enabling the locator to trace cables feeding illuminated street furniture as well as other street lights.
WARNING! The live cable connector is for use only by operators licensed or permitted to work on live cables.
The use of a street light column as a means of applying a signal to other power cables on the same electric circuit is a possibility. The signal may be weak using this method because it may have travelled some distance back to the sub-station and out again on the other system. With the locator used on a high sensitivity setting it is often possible to locate a cable, which would otherwise have been difficult or inconvenient to energize with the transmitter signal.
Finding a good ground point
When using a Direct Connection, it is important to get the best possible grounding for the transmitter. This provides the lowest resistance ground path and the best output signal. If it is not possible to use the ground stake the following are examples of good alternative ground points:
- Metal manhole covers
- Metal drainage grates
- Metal railings
- Metal fence posts
Double ended connections
Large diameter water pipes and gas distribution pipes that are laid in sections sometimes have insulated joints between the sections and can be difficult to locate using a single ended connect. This is because when using a single ended connection ground return, signals can often cause confusion by returning to the transmitter along other lines. The problem sometimes occurs when return signals appear stronger than on the target line, usually because the target line is deeper than the line carrying the return signal, or the return path may be a better electrical conductor than the target line.
Applying a double-ended transmitter connection is a useful technique for positively tracing and identifying a target line in a situation such as a heavily congested industrial site, provided there are access points at each end of the line.
Making a double ended connection
Connect the transmitter to an access point on the target line. The transmitter ground is connected with a long cable to another access point further along the line. A complete circuit is achieved without using ground as a current return path. The long cable should be kept as far away as possible from the expected route of the line.
This method of applying the transmitter signal is ideal for positive identification of a target line. When a connection has been made to two points on the same line, the same level of current should be detectable around the circuit. The locator display should remain constant if the depth of the line does not change.
|Making double-ended connections|