Cable Locating techniques
Identifying target utilities involves several steps, including induction, signal coupling, nulling utilities, signal and ground connection, and finding a good ground point. Induction is the most common method for locating target utilities, as it allows for the identification of each line separately. To trace lines, the transmitter should be placed on its side and in line with a conductor, ensuring the conductor is directly below the transmitter. This will "null out" the signal directly below the transmitter, enabling other conductors to be located.
Unwanted signal coupling is another common location problem, leading to errors in the marked position and depth of the target line or marking the wrong line. To reduce coupling, users can avoid applying the signal by induction, identify points where lines may be bonded or in close proximity to each other, work toward these points rather than away from them, use a low signal frequency where available, and use a double-ended connection to bypass the ground return if possible.
When using single-ended connections, 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, as 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 and interference from services can also be achieved through signal and ground connections. Manhole covers can be used as a ground return when locating on hard ground, such as roads. Direct connecting to a metal street-lamp column is almost as effective as connecting to the cable sheath itself. If the lighting column is made from concrete, make the transmitter connection to the cable sheath unless the cable is earthed to the inspection doorframe.
Finding a good ground point is crucial when using a direct connection, as it provides the lowest resistance ground path and the best output signal. Alternative ground points include metal manhole covers, drainage grates, railings, and fence posts.
Double-ended connections are useful for positively tracing and identifying a target line in heavily congested industrial sites, provided there are access points at each end of the line. By connecting 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, achieving a complete circuit without using ground as a current return path.
This method of applying the transmitter signal is ideal for positive identification of a target line, as 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.