Barney’s Blog no 9 - Using A Pipe and Cable Locator

Using a pipe and cable locator to locate pot ended cable and short lengths of metallic pipe.

Pot ended cables are perhaps the most dangerous and difficult cable to locate and several accidents have occurred because excavators have damaged them and themselves.

 

Locating using passive signals alone (using power frequency and radio frequency modes) can only find cables carrying current and/or reradiating radio signals. Generally pot ended cables cannot be located with the universal cable avoidance tool because pot ends do not carry current even though they are under voltage, so they do not radiate any power frequencies. To be detectable in Radio Frequency, the cable has to be sufficiently long to have a radio frequency current flowing through it, but because pot ends tend to be short length and are ungrounded there is no current path for the RF signal so with a conventional passive locating procedure they are "invisible".

Conventional cable avoiding training methods using a transmitter teach the operator to use connection, a signal clamp, and the induction method, standing the transmitter on the cable/pipe that needs to be traced and depthed. The method using induction relies on the transmitter being placed accurately on the pipe or cable to be traced.

Using conventional active location modes can sometimes help, but only if the pot ended cable is long enough; usually the standard cable avoiding tool transmitter is not powerful enough to transmit a signal that runs along the cable and up to a pot ended cable connection, and unless the cable is long enough to carry a current flowing by capacitance to ground process, there is no signal to detect.

Recently I have been using the Seba Easyloc locator system, Seba's version of the cable avoiding tool, and this has some useful extra features allowing slightly different locating techniques and methods to solve this problem.

The Easyloc's transmitter uses the common 33 kHz active frequency and the higher output power is 0.5 Watt is one principle advantage over the universal system. On induction its transmitting antenna outputs a reliable amount of signal into the ground and this can be helpful to find pot ends and service connections into buildings.

This application using the Easyloc equipment requires a different approach for location of short lengths of cable and pipe; simply the receiver and transmitter change places.

Usually the line of the main cable or pipe in the street can be located easily with either passive or active methods; more difficult if the chosen cable is amongst or below other pipes and cables. Available utility maps will help, and these should, as always, be part of the job.
Using the transmitter for the more precise active location method, pinpoints the centreline locate position of the cable with the known pot end, or the metallic water/gas pipe.
Firstly the main cable to which the pot end is connected or the short length of pipe connected to the metallic main pipe is precisely located, by using any of the usual active location techniques. Then the Easyloc receiver is held, switched on using the transmitter operating frequency, at the exact same position above the locate position.

Note that the position chosen for the Easyloc receiver should be at least 20M away from the anticipated position of the connection, to avoid direct airborne signal from the transmitter, otherwise there is a risk that the induction signal from the transmitter may mask the change in signal response when the transmitter is used to search for the connection.

Next, a colleague takes the Easyloc transmitter and uses it to sweep for the short pipe or cable. When the handle of the TX is above and in line with the target, the colleague with the locator receives the strongest signal. So it helps if the approx orientation of the pipe/cable is known, then the handle of the TX can orientate in the same direction.
Typically the Easyloc transmitter is carried close to the ground with its handle at 90 degrees to the main cable/pipe, and moved along a sweepsearch parallel to and approx 1-2M from the main cable/pipe.

When the Easyloc receiver operator sees an increase in signal he should alert the colleague, who can now stop the sweep and use the transmitter to pinpoint the cable/pipe position by moving the Easyloc transmitter across the ground until the maximum signal is received. Transmitter user marks the position, and then moves the transmitter (with its handle inline with the target) along the line until the signal is reduced dramatically, and that indicates that the end of the short length on pipe/cable/pipe has been reached.

The method of swapping position of receiver and transmitter has other applications; for instance finding a missing metallic water service into a property if the position of the main is known. Apply the same technique with the Easyloc receiver on the water main in the street, and the Easyloc transmitter user sweeps across the property near to the building, until the signal response increases.

Please note: With all location technique, results will always depend on the detectable magnetic field, and always the orientation of buried utility lines; this technique described above will help to solve pot end and short pipe length location problems.

On new utility installations there are other possibilities to install buried passive markers for the pot ends and other important underground connections.

Seba KMT produces a variety of marker balls and discs that are attached to the utility line and buried at installation. These are colour coded for the specific utility and use different frequencies depending on the utility type, to allow the marker to be located on the surface and the utility type defined.

In the Seba range there are 2 marker locators available, The FLM 700 is a dedicated marker locator that will also provide depth indication to the marker, and the type Vloc-Pro ML equipment from Vivax; This equipment can be used as a normal pipe and cable locator and it adds built in marker locating capability.

A more simple approach to mark the end of a pipe or cable, and the same method can be used for property boundaries, is to use an iron or steel stake driven into the ground to mark a specific position. The stake can be located with a magnetic locator like the new Vivax VM 880, generally up to 1M deep. The Vivax VM 880 can also be used to find lost manhole covers, tanks, pipe joints, valves etc providing they are made from ferrous metal, but I already mentioned those application is my earlier blogs.

This Easyloc equipment has also recently received Rail Track approval certificate no. Certificate of Acceptance PA05/03423

If you need a more detailed explanation, you can contact me at barney@mansls.co.uk