Factors Effecting the Quality of the Locate

There are several other things that effect the quality of the locate other than mode of locating which we have just discussed.  The type of conductor, the different frequencies used, the size and depth of the line, other utilities in the area, soil conditions, equipment or other utilities generating electromagnetic fields.

First let’s look at how the type of conductors that effect the locate.  The list generally represents the most optimum conductor to the least optimum conductor.  Cable TV lines are generally shallow and have a conductor that is copper and insulated.  It generally is the easiest to locate.  The gas company uses a number 14 insulated wire to trace their lines when the use plastic for their lines…it is an excellent conductor to trace utilizing RF equipment.  I would like to point out fiber optics on the list.  As you know fiber has become the communication line of choice now and many miles of line have been installed underground in the past few years.  When fiber came out many years ago, no thought was given to having to locate the line, and fiber by itself is not locatable using RF techniques and is generally too small for radar to pick up.  Then fiber came out with a shielding material, which afforded the opportunity to locate the fiber, however, after being in use for a period of time there were enough lightning strikes to where they stopped shielding the fiber.  Currently the practice is to trace the fiber lines.

The depth of the line effects the locate in that RF equipment is generally limited to a range of 10-20 feet in depth. 

The size of the line can effect the locate in that a very large line close to the surface would create a very large electromagnetic field that would make it difficult to locate the target line.

Soil conditions can effect the locate in many ways.  If there are buried metallic objects, the electromagnetic field would be disrupted, or if the soil is very dry, it would be difficult for the grounding system to work.  And other unknown conditions could be present.

Mode of locating.  This we discussed earlier…whether you use the direct connect or inductive modes.

Other utilities in the area.  Other utilities in close proximity to the target line will definitely have an effect on the success and quality of the locate.  This can be compensated for by the techniques and procedures used.  In a known congested area, the process would be to research records, and then locate the easiest utility first utilizing the lowest frequency possible.  I would like to emphasize the importance of research.  On every project it is important to research as completely as possible the information that is available.  It will be a start in developing the picture of what is in the area to be located.  However, we must rely on the information given to us by our equipment to tell us what is actually there. 

I want to take a moment to also emphasize the importance on keeping records of abandoned lines.  It can not only cause problems in the locate process, but after the active lines are noted correctly on drawings, a contractor may uncover an abandoned line thinking it is the active line and dig and sever the active line.

Equipment or lines generating electromagnetic fields such as transformers or overhead power lines can effect and interfere with the signal received by the locate receiver. 

The frequency chosen to use to locate is important.  Generally, utilizing the lowest frequency that will locate the line is always the best. 

It is the best because the higher frequencies have will jump to adjacent utilities in a congested area.  However, when locating ductile iron piping you should use at least the medium frequency on the locator so that it can jump the gasket or mechanical joints that connect the piping.  Low or medium frequencies are generally used for communications, power, and any trace wire.  Low frequency can travel far from a single connection point…often distances in miles.  The telephone company and other such utilities will have a transmitter located at their central switching center and technicians in the field call in to have the transmitter turned on when they are in a certain area and need a locate.

There is other information that you as engineers, facility managers, and property owners are interested in from time to time.  How deep is the line?  What is the accuracy of the locate?  What is the condition of the piping?  What size is the piping?  What is the direction of flow? 

The important thing to know is that accurate information can be obtained about underground utilities.