Cable Locating with Passive Markers
By Ralph Marsh
Outside Plant Consultant
Austin, Texas
In telephone field work, it often takes substantially more time to locate and excavate a cable, underground splice point or other feature than it does to make the necessary repair or cable network change. The challenge is always to accomplish rapid and accurate locating in order to minimize digging effort, avoid unintentional damage, and provide maximum service to the customer.
Underground locating can present serious challenges. For example, in many areas there is an almost unmanageable concentration of underground facilities, including telephone, electrical power, CATV, water, sewer and gas. It can be very difficult to isolate one utility from another for accurate excavation. The cost is digging is high, and inaccurate, unproductive efforts are to be avoided, both because of cost and the danger of damage and potential safety hazards. Finally, the increased use of high bandwidth services increases the value of communication. Customer services must be restored as quickly as possible.
Electronic markers are passive devices that are buried along with cables to mark a cable path and any underground feature such as buried splices, buried service drops, conduit stubs, fiber optic facilities or load coils that may be excavated in the future. They are used by other utilities as well, and each utility uses markers that are tuned to specific frequencies so individual utilities can be identified. Selective marking is particularly in congested areas where multiple services share the same or adjacent trenches.
Passive underground markers have been adopted by a number of service providers, and this buried plant tool contribute measurably to improvements in locating efficiency compared to other methods. Where feasible, this locating method offers tangible benefits. Unlike stakes, flags or painting, buried markers will not be inadvertently removed or damaged. When markers are placed during construction they can save time and improve productivity for the life of the installed plant, contributing to productivity, improved safety and damage control results.
The challenge of fast and accurate telephone locating can be complicated by a concentration of other utilities within a small right-of-way, and by inaccurate underground records or unmarked facilities. Most locating equipment gives only an approximation of position and depth, and when there are high power lines overhead, a high water table, or pipes and metal objects in close proximity, it can be very difficult to pinpoint specific buried facilities. In contrast a passive marker provides a very precise and dependable indication of both location and depth of buried features.
These markers are produced in near-surface, mid-range and full-range models. For example, small three-inch long near-surface units are used down to about two-feet below the surface, four-inch diameter self-leveling marker balls are used in trenches at depths of five feet or less, another type of marker is used for objects 6 feet or less below grade, and large 15-inch diameter markers can mark facilities as deep as 8 feet below grade.
Since markers are passive, they can function indefinitely with no external power, and they resist moisture, chemicals, minerals and temperature extremes. Frequency and color-code guidelines for underground markers have been assigned by the American Public Works Association to distinguish between various utility services.
Portable, hand-held equipment is used to find markers by transmitting a radio frequency signal into the ground. Proper use of a marker locator can be learned in just a few minutes, and no complex interpretation of results is required. When a marker is found, the reflected signal causes the receiver to give the operator both a visual reading and an audible tone. Unlike other kinds of locating equipment, these locators are minimally affected by nearby metal objects, electrical power fields, or buried markers tuned to other frequencies. Reusable plastic markers are safe from accidental damage and can be easily found.
For new construction, markers can be buried at regular intervals along straight cable runs, points of major change in direction, and at features such splices and underground terminals. As-built plans show the location of each marker ball, and a locating crew can dig directly over the desired location to reach the buried feature with minimum excavation. This can cut locating time by as much as 80-percent, and reduce annual locating and excavating labor costs by thousands of dollars.
Some of the larger operating companies in the U.S. are currently selling off selected rural properties to smaller firms to consolidate their holdings and concentrate their efforts on urban service. The buried plant records for such rural areas may be missing, or of questionable accuracy. In one recent instance, the purchaser found that while no accurate records were available, the former telco owner had fortunately placed passive markers to pinpoint key facilities such as hand holes, manholes and buried splices. This made it possible for the new owner to quickly and accurately find these buried resources by merely purchasing a marker locator unit, thereby avoiding the much higher costs of locating and digging.
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Passive underground markers are used to efficiently mark and locate various telephone plant features.
New Telco Marker Extends Range to Five Feet
The traditional depth of a mid-range buried marker for underground locating is four feet. This range has been extended to five feet (1.53m) by a new unit from 3M Telecom Systems Division, the 1401-XR four-inch ball marker. According to Tim Parkinson, 3M Senior Technologist, this four-inch unit is sized to fit easily in a standard four-inch trench without need for additional digging - unlike competitive units that are slightly larger than four inches.
The four-inch ball marker is placed in a trench to permanently mark a cable path or other feature such as a buried splice, service drop, conduit stub, fiber optic facility or load coil. It is fitted with eyelets so that an installer can tether the marker to a pipe, conduit or splice case with an optional tie-down strap if desired.
"This device is self-leveling," Parkinson explains. "The passive antenna inside the ball is suspended in a liquid, which allows the antenna to remain properly oriented for maximum signal strength and accuracy, regardless of the position of the ball. It is filled with a environmentally-benign propylene glycol solution that is readily biodegradable and would not harm humans, wildlife or the environment in the unlikely event that the ball marker was damaged."
This new four-inch ball marker can be located with a 3MTM 1264 EMS II Marker Locator, or a 3MTM DynatelTM 2200 Series Locator equipped with a single or multiple frequency marker locating accessory. Like all markers for the telco industry, the new 3M ball marker is color-coded orange to industry standards for accurate identification, and tuned to the telco frequency to prevent confusion with markers intended for the buried facilities of other utilities.
In addition to the new ball marker for buried facilities down to five feet, 3M also supplies near surface markers designed for applications down to two feet (0.61m), mini-markers for down to six feet (1.8m), and full range markers that work down to depths of eight feet (2.44m).
The new 3M four-inch, self-leveling ball marker functions at a depth of five feet, measured between the bottom of the locator receiver antenna and the bottom of the marker.
