Experts estimate that there are as many as 150,000 cellular sites in the continental United States. Each is equipped with special transmitters, antennas and other equipment. Some of these sites are owned and operated by the wireless provider. Others have been constructed by a third-party owner/operator. Regardless of the landlord, as cell phones and other wireless devices become more ubiquitous, so do the towers. And in most cases, an electric utility has meters running inside their gates.

Working Inside a Cell Site

At first glance, tower sites appear to be free of the intense utility congestion linemen encounter everyday in underground urban and suburban corridors. Utilities such as Duke Energy face tight, congested utility corridors every day. It is common to have cable in the same (joint) trench with other utilities, sharing rights-of-ways in the urban corridor with CATV and telephone cable and paralleling gas, water and sewer pipeline. But cell sites are usually more remote, located away from other utilities, and would appear to be less congested.

On a recent line locate assignment, a Duke line technician found out just the opposite while trying to locate an underground power junction box in a cell site that needed to be removed.

Similar to substation work, cell sites are small footprints with enormous amounts of critical cable and equipment installed on racks and in small buildings inside the cell site. With the increase in opposition to new tower construction, more and more carriers are sharing space on existing towers. In addition to wireless-related cable and equipment, each customer has heating and cooling capabilities. Each tenant has a separate service line and meter to monitor power usage. This co-location has created an even more congestive environment for linemen to perform maintenance and repair assignments.

Locate Technicians

Most of Duke's North Carolina One-Call Center requests for locates are subcontracted to a third-party locate contractor, but in critical corridors in larger markets, such as Charlotte and Greensboro, Duke's locate technicians locate their own lines. Duke locators, who are also journeyman linemen, also do many of their own locates in industrial parks and at cell tower sites.

On this assignment near Charlotte, a Duke locate technician discovered that locating an underground power line inside a cell site can be a line technician's nightmare. It turned out to involve not one but multiple congestion challenges.

Locating a Power Junction Box

The ticket called for locating a power junction box inside the cellular antenna site off W.T. Harris Blvd. in Charlotte. One of the customers had been taken out of service, and the junction box and the energized line had to be located and taken out of service. The junction box had been in place for many years and had been covered over with soil and gravel. Prints were not available so the locate technician was dispatched to find the underground line.

The W.T. Harris Blvd. monopole cell site contains several equipment buildings fabricated to house the wireless carrier's antenna support equipment. Duke's meter banks and cable run both overhead in racks and underground to the steel tower structure at the north side of the 40-ft by 60-ft site. The cell site had been expanded several times to allow the addition of customers. Electrical services had been added during the different expansions, which meant not all the services were fed from a common junction box. Unfortunately, the prints had not been kept current with the additional infrastructure. The equipment and buildings were stacked like a 3-D jigsaw puzzle in a space the size of a postage stamp.

Site Investigation

When prints are not available, the first thing a locate technician does is survey the area looking for every available sign that might point to the location and direction of the cable. But there are no manholes, pedestals and fresh trench lines inside a cell site to give the locator clues for finding where the cable might be. Given the fact there were several energized conductors in place and multiple junction boxes, probing and excavating was a last resort.

Another obstacle at a cell site that is invisible to the locator's eye is the metal grounding grid running underneath and beyond the entire cell site perimeter. Every structure, support brace, equipment cabinet and conductor is grounded at some point to this underground grid.

First Attempt: Conductive Mode

The Duke locate technician began the locate process using the company's cable locater, which included multi-frequency transmitter/receiver in the direct-connect or conductive mode. The idea was to locate all the cables to find a common area where the service wires meet. This area should be the junction box.

Under normal circumstances, the conductive process is the most reliable way to trace a buried cable. A lead wire was clamped directly to a bar conductor at the meter and plugged into the appropriate jack on the locate box. This is often an ideal access point to induce a frequency-amplifying signal onto the natural magnetic field of the conductor.

A grounding rod was planted in the ground at the opposite end of the survey area and the direct-connect cable attached. In the conductive mode, the signal transmitted by the locate system was being picked up by the chain-link fencing and other metal structures nearby. The underground grounding grid was also causing “bleed over” or “jump off.”

Bleed over is a common occurrence for locators, especially around adjacent cable and steel pipe. In this case, the bleed over was confusing the locate system device, giving irregular readings for the locate technician on both the low and high frequency. Even on the low frequency, which often minimizes jump off, the signal was unstable and unreliable.

New technology embedded in today's locators (semi-automatic gain signal management) normally allows the receiver to filter out unwanted signal from interfering radio frequencies, such as the infrastructure and structures at the cell site. In the conductive mode, the locator was able to locate some of the services, but the inconsistent readings gave the locator very little to go on, and the location of the power junction box was still unknown.

Plan B: Inductive Coupler

When the conductive procedure failed to accurately locate the underground cables, Duke's locator had another option in his tool bag — an accessory called a “coil inducer clamp.” The inductive process is often used to locate defective or damaged underground cables. Instead of attaching to bare wire, the coil clamp can be clamped around the insulated conductor or conduit. The signal can penetrate PVC and cable insulation, affixing a recognizable signal to the conductor, which flows to the end of the cable.

With the inductive coupler attached to the conductor at the meter riser, the locate technician switched the transmitter/receiver to the inductive mode. In the inductive mode, the locator box induces a low-frequency signal on to the conductor. Instead of looking for an electromagnetic signal from the conductor, the receiver is actually looking for a familiar frequency signal being carried on the conductor.

Unlike the direct-connect method, the signal does not require a return loop to the receiver. The cable acts as an antenna radiating the signal and, in some circumstances, is less susceptible to bleed over and interference from surrounding magnetic fields, for example the grounding grid, the fence and the surrounding cable and wire at the cell site.

Junction Box Found

The Duke locate technician used the tried-and-true surveying technique to pinpoint the location of the cable. Walking over the survey area, the locator moved from left to right and forward and backward to find the intersecting x and y axes where the signal was strongest. The transmitter/receiver can be operated at three frequency levels: high (80 kHz), medium (38 kHz) and low (9.5 kHz). The low-frequency range proved to be the most accurate.

Historically, the conductive process has been the most dependable method for locating buried cable, but in this environment, the inductive process prevailed. At the end of the day, all four services were clearly located and marked. With paint markings on the ground, the location of the junction box became obvious. It turned out to be buried under 2 ft of gravel at the foot of one of the meter panels, approximately 10 ft east of the antenna tower.

NUMBER OF CELL TOWERS

At an RBC Capital Markets North American Technology Conference, American Tower Corp., an independent developer/provider of wireless sites, reported that the total number of cell towers in the United States among the six largest wireless providers was approximately 143,000. Here's the breakdown:

Sprint Nextel: 45,000
Cingular: 44,000 (other estimates show 50,000)
Verizon: 22,000
T-Mobile: 24,000
Alltel: 8,000 (Pre Western Wireless).