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Personal Communication Services (PCS) Protection in Power Transmission Corridors

Can be a dangerous location to place a wireless cellular tower antenna without 'Expert Engineering' support

by Ernest M. Duckworth Jr., P.E., President of LPGI



PCS providers are joining with power companies to acquire space for their antenna sites within power company high voltage transmission corridors. These site locations require T1 Carrier/telephone communications service and must be protected with a High Voltage Interface (HVI), similar to the requirements outlined in IEEE Std. 487-2000 if wire-line communications are being used. The entrance of 120/240 AC power to these locations, to locally power wireless equipment, also requires special engineering design.

A particular location along a power line corridor will generally experience many more faults in a year than a substation location, because of the fault current distribution. Most of these faults are initiated from lightning. Although a corridor may experience more faults than any one substation, the fault current at any tower location is expected to be generally less in magnitude, because of this current distribution from the Overhead Ground Conductors (OGC).

This lower fault current at any particular tower location is countered with a generally higher resistance grid ground built at the base of these towers for the cellular site. Thus, the expected Ground Potential Rise (GPR) at these antenna sites is comparable to the GPR expected at most power plants and substations. Recently calculated GPRs have been between 3kV Peak and 30kV peak, well within the capability of wire-line isolation products on the market today. A fiber network design all the way back to the serving Central Office (CO) is the best alternative available, if budget permits.

If wire-line communications is to be used, the dedicated cable serving the antenna site should be high dielectric cable of approximately 150 feet in length and perpendicular to the high voltage corridor. This cable should be in PVC conduit for at least the last 50 feet coming into the corridor. This cable run should be kept away from the tower legs to prevent lightning damage from arcing. Lightning will arc 8 feet in 100 meter-ohm soil and will arc 25 feet in 1000 meter-ohm soil.

To insure the complete safety of the general purpose telephone cable plant (serving other customers), the dedicated cable can be spliced to gas tube protectors where it meets the general purpose cable. Thus, if the HVI fails, the general purpose cable plant would not be exposed to the fault current.

It is important to remember that a HVI is absolutely required by IEEE Std. to protect the safety of personnel, equipment and cable plant in High Voltage Locations from the dangers of a remote ground (the other end of a wire-line communication circuit). This remote ground comes from wire-line communications serving these locations. The only way to eliminate the danger of a remote ground is to provide a fiber network all the way back to the serving central office. Fiber optic extensions are not a solution to eliminating a remote ground.

It is equally important to remember that these HVI locations should not be worked on (maintained) during lightning storms. In addition, working at a HVI requires proper protection of personnel by utilizing special 20kV rubber gloves and 20kV rubber mat. A fiber optic network solution does not remove the dangers to personnel working at these locations, particularly during bad weather conditions.

Require expert engineering support? Contact us.

LPGI & Affiliates
962 Coronado Drive
Sedalia, CO 80135-8303
303.688.5800
Fax: 303.688.5551

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