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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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