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Lightning strike damage to PCS cell sites and associated
equipment, radio and microwave towers, TV antenna sites, and
other antenna structures results in losses of over $225 million
annually in North America. A single July 2003 thunderstorm in
the Northwest (from Seattle to Denver) temporarily terminated 38
PCS working cell sites. The real disappointment here is that
most of this damage and resulting downtime could have been all
but totally eliminated.
Insurance costs on these towers and associated equipment will
become astronomical due to the constantly reoccurring damage.
It's difficult to understand why PCS providers continue to
accept rising insurance costs and poor service reliability, when
cost-effective protection is readily available.
Why are these tower locations so susceptible to
lightning-induced damage? There are two reasons: (1) antenna
sites are not correctly grounded to properly conduct light
strike energy, because no code demands it, and (2) the wire-line
communication service providers rarely install isolation
equipment to protect communications services from a lightning
induced Ground Potential Rise (GPR), also because no code
demands it.
Why aren’t antenna sites properly grounded to protect against
lightning strike energy? Because the National Electrical Code
(NEC) is the controlling code for grounding, and it does not
address lightning strike energy, only electrical supply energy!
Section 810-2 of the NEC, Grounding Conductors, describes how to
ground an antenna with a single ground rod and a #6 gauge wire
to insure the safety of personnel in case the antenna should
become electrically energized from contact with the electric
power system. This method of grounding is completely inadequate
to dispose of an average 30,000 Ampere lightning strike. This
method of grounding for lightning strike energy is as effective
as trying to use a one inch diameter pipe, five miles long to
supply water to all of New York City!
Why aren’t wire-line communication service providers isolating
their communication equipment from lightning induced GPR?
Because IEEE Std. 487-2000 is the controlling standard for
isolation against a GPR and it applies only to electric power
fault energy. Lightning strike energy is only mentioned in
passing. At sites other than electric power locations the
requirement for isolation of wire-line communications is not
addressed!
IEEE Std. 487 describes how to isolate wire-line communication
services from 300-Volt to 30,000-Volt GPR due to a faulting
power line. However, Standard 487 does not address a lightning
induced GPR at a non-power site, which is many, many times more
likely to occur than a power-line-fault GPR.
Thus, with respect to lightning strike GPR, we have two
controlling codes that fail to adequately address proper antenna
grounding and isolation of communication services. These code
failures mean real trouble for the PCS provider! Why? Because
low bid wins the contract and no one is going to bid more than
the minimum requirements specified in the codes!
Ironically, the two solutions to properly protect against
lightning induced GPR at antenna sites; i.e., current division
grounding and isolation of communications lines, can be met with
relatively little extra cost. In fact, these additional costs
are minuscule when compared to the total site construction cost
and the costs of ongoing GPR damage repair.
Instead of providing a single ground rod and a #6 gauge wire
from one of the tower legs, it is far superior to install a star
grounding system (ten or more legs 40 to 80 feet) off of a
ground ring that will divide the current from a lightning strike
into smaller segments. This grounding scenario will not only
provide a lower-impedance ground, but will also lead
lightning-induced current away from the antenna building,
thereby reducing the resulting GPR of the building’s grounding
system. The approximate cost for grounding the antenna in this
manner is $4,000.
Instead of providing gas tubes on the wire-line communications
services entering the antenna building, install isolation
equipment on all entering pairs. This will isolate all wire-line
services from remote ground in the event of a lightning induced
GPR and thus protect all associated equipment from damage. The
use of gas tubes does not protect the communications equipment
from on-site GPR and in reality allows GPR current to flow in a
reverse direction from the site, which damages equipment and
cable alike. The cost for isolating the communication pairs is
only about $700 per 4-Wire T-Carrier circuit or 2-Wire voice
pair.
Thus, for less than 1% of the site construction budget you can
effectively eliminate all future communication equipment damage
from lightning strike energy. The average damage resulting from
the first lightning strike on an improperly protected antenna
site is approximately $25,000 or five times the cost of properly
protecting the site!
In Cell Site construction bid specifications, future site owners
should require a high-quality grounding system such as a star
configuration grounding system and require isolation protection
for all wire-line communications service. If they don't, they're
asking for serious unnecessary trouble and expense down the
line.
LPGI & Affiliates designs grounding systems to prevent
equipment damage from lightning and it can be accomplished with
minimum field support from the office. Give us a call at
303-688-5800.
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