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by Ernest M. Duckworth
Jr., P.E., President-LPGI & Affiliates
There is a 50% probability that a first strike from lightning
will not exceed 30kA. This is from Anderson-Eriksson,
"Lightning Parameters for Engineering Applications".
If the self-inductance of a grounding system (ground ring, etc.)
is estimated very conservatively at .5x10-6H and lightning takes
the form of a pulse which has a typical rise time of 2x10-6S,
then from V=Ldi/dt; the estimated ground potential rise (GPR)
will be 7.5kV.
Thus, a grounding system would actually have potential on it
instead of being zero, and be the cause of equipment damage
instead of preventing damage. Values of GPR could easily double
or triple for poorer grounding systems or if the lightning
strike magnitude were up towards its maximum of approximately
200kA! 200kA magnitude lightning strikes are rare, but 100kA
magnitude strikes aren't!
A GPR of 7.5kV on a
grounding system of any structure (tower, building, etc.) will
result in damage to equipment that is tied to a remote ground.
Thus, any equipment that is connected to communication pairs
that lead of the site and back to a Central Office is in
jeopardy of being damaged or destroyed! In addition, there is a
chance that a fire could result that may burn the structure
down!
If we are considering a very large structure with many (1000+)
communication pairs, such as a Central Office, the effect will
be greatly reduced with the many multiple paths, which divide up
the current, to remote ground. However, if we are considering a
small structure with relatively few communication pairs to
remote ground then we must consider isolating the conducting
path as well as harden the grounding system to lightning strike
energy.
Wireless cellular towers and particularly susceptible to damage
from lightning strikes and approximately 6.5% of them are
damaged each year. If there are 100K towers in North America
then 6,500 towers are damaged from lightning strike energy every
year! If the average cost of repair is $10K then $65M every year
is being spent by the wireless cellular industry alone, on
lightning damage.
This accepted cost of doing business is absurd! The excuse that
the insurance company will pay for the losses is also absurd!
All of these losses and down time can be virtually eliminated
with proper design of the grounding system and isolation of the
wire-line communications. And the cost? Carolina West Wireless,
in North Carolina has been doing it for seven years now for
approximately $3.5K per tower site.
What is 98% of the wireless industry doing today to protect
against a lightning induced GPR at their tower sites? Nothing,
absolutely nothing! Basically they point the finger at the local
telephone company and say that it is their responsibility! So
what is used as communication pair protection at tower sites?
Standard shunting protection for the cable pair termination,
such as gas tubes, because that is all the local telephone
company has to do.
Gas tubes, MOV's, SCR's, SAS's, etc. will not protect equipment
from a GPR by isolating the conducting path. These devices
merely offer an additional path to remote ground through the
communication pairs. In fact, they guarantee a connection to the
communication path in the reverse direction from which they were
intended to operate! This guaranteed connection also guarantees
damage to equipment.
Want to get out of the "Circle of Distruction"? Call
us.
LPGI & Affiliates
962 Coronado Drive
Sedalia, CO 80135-8303
303.688.5800
Fax: 303.688.5551
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