Special Grounding at Wireless Tower Sites
will Prevent 99% of all Lightning Damage
Abstract: The average
(30kA) lightning strike will result in approximately $25k in
equipment damage throughout most of the
. A hilltop standard
60 foot PCS tower can expect a hit once every fourteen years, a
hilltop150 foot tower can expect a hit once every other year,
and a hilltop 300 foot tower can expect 1.89 hits every year.
These lightning strike frequencies may be halved if on
flat terrain, doubled if on the east coast, and tripled if along
the gulf or in
. If the tower and
equipment building are grounded and communications specially
protected against lightning strike energy, equipment damage will
be virtually eliminated, no matter where the tower is
providers that own 100 towers on flat terrain (150 foot in height) may
expect a lightning hit every four years to those 100 towers, or saying
it another way expect 25 of those towers to be hit every year.
If you have designed a 5 ohm tower ground, a limit found in
most wireless specifications and in Motorola’s R56 Standard, expect
an average of $25k in damage from each lightning hit.
In a year, your maintenance bill, from owning the above 100
towers, is going to be $625K. If
you are one of the ‘Big Seven’ and own a 1000 towers, your yearly
maintenance bill is going to be $6.0+ Million!
There is a
difference between minimum design standards found in the National
Electrical Code (NEC) and good engineering practice. The scope of the
NEC does not cover lightning and there is not a hint of what Ground
Potential Rise (GPR) is or does. Nor is GPR mentioned in Motorola's
R56 current standard either! In
fact, there is no code book or published standard today that relates
to the engineering design of equipment locations to prevent damage
from lightning strike energy. However,
this type of information is available in the "Guide for
Protection of Equipment and Personnel from Lightning", in ASCE's
Journal of Performance of Constructed Facilities, August 2002, by
30kA lightning strike will produce a 7.5kV GPR into the earth. If you
do not have a grounding system that is properly designed to bleed off
lightning strike energy, you will be part of that average yearly loss
mentioned above. In addition, if you have not isolated wire-line
communications that entered your tower site, a grounding system of
near zero ohms will not help either! Proper grounding and proper
isolation go together.
100 towers would have be properly grounded and isolated, you would not
have had a $625K maintenance bill for the year or $6,250 per tower in
lightning maintenance damage every year. This is quite a maintenance
cost to your company that does not have to exist at all.
total site grounding system that centers around a single point ground.
A single point ground is an absolute must for equipment
protection against a lightning induced ground potential rise (GPR).
Without a tower site single point ground (SPG), equipment
damage from lightning will be a perpetual problem.
resistivity measurements must be made at the tower and building
location in order to design a grounding system that will meet
lightning GPR specifications. Available real-estate, soil conditions,
physical location, typical weather for the area all needs to be taken
ground and building ground should be designed to be less than two (2)
ohms each and the combined total ground should be less than one (1)
ohm. These measurements
must be made prior to connecting the power multi grounded neutral (MGN)
to the grounding system.
grounding system design requires radial grounding (spider legs) that
will move the lightning induced GPR away from the equipment building
and the single point ground location.
The engineering design of a proper grounding system and
isolation of wire-line communication facilities to protect against
lightning induced GPR will save many maintenance dollars over the life
of a tower site.
not least, protect the grounding system from corrosion by designing
for a 40+ year life. Grounding
systems that become green dirt over the years are very difficult to
maintain, and understandably become very expensive to correct.
Of course a deteriorating grounding system also costs more in
maintenance dollars over all of those deteriorating years.
Ernest M. Duckworth
Jr., P.E., President of LPGI & Affiliates
962 Coronado Drive
Sedalia, CO 80135-8303