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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 US . 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 Florida . 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 located.
Wireless Providers
Wireless 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 E. Duckworth .
Basics
An average 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.
If those 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.
Engineering Design
Design a 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.
Soil 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 into account.
The tower 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.
The tower 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.
Last but 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
303.688.5800
Fax: 303.688.5551
----- Original Message -----
From: "Tommy Alderman" <aldermant@windstream.net>
To: "'Tower and HF antenna construction topics.'" <towertalk@contesting.com>
Sent: Sunday, August 01, 2010 4:02 PM
Subject: Re: [TowerTalk] Vertical antennas and lightning
> This then, means that ICE lightning protectors, with their center
> conductor
> at DC ground, are really not good protectors to use? Doesn't it also bring
> in to question the advice to connect coax shields to both the top and
> bottom
> of a tower?
>
> To me, what Dave says, answers the age old question about antennas of any
> kind: 'To ground or not to ground?".
>
> Tom - W4BQF
>
>
> Subject: Re: [TowerTalk] Vertical antennas and lightning
>
> Unfortunately in this case the 'bleeding' when there is a charged cloud
> over
> head is putting charge ONTO the antenna from the ground to equalize it
> with
> the rest of the charge being attracted to the base of the cloud. So it
> make
> it MORE likely to start the upward streamer that completes the path.
>
>
> David Robbins K1TTT
> e-mail: mailto:k1ttt@arrl.net
> web: http://www.k1ttt.net
> AR-Cluster node: 145.69MHz or telnet://dxc.k1ttt.net
>
>
> _______________________________________________
>
>
>
> _______________________________________________
> TowerTalk mailing list
> TowerTalk@contesting.com
> http://lists.contesting.com/mailman/listinfo/towertalk
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