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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



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