LIGHTNING IS A MAJOR SOURCE OF GROUND POTENTIAL RISE (GPR) & DAMAGE TO EQUIPMENT

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(Free-Press-Release.com) July 7, 2008 --
There is a 50% probability that a lightning strike will be approximately 30kA. (Anderson & Eriksson 1980) If the self-inductance of the earth 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 the equation, V=Ldi/dt; the estimated GPR of a 30kA strike will be 7.5kV. Values of GPR could easily triple for higher current lightning strikes or strikes passing through higher inductance.

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LIGHTNING IS A MAJOR SOURCE OF GROUND POTENTIAL RISE (GPR) & DAMAGE TO EQUIPMENT

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If the inductance of a grounding system is 10-6H, then a GPR of 15kV may result in a grounding system from a 30kA lightning strike. Thus, any grounded equipment that is connected to wire-line communication pairs is in jeopardy from outgoing currents seeking remote ground.

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 to remote ground, because of current division. However, if we are considering a small structure with relatively few communication pairs (20 or less) to remote ground then we must consider isolating all wire-line conducting paths.

Gas tubes, MOV's, SCR's, SAD's, SAS's, etc. are ground shunting devices and thus, will not protect equipment located on the GPR ground plane. Also, the firing speed of these devices is of no consequence. These devices merely offer an additional path to remote ground through the communication pairs for any and all outgoing currents. In fact, they guarantee a connection to the communication path in the reverse direction from which they were intended to operate.

The only GPR solution for protecting equipment connected to wire-line communications is through isolation, using fiber optic cable, optical isolators or isolation transformers. (IEEE Std. 487-2007) These devices isolate and thus prevent current flow. If there is no path for outgoing currents to flow, there will be no current flow. If there is no current flow there will be no harm to equipment or associated working personnel.

Divide and Control

The control of dissipating lightning strike energy requires division. This is an absolute must for success, because of the magnitude of the current and the resulting surge impedance of any single dissipation path. Ten radials connected to a ground ring bonded to an antenna, will divide lightning current up into ten smaller segments. This will help ensure that the lightning will more likely follow the designated paths for dissipation into the earth and lower the resulting GPR to the adjacent equipment building grounding system.

The optimum length of these ten radials, using #2 AWG Solid Bare Tinned Copper (SBTC), is approximately 25 m (80 feet) each with interconnecting 3 m (10 foot) ground rods, spaced every 6 m (20 feet). Note: The ground Rods may not be necessary to improve the grounding system. Longer length radials will offer little dissipation improvement, because the lightning strike energy will not remain on the radials for much over 25 m ( 80 feet). (Block 1993) In very limited spaces, the recommended minimum grounding system is at least 60 m (200 feet) of buried bare ground conducting wire composed of five radials, each 12 m (40 feet) in length, with interconnecting 3 m (10 foot) ground rods, spaced every 6 m (20 feet).

A greatly improved copper wire grounding system can be easily achieved by the use of conducting cement placed around the radials at the time of installation. The cement will harden into concrete, both protecting the grounding system (giving it many years of additional life), and making the system a much better (lower) ground resistance.
Consider using only Sankosha Lo-ohm conductive cement. EMC is the distributor.

More information can be found online at http://www.lightning-protection-institute.com

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