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"The Circle of Destruction" ---- Ground Potential Rise (GPR) from Lightning Strike Energy

A common situation where a grounding system can be the cause of equipment failure and personnel can be electrocuted instead of being protected

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
Fax: 303.688.5551