First, it is important to understand that there is a difference between minimum design standards and good engineering practice, as related to lightning strike energy and power line faults. Minimum addressed requirements do not always follow good engineering practice, particularly in the design of facilities that may be subjected to Ground Potential Rise (GPR) from lightning strike or power line fault energy.

IEEE Std. 487-2000 states that you do not have to use isolation on wire-line communications if the GPR is less than 1000V-Peak Asymmetrical, because not doing so will not result in a fire. However, a GPR under 1000V will still damage cable and equipment, and possibly harm personnel. Also, this standard does not directly address the need for isolation of wire-line communications in locations other than in power-type high-voltage environments, because of its mandate to only address those environments. Thus, there is presently no standard that addresses the dangers of GPR resulting from lightning strike energy in non-power environments.

To protect personnel and equipment from the possible effects of a GPR, you should consider using isolation equipment on all wire-line communication services where the calculated GPR will be over 300V-Peak. If you have any highly reliable Class-A services that you want to continue to function at all times, even during a GPR, then you must use isolation no matter what the calculated GPR.

Any tower, antenna, or other object that is higher than surrounding objects, is at risk of being struck by lightning. Thus, these locations are susceptible to the resulting GPR from lightning strike energy passing down through the impedance of that grounding system, and isolation must be considered if equipment damage, personnel safety or communications reliability is a concern.

As you may be aware, standard grounding devices, i.e., gas tubes, MOVs, SADs, etc., that shunt energy on wire-line services to ground, will not protect those services from a GPR. In fact, these very fast acting devices will actually fire backwards placing the current from a GPR back on the line.

Obviously, in cases related to power line faults, the better the cell site grounding system, the lower the GPR. This is basic "Ohms Law" --Voltage is equal to current multiplied by (x) resistance, in cases related to power line faults. However, in the case of lightning strike energy, the equation is: V=Ldi/dt. Very conservatively, an average 30kA lightning strike will produce a 7.5kV GPR. A 100kA lightning strike will produce an approximate 25kV GPR. Although rare, super strokes of 200kA or more are known to exist.

However, don’t be fooled in believing that good grounding systems can be had easily. No matter how hard you attempt to provide a good grounding system for the cell site equipment that matches the grounding system of the object that it is using for an antenna, you will probably not be successful. Consider this: for every foot of 4/0 ground conductor, there is approximately 1.5kV of voltage drop at lightning frequencies. Thus, as an example; a cell site grounding system could be grounded to a water tower grounding system (very good ground) located just ten feet away with a 4/0 ground conductor, and still have a 15kV difference in potential at lightning strike frequencies.

Maintaining reliable, safe and maintenance free wire-line service, year after year, requires the use of isolation equipment in GPR locations and very good grounding systems. Don't be lulled into a false sense of safety just because your grounding system is tied into another very good grounding system. Remember, at lightning frequencies, just a few feet of heavy ground conductor (4/0) can represent a significant difference in potential between two grounding systems.

If you require your cell site to function through a lightning strike to the tower, call LPGI & Affiliates at 303-688-5800. We can engineer a design that meets IEEE Std. 487-2000.