Thank you for your excellent question regarding proper grounding around a tower. It is our pleasure to help.
There are many different types of tower, and many different reasons to be providing grounding. For a typical Radio/Cell/Broadband tower, grounding is used to resolve some of the following electrical engineering factors:
- Electrical safety grounding
- Reduction of Potential Difference or Ground Potential Difference (GPD)
- Reduction of the resistance-to-ground and/or impedance-to-ground
- Reduction of electrical noise, interference, harmonics, etc.
- Safety during electrical faults – Ground Potential Rise, clearing time
- Lightning safety – Electromagnetic fields, frequency spectrum, time domain
- Human Safety – reduction of Step & Touch Voltages
Each of the above issues is dealt with to a greater or lesser extent in a number of industrial standards related to the grounding of such towers. We are aware of no standard regarding a tower that considers a single 8-ft ground rod as acceptable. In fact, under the new National Electrical Code, it could be easily interpreted that a single 8-ft grounding rod is insufficient to meet it’s simple grounding requirements.
There are quite possibly several dozen codes and standards that govern grounding for towers in some form or another (IEEE, NFPA, MILSPEC, Motorola R56, IEC, ANSI, TIA, EIA, etc.), so we will not try to list them all here; additionally most companies have internal standards that have minimum grounding requirements for towers that are above and beyond those in the standards. But that said, you can meet the vast majority of grounding requirements by simply installing a ground ring (sometimes called a loop or counterpoise) around the base of the tower. Typically, the ground ring runs at least 1.5-ft below grade and 3-ft outside the perimeter of the tower, encompassing all of the legs of the tower. This conductor will be bonded to each tower leg via exothermic welding, irreversible compression fitting, and/or a double-bolt connection. The conductor itself will be bare copper placed in direct contact with the earth and typically ranges in caliber from a #2 gage to 4/0 AWG (or higher), depending on the current levels that are anticipated on the conductor (i.e. electrical utility fault and/or lighting strike). This ground ring must be supplemented with 10-ft ground rods (electrodes) at regular intervals (such as every 20-ft), with ground rods always placed immediately next to the footing. And of course, the ground ring around the tower must be bonded back to wherever the equipment is located.
Some helpful links:
The main trouble with all of these regulation, codes, and standards, is that none of them take into account local soil conditions nor do they analyze the specific setup for your tower. Each of the standards simply provides a “cookie-cutter” solution that they think will solve the majority of the problems in most cases. Unfortunately, the “cookie-cutter” solution seldom resolves all the issues, and can leave your project exposed to future problems, if it is not properly analyzed.
The Ground Potential Difference (GPD) issue in particular, is important to analyze and understand for simple daily operation of your electronic equipment. A Ground Potential Difference (GPD) that is too high, will cause major problems with signaling and data transfer rates between the tower and the electronic equipment. Another issue that is of concern is the resistance-to-ground (or impedance-to-ground) of the grounding system, which if not properly analyzed can limit signal range, in certain cases.
How far is the run from the tower to the equipment room? What is the voltage drop across the conductors? How much current will leak into the surrounding soil before a surge of electrical energy hits the equipment shelter? What is the resistance-to-ground of the system? What is the potential difference (GPD) between by tower and the equipment shelter? Will personnel be safe should they be standing near the tower or touching it during either an electrical fault from the utility company, or a lightning strike? How much current will be on any one grounding conductor and can it handle it without critically burning open? What electrical noise/harmonics/Electromagnetic Fields am I likely to encounter and can my grounding system handle it?
Questions such as the above require professional assistance and a serious engineering study to properly analyze your needs. E&S Grounding Solutions is of course more than happy to help you with these important engineering issues. But if not us, please find someone who can help you with your grounding needs. Feel free to call us at 310-318-7151 (California time) and someone will be happy to speak with you about your project, free of charge.
The Engineering Team at E&S Grounding Solutions
Photo Credit: http://www.flickr.com/photos/rutlo/3110766471/sizes/o/in/photostream/