People depend on telecommunications for 911 emergencies, stock-market dealings, and talking to loved ones. If a communication company's grounding systems falter, then equipment fails, calls drop, and people may be inconvenienced or even suffer. With the sensitivity of equipment steadily increasing, it is even more critical that a good ground is installed right "the first time."
Good grounding has other benefits, such as enhanced personnel safety; reducing system noise; and protection from lightning, unwanted voltages, currents, and power surges. Without a proper low-resistance ground, standard protection devices such as breakers-or transient voltage surge and lightning protection systems-are rendered ineffective. Communications equipment manufacturers such as Ericsson, Lucent, Motorola, and Nortel may void their equipment warranties at sites where the ground system performance does not meet their explicit earth grounding requirements, typically 5 ohms or less.
Only through proper electrical site protection can telecommunications companies assure effective grounding and the best protection for their cellular sites and switches. This may take relearning the basics and grasping the engineering design and testing process, but designing a proper ground will result in years of maintenance-free high-quality performance and eliminate the need for rework or enhancement.
Important considerations The first step in designing a proper ground starts with soil-resistivity measurements. It is a crucial first step on which the remaining steps in the process are based. Although not difficult, measuring soil resistivity can be time-consuming and requires some training, a four-pole ground resistance test meter, reels of conductor, and four probes. Soil resistivity measurements must be taken in at least three different directions at four or five probe spacings, even on the smallest land areas. This involves driving probes into the earth several times in each direction. The more probes and data, the more accurately and effectively the designer can model the site. Variations of soil resistivities can range from 500 ohm-cm in clay, to 5000 to 1-million ohm-cm or higher in limestone. Even in adjacent lots they dictate the ground system performance within each site. When all the soil data are collected, the technician should forward the information to a qualified design firm.
The design process Armed with reliable resistivity data, a site map, and a geo-tech report to identify rock beds, a designer can complete the ground system design with high accuracy. A sophisticated computer program utilizes this information to model the soil and grounding system and makes a recommendation for the quantity, type, length, and shape of the ground rods, including rod spacing and placement. Using these models, applications engineers can write recommendations that include CAD drawings detailing rod placements and anticipated performance levels.
Installation and verification At this point, the drawings are given to an installation contractor, but the job is far from over. After the grounding system is in place, the verification process begins. Verification testing is required to ensure the predicted ground system performance has been achieved. This validates the design, installation, and equipment manufacturer's warranty. Although seemingly simple, conducting the test is often a problem, and the results are frequently rendered invalid.
The most reliable post-installation testing procedure involves the Fall-of-Potential (three point) method. Utilizing a digital ground resistance meter, two auxiliary electrodes are driven into the soil at predetermined distances as per testing specifications in a straight line from the ground rod under test. The meter supplies a constant current between the ground rod under test and the most remote electrode.
A series of measurements of the voltage drops between the ground rod under test and the remote electrode are made by moving the intermediate electrode in steps away from the ground rod under test. The meter reads the resistance at each distance. The verifier then plots the data and decides if it is a valid test. If test results seem "too good to be true," they probably are.
One of the most frequent reasons for invalid results is that the remote electrode is not extended far enough. If you are testing a single electrode grounding system, then the remote electrode probe must be placed at a minimum distance of five times (10 recommended) the length of the ground rod under test (or the diagonal of the grid under test). Often this is not done be cause it is impractical-there may not be that much surface area at the top of a mountain.
Not disconnecting the earth ground-to-neutral bond is another cause of invalid results. There have been cases where a single 10-ft driven rod tested out at 65 ohms with the neutral bond disconnected. However, with the utility neutral connected, the resistance dropped to 2.5 ohms. Certainly, breaking the ground-to-neutral bond cannot always be done, but performing the test without breaking that bond is a waste of time and money.
Alternative testing methods Fortunately, the utilization of a ground resistance clamp-on meter provides an alternative method of verification testing. The meter takes advantage of the connection between site ground and the utility neutral-exactly the opposite of the Fall-of-Potential test. The jaws of the meter contain two current transformers (CTs). When clamped around a ground conductor, one CT induces a high-frequency fixed voltage into the conductor. If a continuous circuit exists, a resulting current flows. A second CT then senses and measures the flowing current. Since the meter already knows the amount of voltage induced, it can automatically calculate the resistance in ohms-and then display the results.
Although the clamp-on meter permits testing a grounding system without disconnecting the utility, the meter must be clamped in a place that will force the induced current through the grounding system. If a metallic alternative path exists, the current will follow a path of less resistance (not through the grounding system). When this happens, the meter will read an error indication of 0.7 ohms.
In instances where it may be electrically unsafe to attach a clamp-on meter, or for locations that are difficult to access, a AEMC Model 3780 Ground Integrity Resistance Tester can be used. Here, the sensing unit would be separated from the reader unit. For example, the sensor can be buried in spots inconvenient or even unsafe to access, but the reader may be easily and quickly connected for instant ground resistance readings. This negates the possibility of reading the wrong lead, and it also allows easy access to inaccessible points via a 6-ft connector lead. In effect, you end up with a permanently installed grounding monitor to ensure a lifetime of grounding system checks.