WELL-GROUNDED STRATEGIES: GET A GOOD GROUND THE FIRST TIME AROUND

Every day people depend on telecommunications for 911 emergencies, stock market dealings and conversation. If a telecom company's grounding systems falter, then equipment fails, calls drop and people are inconvenienced. Companies will have a difficult time staying alive in an increasingly competitive industry if their service levels decrease. With equipment sensitivity also steadily increasing, installing a ground right the first time is even more critical.

Good grounding has other benefits, such as enhanced personnel safety, reduction in 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 telecom companies ensure 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 maintenance-free years of high-quality performance and eliminate the need for rework or enhancement.

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 training, practice and the right equipment, including 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 the designer can model the site to provide the most effective design.

Variations of soil resistivities can range from 500 ohm-cm in clay to higher resistivities in limestone - 5000 to 1,000,000 ohm-cm or more. 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 very accurately. A sophisticated computer program uses this information to model the soil and grounding system and recommend 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 computer-aided design drawings that detail rod placements and anticipated performance levels.

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 will ensure that 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 problematic, and the results are frequently rendered invalid.

The most reliable post-installation testing procedure involves the fall-of-potential (three-point) method. With the help of 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 being tested and the most remote electrode. Measurements of the voltage drops between the ground rod and the remote electrode are made by moving the intermediate electrode away from the ground rod in steps. The meter reads the resistance at each distance. The verifier then simply plots the data and decides if it is a valid test.

If the test results seem too good to be true, they probably are. The first of the two most frequent reasons for invalid results is that the remote electrode is not extended far enough. When testing a single electrode grounding system, the remote electrode probe must be placed at a minimum distance of five times the length of the ground rod (or the diagonal of the grid under test) although ten times the length of the ground rod is recommended. Often this is not done because it is impractical - there may not be that much surface area at the top of a mountain.

Another cause of invalid results is that the earth-ground-to-neutral bond is not disconnected for the test. Cases have occurred in which a single 10-foot 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 use 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. Because the meter already knows the amount of voltage induced, it can automatically calculate the resistance in ohms and subsequently display the results.

While 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 might be electrically unsafe to attach a clamp-on meter, or for locations that are difficult to access, another of ground resistance tester is helpful. In this case, the sensing unit would be separated from the reader unit.

For example, the sensor can be buried in spots that would not be convenient or even safe to access, but the reader may be easily and quickly connected for instant ground resistance readings. This not only negates the possibility of reading the wrong lead, but it also allows easy access to inaccessible points via a six-foot connector lead. In effect, the permanently installed grounding monitor ensures a lifetime of grounding system checks.

By following the processes described above, your grounding program will provide dependable results right from the start. Treat the ground system as an important part of your telecom facility - the same as the batteries, radios or fire alarm systems - and you will be rewarded with increased system performance, protection and reliability.