How can you perform a ground test when you can’t drive a spike?
A new customer has just awarded you a service contract to perform standard electrical maintenance for a large suburban mall. As part of the agreement, you must routinely test the pole grounds on parking lot lighting. But the site has acres of asphalt and concrete roadway or pavement, with little soil except what's around the potted shrubs. Many of the poles are so far from the fringes of open soil that you'd need prohibitively long test leads to reach a spot with access to the soil. Under the circumstances, you'd be unable to meet the critical placement requirements of the potential probe necessary to construct a fall-of-potential graph, but your contract calls for you to perform a full fall-of-potential test, as described in IEEE Standard 81, with the resultant graph included. What can you do?
Lack of penetrable soil is a common problem. Job sites such as airports, large commercial and industrial parking lots (see Photo on page 54), and congested urban areas afford little or no access to soil. Breaking up cement and asphalt surfaces to gain access to soil is costly, time-consuming, and often not permitted. But even in open areas, the soil may be rocky, baked, or extremely shallow, making it impossible to drive the test probes.
Fortunately, modern test equipment allows you to perform standard tests in the worst situations without altering the prescribed method or sacrificing reliability. Just as advances in microprocessors have revolutionized personal computers, they've also greatly changed ground-testing equipment. These technological leaps have made possible advanced filtering logarithms and other means of dramatically increasing the sensitivity and accuracy of these instruments.
Today's instruments need only a fraction of the test current and voltage drop once required to perform a rigorous and reliable test. This new design has also made it possible for ground tester test circuits to tolerate high contact resistances while delivering a dependable reading. This resistance threshold enables you to perform the test without breaking the surface of the roadway or pavement — what some people refer to as the “lazy spike” method.
Try laying the test probes, or spikes, parallel to the surface being tested so the full length of each probe is in contact with that surface. A tester that affords maximum sensitivity should have an indicator advising you whether you've made adequate contact. This may be a lighted LED or a display message such as “current terminals open circuit.” You should have one indicator for the current probe and another for the potential probe. If these indicators are not engaged, the test is set up accurately and you can proceed with confidence.
But suppose you have an indication of too much resistance (via the LED or screen message). Remember, all you're trying to do is get a better contact with the soil. This is similar to getting a better “bite” with a set of alligator clips, except the methods must accommodate asphalt or concrete rather than wire or metal casework. Just throw water on the probes. Watering a probe increases contact with the test surface, and thereby reduces resistance by filling in the gaps between bumps in the test surface and gaps on the surface of the probe. The effect is even greater if you use salt water.
Don't be concerned you may influence the test result — you're only meeting the threshold of tolerance as indicated in the tester's specifications. That done, the test circuits automatically balance to give the correct reading. Remember, you're testing the resistance of the ground electrode, not that of the probes. If you water the electrode, you'll influence the result, but the probes are only facilitating the test; they aren't part of the measurement. Watering the probes permits the measurement without changing it.
Unfortunately, you can't always count on water to help. If you have enough information to fully prepare for the job, you can use a coiled metal chain or a flexible metallized-grounding mat. Clip the test leads onto the chain or mat and water it down. Either will contour much better to the surface — no matter how rough — and provide a much larger area of direct contact than a rigid spike would. Don't forget, you still must watch for the warning indicators.
A high-quality tester will monitor its degree of contact, and if you don't meet the threshold for measurement within the specified accuracy, the tester will let you know. The correction factor when using the metal chain method is easier than correction with driven probes: Just add more chain. Instead of hammering away at a probe or tamping the surrounding ground — methods that have their physical limits — you can add chain almost without restriction until you've made enough surface contact. With this setup, you can perform all of the standard tests by moving and placing the chain or mat just as you would pull up and redrive a spike.
Ground testing is no longer an add-on function you can perform when convenient and circumvent when not. The sophisticated electronics associated with data- and telecommunications, process control, and computer room performance are susceptible to noise and harmonics, making a good ground essential. In these demanding situations, it's no longer acceptable to merely run a test lead to the nearest fence post or spigot. You must test and verify ground to high standards and accuracies. In difficult environments, lazy spike capabilities will prove very beneficial.
Jeff Jowett is a senior applications engineer with AVO International, Valley Forge, Pa.