Often clouded in confusion, ground electrode testing can offer results that are valid and useful if you understand some basics.
Confused by the how and why of ground electrode testing? You're not alone. Most people perform such tests improperly or skip them altogether. Either approach is risky. Armed with the right information, however, you can produce accurate test results. And based on those results, you can make the changes necessary to ensure your grounding system provides a safe environment for the most sensitive equipment.
The standard three-point fall-of-potential or slope test on a “totally isolated electrode,” as described in the IEEE Std. 81-1983, will determine the effectiveness of a grounding system. You can test nearly any site with a standard three-point ground tester. This equipment works at a safe output of up to 50 mA and — when not affected by on-site stray voltages — provides accurate values if properly employed. However, people often perform the test without shutting down power or isolating the grounding system from the utility neutral. Such a mistake will only provide you with a test of the utility neutral ground.
You will need one auxiliary potential probe (P2) and one auxiliary current probe (C2) to perform a three-point test. Connect the C1 and P1 either directly or internally through the meter to the electrode under test. To perform a valid test, place the C2 probe 100 ft from the potential probe. If the system under test is larger than one or two 10-ft electrodes, you must determine the width and length of the system as well as the diagonal dimension. Place the C2 probe at least three times the length of the diagonal from the system under test. For example, if the system is 30 ft by 40 ft, the diagonal will be 50 ft. You should place the C2 probe a minimum of 150 ft (3 ft × 50 ft) from the system under test. If the P2 values remain within 5% to 10% of each other at 52% (52 ft), 62% (62 ft) and 72% (72 ft) distances, you can assume the results are accurate. Use the 62% reading as your base value.
To get an accurate test, place the auxiliary probes on opposite sides of the electrode under test and take the average of the readings. If the test values differ greatly, rotate the position of the probes 90° around the electrode and take two more tests. Now average the four results. Keep in mind, dry conditions or highly resistant soils can make it difficult to get consistent readings. Adding salted water around the test probes or employing deeper test probes may be necessary. If stray voltages affect your readings, running a test 90° from the first test should cancel out these objectionable currents. It may be necessary to use a tester that provides adjustable or variable test currents.
An alternative to this kind of testing is the clamp-on ground tester, which is effective in many grounding situations and doesn't require you to turn off the power — nor does its effectiveness depend on how well you lay out your probes. However, the instrument is useful only if you follow the instructions that come with the tester. Doing so will yield a valid test or will render an indication the test is invalid — usually, a reading of 0.7 ohms.
When used properly, the clamp-on gives consistent readings and is an excellent tool for periodic checking of an existing system against baseline readings. If you are reading anything less than a few ohms, you're probably reading a ground loop. In that case, you need to look at the system and make sure you're at the right point. Don't overlook the three-point test during new construction either — you may never get another opportunity to conduct that particular test.
If possible, perform these tests during the driest and least conductive period of the year. If you want the system to be more effective year-round, install longer grounding electrodes deeper into steady-state layers that are less affected by weather conditions.
The importance of implementing a system as described is critical if you want to maximize the protection of electronic equipment and people. Applying grounding-testing theory provides an enormous return on investment — provided you use the proper equipment and apply the right techniques.
Brooks is the president of Ground Testing Inc., N. Billenca, Mass.
Sidebar: Considerations for Grounding Grid Effectiveness
Use wire at least as large as specified in NEC table 250.94.
Before energizing, perform a continuity test between the grid and the neutral ground connection.
The ground grid wire must be mechanically continuous to the neutral ground bus.
The ground grid connection may not have any sharp bends in it.