Ground Testing Methodology

Until the last decade, the basic methodology for ground testing had remained fundamentally unchanged since it was devised in the early twentieth century . A current is injected into the earth from the test electrode to a remote probe, and the voltage drop caused by the soil to a designated point is measured . Ohm’s Law calculates the resistance . Initially, separate current sources and voltmeters were employed, and the calculation was done with paper and pencil . Soon, dedicated instruments appeared in which these functions were properly combined to best fit the application, reduce operator error, and increase the speed and efficiency of the test.

Along with the development of instrumentation, the procedure was also standardized . By “walking” the potential probe toward the current probe and graphing the measurements, a profile could be developed that reliably indicated the ground electrode’s resistance . This procedure is termed “Fall of Potential,” and is described by IEEE in their Standard #81.   This procedure has laid the basis for ground testing from the dawn of the Electrical Age to the present . Various other popular methods, such as Slope, 62% Rule, Intersecting Curves, and others, are derived from Fall of Potential, with modifications to deal with special situations or to improve productivity . They all employ the same instrumentation . That instrumentation has been steadily improved in terms of convenience, safety, ease of operation, accuracy, and various features . But the basic design is still determined by Fall of Potential.

Stakeless Method

During the last decade, a new technology appeared in the form of the clamp-on ground tester . Its raison d’etre was speed and convenience . Fall of Potential can be labor intensive and time consuming . Even the expedient methods still require the stringing of leads and driving of probes . The clamp-on tester represented a quantum leap in convenience; just clamp it over the ground rod and take a reading ! This convenience is both an advantage and a drawback . Properly understood, the clamp-on ground tester can be an indispensable tool . But its simplicity makes it too easy to ignore instructions and use improperly . Users had a tendency to clamp around any convenient point and take away the reading without question . As a result, the method itself became unwelcome in some industries . Therefore, its effective use mandates that the operator be aware of its principle of operation, where and why it works, and the applications for which traditional methods only must be employed.

The clamp-on method derives from the original “shortcut” procedure: the “2-point” or “dead earth” test . This test was made popular by its ease, simply connecting a tester between the test electrode and a remote ground, and measuring a loop resistance . The method is easy, but loses accuracy because all the other elements in the loop are part of the measurement . It is fraught with the danger of inaccuracy because the remote probe must be of negligible resistance, which may not be the case . The clamp-on avoids this problem by utilizing multiple returns in parallel . By the Law of Parallel Resistances, the return resistance virtually cancels out, and the tester measures the resistance of the soil.

The jaws of the clamp-on contain two windings . When clamped over a ground rod and energized, a CT induces the test current by electro-magnetic coupling . The current travels through the soil and returns to complete the circuit through all available system grounds . Since these are normally plentiful, their total resistance is negligible, and the test measures principally the resistance of the intervening soil. The tester accurately measures the current flow, and the second winding senses voltage drop around the loop . Ohm’s Law does the rest . The method can be an enormous time saver, but unlike traditional 3-point testing, it cannot be used everywhere.

The clamp-on should not be used to perform the following . Instead, rely on the traditional three and four terminal instruments.

• Commission new grounds, as they will not yet be connected to the utility, and hence no return path exists for the test current.
• Measure soil resistivity (electrical conductivity properties of soil itself); this requires a 4-terminal tester.
• Test any complex ground where a metallic loop exists; test current will return through metal and not be forced into the soil . These include ring grounds, counterpoise, substation grounds, and various other multiply interconnected grounds.
• Perform any test where a client or third party require conformance to a reference standard; the method has not been incorporated into any independent standard.

The clamp-on should be used to:

• Test installed grounds without having to disconnect from the utility.
• Test any grounding electrode configuration where there is a return path that includes the earth.
• Perform a ground test without the necessity of any auxiliary leads or probes.
• Test the resistance of a single rod in a series or array.
• Simultaneously perform a bonding test of the grounding conductor (necessary to complete return path).
• Check errant current flow to ground for operator safety and give an overview of ground system dynamics.

In summary, both types of testing...traditional and clamp-on...have unique strengths. They are by no means competing or mutually exclusive technologies . For their effective utilization, their differences must be understood . Merely clamping onto any accessible site does not constitute effective ground testing . Traditional methods can be used in all situations; clamp- ons have dedicated applications . Traditional testers afford highest accuracy; clamp- ons give a series resistance with a small contribution from other elements . Traditional testers are labor-intensive; clamp- ons are extremely easy to operate . Traditional testers can perform installation checks; clamp- ons can test connected grounds . Clamp- ons simultaneously check bonding; traditional testers perform this as a separate test . Traditional testers can perform soil resistivity tests; clamp- ons cannot . Traditional test results can be proofed and certified to independent standards; clamp-on results cannot.

A well-equipped ground testing program will take advantage of both technologies . A clamp-on will quickly pay for itself in man hours . A traditional tester will equip the operator for all possible situations.

The wide range of ground resistance testers available from Megger offer both fall-of-potential and clamp-on methods to optimize your ground testing .