Ground Testing FAQs
Category: Instrumentation Vol. 2

What do the test indicators tell me?
Indicators, in the form of red LEDs or black display bullets, tell the operator that threshold requirements for necessary test conditions have or have not been met. Specifically, these are contact resistance and signal-to-noise ratio. The test circuits (current and potential) must be mated with the earth with no more than a maximum threshold of contact resistance. This is quite high but if not achieved, red LEDs let the operator know. What then? Just wet down the area around the probe, hammer it in deeper, or pull up and go off in another direction. Maybe you'll find better soil. As with contact resistance, AVO's models are also top of the market in noise tolerance: 40 V peak-to-peak before saturation. This is double the industry standard. But if the environment is particularly bad with respect to electrical soil transients (noise), again a red LED will warn the operator that the measurement may be influenced by interference. In this case, if the operator is fortunate enough to be working with a model DET2/2 (a display message rather than an LED will be the actual warning with this model), the instrument possesses special features for eliminating the problem. With other models, it may be necessary to delay the test until the noise abates, or pull up and move off in another direction in hopes of striking a quieter environment. The significant point is that for all of these contingencies, the tester lets the operator know that there is a problem so that corrective action can be taken and adversely influenced readings will not be recorded as data.

How do I decide if I need a 3-terminal or 4-terminal model?
Easy! Four-terminal models do it all, resistance and resistivity testing, while three-terminals let you do resistance testing only. Resistance tests measure an installed ground electrode, while resistivity measures the electrical properties of the soil itself. If you will only have to work on grounds already in place, then all you may need is a three-terminal model. But don't overlook planning There may be a situation months or years ahead in which you will need to prospect the soil preparatory to designing and installing a new ground. If so, you'll want to be prepared with a four-terminal tester.

There's one other consideration. Our models measure to a resolution of .1 W in the three-terminals, and to .01 W in the four-terminals. This is because extra range and resolution are much more important considerations in resistivity measurements than for resistance. However, while it is unlikely that a few hundredths of an Ohm will ever make a difference in a ground rsistance measurement itself, this degree of resolution can be useful in removing the uncertainty factor from the digit ahead of it. The instruments' accuracy specification means that when measured only to tenths the measurement could be three counts higher and still be within the accuracy specification. But transposed to hundredths, that same accuracy spec means that the measurement is firmly fixed to a single tenth of an Ohm! Examine closely the specifics of your requirements, and make a determination as to how rigorous you want to be.

What does the DET2/2 offer for its higher price?
This is our top-of-the-line model. It provides extra noise protection from interference by voltage transients in the soil. All of AVO's models provide noise protection in varying degrees, but the DET2/2 offers extra capabilities. Originally, it was used to introduce the highest interference protection on the market, a specified 40 Volts peak-to-peak of noise tolerance. This represents twice the industry-standard 20 Volts which are typically offered by other lines. For a time, the DET2/2 was the only model to offer this added bonus, while other models adhered to the industry standard. But as AVO continually strives to incorporate the latest and best technology as much as possible, all of the newer generation of ground testers have been introduced with this enhanced protection. Now, even the economical three-terminal model (DET62D), as well as the mid-range DET5/4D and DET5/4R, all incorporate the maximum 40 V tolerance.

Three additional features of the DET2/2 give the operator added flexibility to overcome the worst testing environments. These features are: variable test frequency, adjustable test current, and filter. The variable frequency enables the operator to adjust the square-wave frequency of the test current in half-Hertz increments over a range of 105 to 160 Hz. Interference can simply be dialed away by shifting to a different test frequency. The high current mode can be engaged to improve signal-to-noise ratio, and using the filter will integrate unstable readings to clear the effects of superimposed noise. These added features comprise an impressive battery of capabilities that the skilled operator can use to prevail over the worst of electrical environments.

The DET2/2 provides an extra digit of resolution (to 0.001 above 0.010 ) when compared to other models. This extra digit accurately fixes the reading to one-hundredth of an Ohm (the digital uncertainty is confined to thousandths), and can prove invaluable to design engineers who rely on the most accurate data possible when creating a grounding system.

Is there a clamp-on ground tester?
Yes, there are clamp-on ground testers on the market, but AVO does not offer one. Clamp-ons are popular, primarily because they are so handy, but this can be a double-edged sword, as we'll see. Clamp-ons operate by inducing a current onto the test ground (rod), then measuring the voltage drop as the current creates its own test circuit by finding its way back to source via the most available path. Presumably, this circuit includes the soil in the vicinity of the test ground, plus the adjacent elements of the electrical system, such as a pole ground and system neutral. These extraneous elements are expected to contribute only a negligible resistance, so that the observed measurement is composed almost entirely of the soil resistance.

It does work, but the operator should be aware of the limitations. Most obviously, a clamp-on cannot test an isolated ground. There is no return path in place to complete the test circuit. A clamp-on must take advantage of in-place conductive elements in the electrical environment in order to operate, while with a three- or four-terminal tester, the operator creates his own test circuit by judicious placement of leads and probes. As installation tests to "meet spec" must be performed before the ground is brought on-line, clamp-ons are of little applicability for this function.

As a consequence, the clamp-on operator must have familiarity with the electrical system as a whole. Because it establishes its own current path via whatever is available, the clamp-on can "short circuit" and avoid the soil altogether. Therefore, the operator must know the electrical layout so that no such possibilities are in place, or else can be disengaged. By contrast, the operator of a standard tester is in complete control of the test set-up.

Because it measures an entire electrical loop resistance in series, the clamp-on experiences some loss of accuracy at the critical low end, where the best grounds are established. If the test "spec" is not too tough, like the NEC 25 or less, the few tenths contributed to the measurement by other elements in the test loop (such as adjacent ground and return through the system neutral) do no harm to the efficacy of the test. But below 5 Ohms, this introduced error can become an unacceptably high proportion of the total measurement. And there's no way to get rid of it. But with a four-terminal tester, all that the operator need do is utilize all the terminals in a true four-wire bridge configuration in order to measure the ground resistance only. And even with an economical three-terminal unit, the short lead to the test ground contributes far less "error" than does the complete return path required by a clamp-on.

And of course, clamp-ons cannot perform resistivity measurements at all, so are of no help in planning ahead.

AVO's ground testers have remained dedicated to the tested, proven, and traceable method(s) derived from Fall of Potential and described by IEEE. We feel confident that the small investment in set-up time is amply rewarded in versatility, accuracy, control, and reliability.

My test spec calls for the test to be performed with a null balance meter; what do I use?
This is a common reference appearing in older specifications. Null balance testers were designed for the performance of Fall of Potential testing and appear in specifications as a means of discouraging the operator from attempting a less rigorous and less acceptable type of test. Many null balance models, including those manufactured by AVO, are still operating in the field and are capable of performing a perfectly acceptable ground test. Their principle limitation is that they are older technology, somewhat more complicated to operate. Be assured, AVO's present models, while offering improvements in speed, ease of operation, noise protection, and other critical factors, still conform to the essential fundamentals of reliable ground testing. They are fully acceptable as modern replacements for null balance testers, and will meet with the approval of the most discerning and/or demanding client or inspector.

Why would I need .01 Ohm resolution?
At a glance, this capability in many of our models (0.001 in the DET2/2) may look like overkill. After all, ground resistance measurement is based on pass/fail requirements that are expressed as whole numbers. It isn't unheard of for instrumentation manufacturers to update models with leading edge technology just because it's there, whether the field operators can make any practical use or not. But this is not the case regarding the range and resolution of AVO's ground testers. Every digit has its place and purpose. Remember, the last digit freezes the digit ahead of it, so far as typical accuracy specifications are concerned. If a display reads only to tenths, a typical plus/minus accuracy specification of two or three digits could put the actual measurement above the next whole number; e.g., a reading of 1.8, ± 3 digits, could actually be 2.1, but if it were 1.85, this degree of uncertainty is eliminated. And so on.

But so what? First, many standardized test procedures use built-in mathematics which, as in the Slope Method, can be moderately complex. The reason for these mathematical tests is to provide an objective means of recognizing whether the test was adequate or not, and to spot, within reasonable accuracy, the point at which the measurement most closely approximates the theoretical true resistance. Thus, the operator's plain judgment can be replaced by an objective test, in situations where human factor may be deemed, by a client or agency, unacceptable. The accuracy error introduced by calculations becomes narrower as more digits are available with which to work. At critical low resistances, where the operator must introduce into the calculations a series of measurements that represent very small changes in a low value, the confidence level is significantly improved by higher resolution.

Secondly, resistivity measurements which are to be used in engineering formulas to design an effective ground are much improved in utility and accuracy by increased digits. Resistivity measurements become more effective if they can be made to greater depths, thereby possibly revealing sudden unexpected changes in rock strata. At greater spacing of the probes (for increased depth), resistivity measurements are typically lower, and better resolution is needed if these values are to be used most effectively in design formulas so as to eliminate the risk of rework and penalty clauses.

These considerations may or may not be significant to a given situation. But they should certainly be taken into account when selecting a tester.

Ground Testing FAQs
Category: Instrumentation