Ground Testing FAQs
Category: Instrumentation Vol. 1

What is the best model for my application?
It's easy! Selection is based on just four factors: test(s) to be performed, prevalence of noise interference, desired resolution of measurement, and personal preference...

Test or tests - Ground testers perform two categories of measurement, resistance and resistivity.

Although similarly named, they are quite different. 'Resistance' refers to an installed electrode in the soil, while 'resistivity' is a measure of the independent electrical properties of the soil itself. If you only need to test installed grounds, you may need only a three-terminal model (DET62D or 250260). But plan ahead! Don't get caught if your testing requirements change a year from now. To do resistivity, you must use a four-terminal model (250302, DET5/4D or R, DET2/2).

Interference - Voltage transients in the soil can interfere with a test. Our standard models offer the best noise protection on the market, typically twice that of competition. But there are limits to everything! Indicators give warning if excessive noise is present, so that attention is called to influenced readings. Typical field procedure is to wait for the interference to abate, or move the test setup to another area, which is hoped will be ìquietî. Usually, this is sufficient, but if such interruptions are repeated, the delays can become counter-productive. For such inhospitable environments (around substations or heavy industry, especially if a 'cogen' is present), it is worth the extra expense to go up to the top-of-line DET2/2 (250202) model. This model offers enhanced noise protection features, including adjustable test current frequency, line filter, and high test current to improve signal-to-noise ratio.

Resolution - The three-terminal digital model (DET62D) affords resolution to 0.1 (. The four-terminal DET3/2, DET5/4D, and DET5/4R measure down to 0.01 _ . All can test resistance of an installed ground. Whatís the difference between having a tenth and a hundredth resolution? It depends on the rigor of the testing requirements. If the grounding spec is not too rigorous, a tenth of an Ohm is more than enough. Certainly, no specification is going to be written to a hundredth of an Ohm resolution. But if the ground must meet a tough spec, such as a computer-room one or two Ohms, the extra digit serves to fix the digit ahead of it so as to firmly establish the accuracy of the measurement. As an example, both three- and four-terminal models afford the same accuracy: ±2% of reading ±3 digits. But with three-terminal resolution, a reading of, for instance, 0.8 ( could be as much as 1.26 (displayed as 1.3). If the required specification were one Ohm or less, the ground would appear to ìmeet specî when in fact it needs improvement. With a four-terminal model, however, the same reading might be displayed as 0.85, indicating that the ground did indeed ìmeet specî. The choice is yours.

Preference - Within the three- and four-terminal groups, it is largely a matter of user preference. The three standard four-terminal models, DET3/2 (250302), DET5/4D and DET5/4R, differ in power supply; DET3/2 is hand-cranked, DET5/4D stands for ìdisposableî batteries, and the ìRî version indicates ìrechargeableî. Selection between these choices is easy, based largely on the operatorís experiences with these power sources. Rechargeables are certainly convenient, but run the risk of ìhuman errorî, i.e., someone forgets to recharge over night. Disposables can be changed in seconds, which minimizes but does not eliminate human factor. A set of ìextrasî taken to a job site can still turn out to be bad, and valuable time lost. Hand-cranking is still the most thoroughly reliable power source, but asks a bit more of the operator. The good news is that most readings can be taken in well under a minute! The time-consuming factor in ground testing is in the setup, not the measurement.

Of the two three-terminal models, the digital DET62D is the selection of choice. It offers all the latest electronic capabilities possible in an economical, three-terminal unit. But the analog 250260 may still be the choice of those who simply love analog, even though the time-base factor is not so significant in ground testing as it is in insulation testing (measurements should stabilize in a matter of seconds). Plus, due to its longevity, reliability, and popularity, the 250260 has been written into many standard test procedures that are encountered throughout industry, the utilities, and the military.

Why do I need 20 k of measurement capability when a good ground is no more than a few ohms?
MEGGER® Ground Testers have ranges up to 20 k . This is not overkill. It is correct that when testing an installed ground electrode, once the pass/fail threshold has been exceeded by a substantial amount, the purpose of the test may have been largely fulfilled. But there are at least two significant reasons why actual measurements, rather than over-range indications, can be useful. If a bad ground is to be improved, and the common trial-and-error method is used, it can be handy to see what effect an additional rod or another 10 ft of depth may exert, in order to better plan the overall tactic and have less trial and error. And when resistivity (as opposed to resistance) is being measured, units of measurement are not in ohms, but in ohm-centimeters. These measurements may be thousands of ohms.

How do I know if the unit is working?
You need only connect standard resistors between the leads, and energize the tester. Field calibration resistors are available from AVO or any convenient resistors of appropriate ratings can be substituted. General procedure would be to use two resistors, one with a value at the low end of the scale (1 to 5 ), and one at the high end which could be in hundreds or thousands of Ohms, depending on the model. If a three-terminal model is being checked, connect the resistor between a lead from the common terminal and one from the P terminal, with the C terminal jumpered to the P. For a four-terminal model, jumper the two corresponding pairs of P and C terminals together, take leads from the P terminal and connect the resistor between. Also, don't overlook the battery check. All models have one type or another of battery indicator, so there is no reason to be caught in the field with a dead instrument.