Ground Testing FAQs
Category: Applications Vol. 2

Does it matter how deep I drive the probes?
No, not once a threshold value for maximum contact resistance has been met (we are referring to test probe resistance here, not the installed ground that is being tested). It is a common fallacy that driving the test probes deeper will lower the readings. Imagine your readings changing as you drive the probes; what would be the correct value? It's true that the resistance of the ground under test does change as the test probes are moved, but the standardized procedures deal with this and provide a means of determining when the correct reading has been achieved. With respect to the probes, this is not necessary. They need only make a minimum amount of contact with the soil, the attainment of which can be recognized by merely observing the display indicators. Once contact is achieved, the test may proceed. And with instruments from AVO, the resistance tolerance in the test circuits is particularly high, so that threshold contact is readily attained. So much so that it may not be necessary to even penetrate the surface. Just laying the probes flat and watering them down often lowers contact resistance enough to meet the threshold tolerance.

My testing is all on concrete and macadam; how can I drive probes?
The good news is that you probably don't have to. Our models have uncommonly high resistance tolerances in the test circuits (typically 4, 40, & 400 k for the current circuit, 75 k for the potential). Any surface contact of a resistance less than these high thresholds is enough. Therefore, you may only need to lay the test probes flat on the surface and establish contact by wetting the area. Concrete conducts current fairly well and chances are good that you'll have an acceptable test. Macadam is not as good because of the non-conductive tar, but you may still be able to achieve enough contact. You'll know because indicator lights on the tester warn you if the contact threshold has not been met. If that becomes a problem, you can improve your chances by using a contact mat instead of the provided probes. Mats are flexible metallized conductive pads that mate with the surface contours. They are readily available from ground materials suppliers.

I have no room to stretch out the leads; what do I do?
Try another method. This is not as cavalier an answer as it may sound. Good samaritans have anticipated this problem long ago, and developed specialized test procedures to meet it. Test procedures are described in IEEE Standard No. 81 and are readily available from the general ground-testing literature. Most procedures are generalized variations of the comprehensive Fall of Potential Method but some have been devised as answers to specific situations like testing in congested areas. The most-used procedure for this particular problem is the Star-Delta Method. This is an adaptation of the two-point method, with arithmetic built in to compensate for the uncertainties that make more generalized two-point testing unreliable. Specifically, rather than going straight out with potential and current leads, the test probes are arranged in a fairly close triangle around the ground under test. A series of two-point measurements are made between the various pairs of elements in the array (probe to ground and probe to probe), plugged into a series of equations, and then you have your answer. If there are problems in the test setup, the arithmetic doesn't compute to a coherent answer and you know to try again with a different configuration.

What safety precautions should I observe when performing a ground test?
Industry-standard safety practices are always a good idea, if for no other reason than to condition personnel against becoming lax and wandering unprotected from one electrical environment to another. But with current model Megger® ground testers specifically, there are few requirements. The instruments themselves present no potential hazard. While testers of years ago sometimes used high currents and voltages, and some lines still do, all Megger models have taken advantage of microprocessor sensitivities in order to limit both voltage and current within levels safe for human operation. No more than 50 V and 10 mA are produced, except when using the DET2/2 in its high-current mode, in which case a maximum of 50 mA are produced at low voltage. The only possible hazard in a ground test then is from the electrical system if the test is being performed on a ground electrode while it remains on line. Incidentally, because of the low voltage and current, plus uncommon frequency, the ground tester does not introduce any intrusive signal onto the electrical system that might trip protective devices or interfere with operation. The reverse can occur, however. The electrical system can intrude on the tester. That is to say, if a fault condition occurs while the ground test is being performed, the ground electrode will be brought on line by the fault current going to earth and voltages can develop across the tester. These can damage the instrument and threaten the operator. For personnel safety, however, all that needs to be done is to follow standard safety practices, such as wearing insulated electrical gloves and working on a protective mat. To protect the instrument, fused leads can be used.

I've tested a ground and gotten a reading of 520 Ohms. Is there something that I've done wrong?
Probably not. As long as the tester is in calibration and a standard test procedure has been followed adequately, you can rely on the measurement. The problem is with the ground, not with the operator. It is not at all uncommon to discover grounds that are anything but! Because of neglect, deterioration, drastic changes in environment, poor installation, lack of forethought in design, or any of a number of common causes, ground electrodes may be providing little or no protection. To seek out and correct these bad grounds is why the test is being made in the first place. Do not be surprised to find ground electrodes that measure in the hundreds, even thousands, of Ohms. It is a good idea to first perform a few repeat tests so as to confirm the bad news, and then start digging.

I have a large grid to test; how do I know what model to choose?
The size of the grid hardly matters to the tester. It does matter, however, to the operator. The two principle effects of grid size are on lead lengths and method chosen, rather than model of instrument. Any model can, at least nominally, test any grid. Just because you might have an economical model, don't become dismayed by a large grid, and assume a special instrument is required. As long as an accepted test method is followed rigorously and yields coherent results, the measurement is reliable. Large grids do, however, often require long lead lengths and/or special test methods. It is more important that the operator give thought to these considerations than to the instrument itself. In some difficult situations, the high current available from the DET2/2 can be of help in establishing adequate test current, but this can be accomplished in other ways also, such as by driving deeper probes or watering the probe area. A good operator relies on his own experience as well as on the instrumentation. No model is meant to serve as a substitute for human abilities. The combination of an experienced, thoughtful operator and a high quality instrument is always the best tool.

How do I test a counterpoise system?
Although it is notably different in design from a standard ground rod or bed, a counterpoise system is nothing special to the ground tester. Fall of Potential, and its derivative methods, still provide the means of testing. Theoretically, the system should test essentially the same from any point, although local realities can sometimes superimpose their effects. For that reason, it is a good idea to repeat the test in various directions, in order to average out possible localized variations. Contact can be made with the counterpoise system at any convenient, accessible point. Remember, as long as the system was designed and installed correctly, all of its elements should be in parallel with respect to the ground.

Other requirements may prevail as well with respect to the conductors coming off the tower and their connections to the system. Therefore, it is recommended to thoroughly examine specifications. The test may not actually entail a single measurement, but rather it may include both the electrical relationship of the buried system to the soil and of the conductors to the system and the tower. The actual ground resistance measurement remains the same but it may have to be supplemented by verification of complete continuity from the tower to the buried electrode. Of course, this is true of any ground but is often more elaborately and rigorously specified for counterpoise systems because of their demanding requirements for symmetry in order to smoothly conduct high-current faults from lightning strokes. To be thorough, a high-current, low-resistance ohmmeter, such as a Biddle® DLRO®, should be used to check for continuity across bonds, welds, clamps, and similar conductive elements. But as a mere backup or spot check, multimeters, or even a ground tester placed in two-terminal configuration, may suffice. If the system has already been buried, there is a problem, but it is not hopeless. An instrument such as the Multi-Amp® Safety Ground Test Set is a highly specialized unit that can inject a high current into a buried system in order to test that the individual elements comprising it have sufficient continuity through bonds and welds. Do not confuse this with a ground test, however. The electrical relationship with the surrounding soil is separate and must be verified by a recognized ground test.

Can I test my ground conductors with your model?
Yes and no. First, it is essential to identify terms and what all is actually required of a test procedure. It is not uncommon that what someone means by a ground test is actually comprised of two separate measurements, one involving the surrounding soil; the other the continuity of conductors. Read your test specifications and requirements carefully so that nothing is overlooked. Once installed, the ground electrode will be connected to the electrical system by one or more conductors. This connection may vary anywhere from the usual copper conductor extending from the ground buss at the service entrance to a rod driven at the base of the building, to the elaborate and symmetrical connections designed to conduct lightning with a minimum of impedance from a tower to a counterpoise ground. These connections must offer little resistance, otherwise the whole purpose of establishing a ground is defeated. To thoroughly test them, an appropriate instrument is an ohmmeter with a high test current capable of exploiting weaknesses, loose connections, corrosion, poor welds, and such, and revealing them in the measurement. A common multimeter does not do this, because it makes its measurement with only a few milliamps of current. A Biddle® DLRO®, which can output 10, or up to 100, Amps will perform a rigorous test that will satisfy the most demanding inspector. As a handy backup check, a multimeter can be used and even a ground tester, with its pairs of current and potential terminals jumpered together to produce two terminals, can serve this purpose. Mainly, just be aware that your test requirements may include both a separate ground test and a continuity test across the connectors. Incidentally, the ground test should be conducted before connection is made to the electrical system. If made after, the entire power grid becomes part of the measurement, all the way back to the substation ground. This is still useful in determining the overall ground but the actual quality of the local ground cannot be assessed.

I need greater lead lengths than your kit provides; what do I do?
This is simpler than it may look. There is nothing exotic about the leads and probes used to adequately conduct a ground test. The wire is 14-gauge braided copper, covered by rubber insulation. Any suitable wire can be used to extend the length of the test setup, down to 18 gauge. Connection is made to the tester by universal terminals that will accept spade lugs, banana plugs, and even bare conductor. The probe end is a copper alligator clip covered with a protective rubber boot. All of these materials are readily available and can be used to increase the expanse of a test setup. Contact resistance from stringing lengths together will most likely not present as much of a problem as might be expected, because of the high resistance tolerance in the test circuits. If too much contact resistance does occur, warning lights on the tester will give the operator ample indication that some additional steps must be taken. The probes too are not highly specialized and general ground rods, even railroad spikes, can be used to penetrate the soil. Again, the warning indicators will let the operator know if there is any problem with the choice of accessory materials. It is for this reason that AVO offers leads and probes as optional accessories, to allow planners the option of making use of their own materials.

It has just rained heavily; will this influence my test?
Yes. There's little you can do about the weather, except be aware of its effects and work accordingly. Soil conductivity is based on electrical conductance by dissolved ions in moisture, not unlike the action of a car battery. When it rains, the increased moisture dissolves salts in the soil and promotes added conductivity. Resistance goes down. If the only goal was to "make spec", you could try watering the area before the arrival of the inspector. But that only defeats the purpose of installing a ground. Remember, the electrode is only as good as its worst day, because a fault situation can occur at any time. If it has rained all night, and the electrode barely meets spec, chances are that it will not when tested during dry weather. The ground design should be improved. Take all of this into consideration, and plan accordingly.

I have a ground installed in sandy (or rocky) soil; what can I do to test this?
The test is the same, but chances are the results won't be pleasant. It is much more difficult to ground in sandy or rocky soil. Sand does not hold water well and so the moisture that is needed to promote electrical conductivity readily drains away. Rocky soils have poor overall consistency, lots of space between individual elements, and reduced surface contact with the buried electrode. All of these conditions mean that the original design and installation must be more rigorous and thorough than in more agreeable types of soil. If this wasn't done in the first place, chances are that the results of a later ground test will prove unpleasant.

The test itself isn't categorically different, but it may be advisable to take some special steps to make it more successful. In rocky terrain, it may be necessary to use longer, more robust test probes in order to attain sufficient contact with the soil. The tester's indicators will apprise the operator of this. And since ground systems in poor soils generally have to be larger and more elaborate, their electrical field zones are much larger and more diffuse than those of simpler grounds. Therefore, it may require excessive lead lengths to get outside the ground's sphere of influence for a good test. Be prepared to switch to an alternate method that does not require as much distance (e.g., Slope Method). In general, it is a good practical idea to become familiar with several recognized test procedures, some standard and some specialized, so as to be always ready to adapt to an atypical situation if your usual method fails to provide a coherent result.

If I water the test probes to improve contact, won't this influence my result?
No. Remember, it is the resistance of the ground electrode that is being measured, not that of the test probe. The probe is merely a tool. Once a minimum amount of contact is made with the soil (below a resistance threshold, which is indicated by an LED on the tester), the setup is ready to go on with the test. In order to achieve sufficient contact, it is perfectly legitimate to water the area around the probes. This is like sanding an alligator clip with emery paper before connecting to a circuit; just a specialized means of improving contact. Watering the area around the ground electrode lower its resistance too, and this, of course, does influence the test result. If the test setup has adequate spacing, however, the probes will be far enough away outside of the electrical field of the test ground so that watering them has no influence on the test result.

Ground Testing FAQs
Category: Applications
Page