Ground testing do’s and don’ts
Ground testing is often misunderstood by electrical engineers and technicians who do not perform such testing often and regularly. It seems that there are a lot of questions and doubts on testing the ground. This technical article, based on Transcat’s paper, will try to answer the twelve most common doubts related to using ground tester, connections, testing methods, and safety.
Ok, let’s see these doubts and questions. Please note that there are probably tens of other possible problems and uncertainties that could popup during the ground testing procedure, but we shall focus on the most common ones.
- Can I use an insulation tester to do the same test?
- The required measurement is of resistance; why can’t I use a multimeter?
- What is the difference between a two-point, three-point, and four-point test?
- Can’t I simply make a measurement to a reference ground?
- How do I know if my ground is good?
- How often should I test my grounds?
- How do I go about designing a good ground?
- Does it matter how deep I drive the probes?
- My testing is all on concrete and macadam; how can I drive probes?
- What safety precautions should I observe when performing a ground test?
- It has just rained heavily; will this influence my test?
- I have a ground installed in sandy (or rocky) soil; what can I do to test this?
No. This is a common error. Field operators are often issued a Megger instrument from stores, without its being checked to determine whether it’s an insulation or ground tester. Insulation testers are designed to measure at the opposite end of the resistance spectrum from a ground tester.
However, a continuity test can only make an arbitrary measurement between an installed electrode and a reference ground, which is assumed to have negligible resistance. This does not afford a reliable measurement of the resistance the earth offers to a ground fault current.
Even this arbitrary measurement may not be reliable, since a dc continuity test can be influenced by soil transients, the electrical noise that is generated by utility ground currents trying to get back to the transformer, as well as other sources.
For the same reasons that a continuity range on an insulation tester should not be used. Measurements made with a DC multimeter are subject to distortion by electrical noise in the soil.
With a multimeter, one can measure the resistance of the soil between a ground electrode and some reference point, such as the water pipe system, but a fault current may encounter a higher resistance.
Literally, the difference is actually the number of points of contact with the soil. More specifically, these commonly used terms refer to dead earth, fall of potential, and Wenner method tests, respectively. In the dead earth method, contact is made at just two points: the ground electrode under test and a convenient reference ground, such as the water pipe system or a metal fence post.
In the fall of potential method, a genuine ground tester makes contact via the test electrode, plus the current and potential probes. With the Wenner method, no ground electrode is involved, but rather the independent electrical properties of the soil itself can be measured using a four-probe setup and a recognized standard procedure.
This common method often uses an instrument other than a dedicated ground tester. It is referred to as the dead earth method because the reference ground is only being used for the test and is not normally part of an electrical system. It can be the water pipes, a metal fence post, or even a rod driven just for the test.
The method is popular because of its ease and generality, but is not recommended. Since the reference ground happens to be located by a combination of convenience and chance, it is only a matter of luck if the soil resistance to it actually represents the true electrical ground resistance.
Interesting reading and comments…
The most widely used specification is that of the NEC, which mandates for residential grounds a resistance of 25 Ω or less. This is not a particularly difficult specification to meet. Others are more demanding, and may be specified by the engineer designing an electrical system, or by a client, or may come as part of the warranty requirements for advanced equipment.
The most commonly encountered specification for industrial grounds in general is 5 Ω or less. Computers and process control equipment may demand as little as 1 Ω or 2 Ω .
Further reading about what should be the good grounding…
Odd intervals of 5, 7, or 9 months are recommended so that the various seasons will all be encountered in succession. This is because the quality and effectiveness of a ground are profoundly affected by weather and seasons. If quarterly or semiannual testing schedules were used, certain months would consistently be missed, and these could be the ones in which the grounding is most stressed by the weather.
Adopting irregular intervals, on the other hand, ensures that worst case seasons will be revealed. Since a ground fault, potential fire or accident, can happen at any time, your protection is only as good as the ground condition in the worst time of year.
Traditionally, the trial-and-error method was used and could be enhanced by an individual’s experience. This consists of designing the ground during the process of installation, and repeatedly testing it in progress, until the desired spec has been met.
The trial-and error method is still frequently used and often works well. But its limitation is that it is subject to the Law of Diminishing Returns: more and more work for less and less reward.
In optimal soil conditions, a satisfactory ground can be achieved with only a few retests. But in more difficult environments, one can end up wasting the day without realizing the goal.
If you find some time, read interesting guidelines and best practice in power substation grounding…
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?
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.
The good news is that you probably don’t have to. Some ground tester models have uncommonly high resistance tolerances in the test circuits (typically 4, 40, and 400 k for the current circuit, 75 k for the potential). Any surface contact of a resistance less than these high thresholds is enough.
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.
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 for example, with the Megger’s models of 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 for example the Megger’s model DET2/2 in its high-current mode, in which case a maximum of 50 mA are produced at low voltage.
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.
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.
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.
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.