Support for DET4 four-terminal earth resistance testers
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Troubleshooting
Corrosion on the battery contacts is a frequent issue. This can be cleaned off and operation should be restored.
The PC board has two resistors on opposite sides of the orange relays that may fail. If P4 (four-terminal selector position) and P3 (three-terminal selector position) do not work but P2 (the two-terminal continuity position) does work, this is likely the problem. Please send it to a Megger Repair Centre.
You can substitute a throwaway battery for the rechargeable one in DET4 models designated with R in their alpha-numeric designations. These testers recognise 9.6 V rechargeable batteries from 12 V throwaways. The tester will automatically disable the charging circuit if an attempt is made to charge a throwaway battery to prevent damage.
To enable the charging circuit again, you need to follow these steps:
- Switch the tester to 4P while holding down the TEST button. Software version # will briefly be displayed.
- The display will show ‘tst’; release TEST button.
- The Charger Enable Screen will be shown.
- The state of the charger circuit is shown by either a cross (X) or a tick (check mark) below the letters CHg. A cross indicates that the circuit is disabled.
- Re-enable the charger by pressing the TEST button once. The cross should change to a tick.
- Switch the DET4 off to save the new setting.
Interpreting test results
Before proceeding with your analysis, confirm that you followed a procedure. If you randomly place your probes for an earth (or ground) resistance test, your results will probably be meaningless. Only by remote chance would your results be representative.
The Fall of Potential method is the most reliable and accurate means of measuring the resistance of an earthing (or grounding) electrode. After placing the current probe a healthy distance from the earth electrode, this method entails driving a potential probe in the soil near the earth electrode and taking a resistance measurement. Subsequently, you will move the potential probe several times, nearer and nearer to the current probe, taking a resistance measurement at each position. With the test results, you will generate a graph of resistance (plotted on the y-axis) versus distance (between the earth electrode and the potential probe, plotted on the x-axis).
Each current source - the earth electrode and the current probe - has a unique electrical sphere or ‘footprint’ in the surrounding soil. Accounting for the size of the earth electrode’s electrical sphere is critical to correct measurement. The size depends on variables such as soil type and composition, moisture content and temperature, and size and shape of the earth electrode. What we emphatically don’t want for correct measurement is for these spheres to overlap or coincide.
If the current probe is adequately spaced from the earth electrode, the resistance should rise initially, level off in the middle of the graph, and then rise again as the potential probe nears the current probe. The resistance reading of the horizontal section is your earth resistance measurement.
Typical: In the U.S., the National Electrical Code (NEC®) defines a max limit of 25 Ω. But that’s very forgiving and primarily for residential grounds. For example, you wouldn’t want a 25 Ω earth for commercial and industrial earthing. Ideally, you’d like to have an earth resistance less than 5 Ω or, worst case, 10 Ω. Meanwhile, requirements are more stringent for demanding situations like substation grounds and computer room grounds, e.g., < 1 Ω. In any case, you should know the earth resistance range you are willing to accept.
If resistance exceeds your defined limit, the electrode has to be improved by adding more rods or driving a single rod deeper.
Trending: If you have previous earth resistance test results, you should compare your results to these. While resistance should trend similarly, local earthing conditions do change. Suppose a business moves in next door and construction ensues. The contractor drives a line down and hits the water table. Consequently, the water table drops for you, your earth gets drier, and your resistance increases. Hence, you need to test earth resistance periodically and react to notable changes in resistance that you discover.
User guides and documents
FAQs
Your results indicate that the electrical footprint of the earth electrode overlaps that of the current probe. The first choice of action would be to get more lead wire and repeat the test with the current probe driven farther away. The goal is to remove the interference from the current probe’s electrical field from that of the earth electrode, which is what we want to measure.
If you believe that you have already spaced the current probe a very healthy distance away from the earth electrode, keep in mind the two conditions that can cause the earth electrode to have a sizable electrical footprint. You may have poor earthing soil. Poor earthing soil includes sandy, rocky, and dry soil and soils lacking natural electrolytes (ions). Secondly, your earth grid may be extensive, such as that for a substation. These conditions cause large electrical footprints that may result in prohibitive distances for the test leads when using the Fall of Potential method.
The most likely answer is that you’re actually reading a metallic loop in the earth system. This is a very common problem as most equipment is bonded to earth, and this bonding frequently creates earth loops. Unfortunately, you may not be able to use the stakeless technique in your application.
The most common method for addressing this problem is the Slope Method. It employs much shorter leads and some mathematics that tell the operator where the earth electrode's electrical footprint reaches its limit on a steadily rising graph.
Another commonly used method to determine earth resistance when there is insufficient distance between an earth electrode and test probes is the Intersecting Curves Method. This method is for the adventurous! It involves constructing three graphs based solely on arbitrary guesses as to probe position. Since all other points are wrong, the three graphs come together only at an intersecting point that signifies the correct reading. You can verify the legitimacy of this intersecting point by recording a resistance measurement at that point.
The Four Potential Method uses considerable mathematics. Six readings are taken and processed through four parallel equations that look for agreement while weeding out random measurements.
The Star Delta Method is specially adapted to extreme limitations of test space, such as urban downtown areas. Rather than going in a straight line, six measurements are taken in a tight triangular configuration around the earth electrode. These results become inputs for a series of equations that look for agreement in signifying the correct reading. The speed and accuracy of mathematics-dependent results have significantly improved with advancements in software development.
As far away as possible – and ideally at least 6 to 10 times the maximum dimensions of the earth system. To provide some rough rules of thumb, for a single earth electrode, the current reference spike C can usually be placed 15 m from the electrode under test, with the potential reference spike P placed about 9.3 m (62 % of the distance to C) away. With a small grid of two earth electrodes, C can usually be placed about 30 to 40 m from the electrode under test; P correspondingly can be placed about 18.6 to 24.8 m away. If the earth electrode system is large, consisting of several rods or plates in parallel, for example, the distance for C must be increased to possibly 60 m, and for P to some 37 m. You’ll need even greater distances for complex electrode systems that consist of a large number of rods or plates and other metallic structures bonded together.
