DET4 series four-terminal earth/ground resistance testers
Stakeless or clamp-on testing capability
Use the instrument like a clamp-on tester in applications where viable, while also being able to operate as a Fall of Potential tester
Attached Rod Technique (ART) capability
Allows Fall of Potential testing without the need to disconnect the ground rod
Multiple, user-selectable test frequencies
Allows the operator to find the most effective frequency for making the measurement
Resistance measurement range to 200 kilohms
Provides the ability to measure the resistivity of any type of soil
About the product
Megger’s DET4 series of four-terminal earth/ground resistance testers offers a comprehensive solution to your earth/ground and soil resistivity testing needs.
To that end, this popular instrument series includes four models with different kit variants that contain various accessories for greater testing flexibility. The four basic units are:
- DET4TD2: Dry-cell battery-powered basic four-terminal tester
- DET4TR2: Rechargeable battery-powered basic four-terminal tester
- DET4TC2: Dry-cell battery-powered four-terminal tester with selectable test frequencies, greater measurement sensitivity, attached rod technique (ART), and stakeless measurement capability
- DET4TCR2: Rechargeable battery-powered four-terminal tester with selectable test frequencies, greater measurement sensitivity, attached rod technique (ART), and stakeless measurement capability
The DET4TC2 and the DET4TCR2 testers are for demanding applications requiring maximum versatility. These instruments support two-, three- and four-pole testing, as well as attached rod technique (ART), stakeless, leakage current, and earth noise voltage measurement. They also allow you to select the test frequency from one of four options: 94 Hz, 105 Hz, 111 Hz, and 128 Hz, making it easy to choose a frequency that minimises the effects of interference, even in difficult situations.
When the exceptional versatility of the DET4TC models is not needed, Megger offers the DET4TD2 and DET4TR2 testers, the former designed for use with replaceable batteries and the latter with rechargeable cells. These competitive instruments fully support the most popular earth resistance test methods – two-, three- and four-pole testing.
All DET4 models are rated to IP54, making them ideal for outdoor working, and they are designed to meet stringent safety standards, rated CAT IV to 100 V. Additionally, all the DET4 instruments are ergonomically designed with a large selector switch enabling you to easily select two, three, or four-pole tests, even with gloved hands.
- Data storage and communication
- Power source
- Test method
- 2, 3, and 4 pole resistance
- Test method
- ART resistance
- Test method
- Stakeless resistance
Product documentsAdditional documentation can be found on the support tab
FAQ / Frequently Asked Questions
You can work with the test spikes at shorter distances from the earth/ground system using the slope test technique. With this technique, the current spike is inserted at an approximate distance of 2 to 3 times the maximum dimension of the earth/ground system. Measurements are then made with the voltage spike at 20 %, 40 %, and 60 % of the distance to the current spike. By using various criteria to evaluate the results obtained from these three tests and performing further tests, if necessary, you can obtain a reliable value for the resistance of the earthing system. Please see Megger’s “Getting Down to Earth” guide for further details.
In most cases, it is only necessary to show that the earth electrode resistance is below some specified maximum acceptable value. However, there are certain applications where high-resolution measurements are required. These include the determination of earth/ground resistance using the slope technique and evaluating earth/ground resistivity over large areas. High-resolution instruments typically use the four-terminal method of measurement and include additional features, such as variable test frequency, that help users to obtain good results even under challenging conditions.
Four probes make a Kelvin bridge, which in grounding is only necessary for soil resistivity measurements. There has to be a uniform current across the sample of soil being measured, so you need two current probes and two potential probes, arranged C-P-P-C. Soil resistivity measurements can be used to establish the optimum electrode design and location, as well as for performing archaeological and geological investigations. For ground electrode testing, three terminals often suffice. Hence, a less expensive model is a smart choice if you are sure you will never need to perform soil resistivity testing. The exceptions are those cases when you need high-resolution earth resistance measurements (see the following FAQ).
Further reading and webinars
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
FAQ / Frequently Asked Questions
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.
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.