Author - Brian Hammerschmidt
When they’re considering earth (ground) testing, people often ask about what type of test leads they should use. Typically, they ask questions about the type of wire, the resistance of the leads, the length of the leads, and voice their safety concerns. Fortunately, most of these things are taken care of in the design of the earth tester itself.
Because Megger earth testers output a maximum of 50 V and a maximum current of 50 mA, there are no electrically demanding specifications for the type of wire that should be used for test leads. Any wire size down to approximately 18 gauge (1.0 mm2) can be used. However, if there is a requirement to meet the IEC61010-1 CAT IV 100 V safety specifications, it’s important to remember that the wire used for the test leads is part of the system, and this must be taken into account. In such cases, specific test leads are available that will meet the requirements.
Looking more broadly at safety, the low test voltages and currents used for earth tests mean that there are few safety concerns relating to the test leads or to the tester.
Manufacturers of earth testers specify the maximum resistance in the current and potential test circuits. For Megger testers this is typically between 50 kΩ and 200 kΩ for both circuits. Since this maximum resistance is so high, there is no practical limit on the length or the resistance of the test leads. When tests are being carried out, most of the resistance comes from probes in the ground rather than from the wire in the test leads – that is, compared to the probe resistance, the resistance of the wire is negligible.
In addition, Megger earth testers warn users if there are poor connections or if the instrument’s operating parameters are exceeded. To summarise, users don’t need to worry about the length and resistance of the test leads as these factors are accounted for in the design of the tester.
Stranded wire should be used for the test leads, because solid wire is difficult to handle, especially when long leads have to be used. This does mean, however, that over time it’s possible for one or more strands to break within wire insulation and this won’t be apparent from a visual inspection. For this reason, it is recommended that a loop test be performed to ensure that the leads are in good working condition before tests are conducted.
Another consideration in earth testing is the probes or earth test spikes. Some users mistakenly believe that the probes must be driven deep into the ground, but the reality is that they simply need to be deep enough for the earth tester to recognize them. Megger earth testers will provide a warning if the probes are not deep enough or if they are not making adequate contact with the soil to allow satisfactory testing.
To make earth testing easier, and to alleviate concerns about test leads and probes, Megger has created its new ETK series of earth test kits, which are designed to simplify and speed earth testing. A carrying bag is provided which protects the kits as well as allowing them to be stored neatly and easily transported.
The test leads supplied in the kits are stored on lightweight reels which have handles and a smooth action for easy reeling and unreeling. These are a big improvement over previous models because they had no handles, the reeling action was not smooth and there was a tendency for the leads to become tangled inside the reels. All of these problems have been solved with the new larger reel design.
The new design also allows the user to connect a lead to the tester, then carry the reel away to the reference point. The cable unreels easily as they walk.
Each reel has clips for holding the test probes and for securing the ends of the test leads. The reels can also be connected together in a daisy chain configuration to meet requirements for longer distance applications. These are, for example, found in large substations where the physically large area of the ground system results in a large ‘resistance area’ requiring long distances to the test probes.
Ideally, the current test probe should be placed at 10 times the maximum distance of the ground system (e.g. 3000 feet for a 300 x 300 ft ground grid) to find the ‘flat’ portion of the characteristic resistance curve when conducting a fall of potential test. The position of the current probe is critical for getting proper measurements; if the distance between the ground electrode being tested and the current test probe is too small, the electrical fields of each will overlap and degrade the accuracy of the measurement.
Other accessories in ETK kits include a calibration checker for the tester and another for the test clamps. These allow the user to ensure functionality and accuracy of the tester and clamps prior to and following testing. Also included are a tape measure, to ensure accurate placement of earth test probes, and test leads with clips for performing continuity tests.
The earth test kits are available in five different versions to meet a variety of needs and applications. Options include voltage and current test clamps that allow the user to perform ‘stakeless’ testing and use ART (Attached Rod Technique) methods as well as measuring leakage currents.
Megger has been a leader in earth/ground testing since the 1930s. Over the decades, the knowledge and techniques developed by the company have allowed it to design and produce the most accurate and trusted testers on the market. The new ETK models provide the most practical and innovative accessory kits available, which facilitate easy and efficient completion of earth testing.