ET online
October 2016
Q and A: Earth resistivity and earth electrode testing

Q and A: Earth resistivity and earth electrode testing

01 October 2016

Not long ago, the need to measure earth resistivity or the resistance of an earth electrode was, for most engineers, a rare occurrence. With the advent of small generating schemes and, in particular, solar and wind energy schemes, this situation has changed. Most of these schemes have their own earthing systems and, to ensure safe operation, these need to be checked. This has led to a large increase in the number of questions our helpline receives about earth testing; here is a selection of the most common.

Q: When I’m measuring the resistance of an earth electrode or system, how far away from it should I place my test spikes?
A: 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.

Q: My earth system is very large, so to make measurements in the usual way I would need very long leads. This simply isn’t practical. What’s the alternative?
A: You can work with the test spikes at shorter distances from the earth system if you use the slope test technique. With this technique, the current spike is inserted at a distance of about 2 to 3 times the maximum dimension of the earth 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, if necessary performing further tests, a reliable value for the resistance of the earthing system can be obtained. There’s no space here for the full details, but they can be found in Megger’s invaluable publication, “Getting Down to Earth”, which can be downloaded free of charge from the Megger website.

Q: I’ve used the ‘stakeless’ technique to measure the resistance of an earth electrode and the result is very obviously too low.
What’s going on?

A: 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 ground, and this bonding frequently creates earth loops. Unfortunately, you may not be able to use the stakeless technique in your application.

Q: What are the applications that require high-resolution measurements of earth resistance?
A: While in most cases it is only necessary to show that the earth electrode resistance is below some specified maximum acceptable value, there are certain applications where high-resolution measurements are necessary. These include the determination of earth resistance using the slope technique mentioned in an earlier question, and the evaluation of earth resistivity over large areas. High-resolution instruments, such as the Megger DET2/2 automatic earth tester, 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 in difficult conditions. As well as earth electrode resistance measurements, these instruments are ideally suited for soil resistivity measurements, which can be used to establish the optimum electrode design and location, as well as for performing archaeological and geological investigations.

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