Bringing testing down to earth

6 August, 2009

In very simple terms, the main purpose of an earth connection in electrical installations is to provide a path for fault currents to flow safely to earth. The objective is to prevent accessible parts of electrical equipment from becoming live and therefore, dangerous to touch under fault conditions. To do this effectively, the earth connection must be of low resistance, so it’s important to be able to measure the resistance of earth connections.

At first sight, this may seem easy. Simply make a second earth connection, with a temporary stake for example, and connect an ohmmeter between this and the earth electrode under test. However, this approach will almost certainly yield meaningless results. Quite apart from the impossibility of separating out the resistance of the earth electrode under test and that of the temporary stake, the result will also be influenced by noise currents flowing in the earth. These currents are typically large enough to swamp the tiny test current used by an ordinary ohmmeter.

Unfortunately, a lot of the new, alternative techniques that have been developed have acquired the reputation of being complicated and difficult, with the result that some contractors avoid carrying out earth testing, preferring to leave it to a specialist. However, this really isn’t necessary as modern earth resistance testers are convenient and straightforward to operate.

Alternative methods

Let’s take a look at some of the test methods that they use. The simplest is to carry out a direct measurement, as described earlier, but with a purpose designed earth tester that uses an AC test current. By choosing a test frequency that is not an integer multiple of the mains supply frequency, such an instrument can provide good noise rejection.

In fact, to allow users to optimise noise rejection, some earth resistance testers offer a choice of test frequency. This is a valuable option, not only when carrying out direct measurements, but also when using any of the other techniques described later.

The results from direct earth resistance measurement with a dedicated instrument are more meaningful than those obtained with an ohmmeter, but there is still no way of confirming that they are accurate or of separating out the contribution of the secondary earth.

A better method is the three terminal or fall of potential method. This uses a connection to the electrode under test and two test spikes that are driven into the ground prior to testing.

One spike - the current spike - injects the test current and is placed as far away as possible from the electrode under test. The other spike - the voltage spike - is driven into the ground at several locations between the current spike and the electrode. At each location, a voltage measurement is taken. Since the test current is known, the instrument can convert these voltages to resistance values.

If a graph is plotted of resistance versus the distance of the voltage spike from the electrode under test, it should have a region where the resistance value hardly varies as the rod is moved. This value is the earth resistance for the electrode under test. This method is accurate but it is time consuming, requires a lot of space and the earth electrode under test must be disconnected from all other circuits during the test.

Addressing shortcomings

To rectifying these issues the clamp-on or stakeless method was introduced. This injects the test current into the earth electrode system via a current clamp and uses a second clamp to measure the resulting current flow in the electrode under test. Note that with some instruments, both clamps are combined in a single clamp head. With this method, the earth electrode does not need to be disconnected from other circuits - indeed, for successful testing, it cannot be.

Still, the stakeless method has several limitations. It only works if there are multiple parallel earth connections to provide a return path for the test current. Therefore, it cannot be used to test isolated electrodes. There’s also no way of verifying the result, so it is unsuitable for checks on installations where no previous test results are available for comparison.

A work of ART

A new and more versatile solution is attached rod technique (ART). In many ways, ART is similar to fall of potential testing and all of the same connections are required. The one crucial difference is there is no need to disconnect the earth electrode from other circuits during the test.

That may seem a small advantage but, apart from the physical difficulty of breaking earth connections, it’s important to remember that earthing is a safety function. There are dangers in disconnecting an earth electrode as a fault current may be flowing and disconnection could give rise to a potentially lethal situation. Furthermore, if equipment is disconnected from the earth electrode to enable a test to be carried out, that equipment may no longer be safe.

The key to the operation of ART testing is in the current measuring clamp (ICLAMP) that is put around the earth electrode under test. The tester is designed to ignore any system leakage and noise currents that may be flowing through the earth electrode. This means that it can accurately measure the test current. After set up, the ART test proceeds in the same way as a fall of potential test. Therefore, it is still time consuming, but there are shortcuts that can be used in appropriate circumstances.

For example, instead of taking readings with the voltage spike at various distances between the electrode and the current spike, it is sometimes sufficient to take a few readings, with the voltage spike around 62% of the distance between them. Therefore, ART testing can provide a good balance between convenience and accuracy.

For earth resistance testing, it is important to have the right equipment and to understand the limitations of the various test methods. Although it’s versatile, even ART testing isn’t suitable in every case. Provided that these limitations are understood (and leading test instrument manufacturers are more than happy to provide comprehensive guidance) earth testing is a straightforward and potentially profitable service for virtually any electrical contractor to add to their portfolio.