Without good earth connections, even the most sophisticated of building lightning protection installations will be ineffective. However, the only way to ensure that the earth connections really are good is to test them. Paul Swinerd of Megger looks at how this testing can be carried out conveniently, and provides guidance on ensuring that dependable results are obtained.
Low resistance earth connections are the key to providing effective lightning protection for buildings. In order to be able to confirm that a lightning protection installation will perform satisfactorily it is, therefore, important to measure the resistance of the earth electrodes it uses. There are many techniques for doing this but some of these are, in practice, rather inconvenient.
For example, the classic technique for measuring earth electrode resistance is the fall-of-potential method. This requires the test set to be connected to the electrode under test and also to two temporary earth stakes.
The first of these stakes – the current stake – injects a test current into the earth, and must be located as far away as possible from the electrode under test. The second stake – the voltage stake – is driven into the earth at several locations along a line between the electrode and the current stake. At each location, a voltage reading is recorded. While the test is being performed, the electrode under test must be disconnected from everything but the test set.
Since the current injected via the current stake is know, the voltage readings corresponding to the different locations of the voltage spike can be converted to resistance values. If these values are plotted against the position of the voltage spike, the resulting graph will show a plateau, where the voltage stake can be moved from location to location with very little change in the resistance value. This plateau value is the resistance of the earth electrode under test.
This method of testing gives reliable and accurate results, but it could hardly be described as fast or convenient. This is especially true in lightning protection applications, where there are usually several electrodes to test and it is often difficult to find suitable locations for the temporary spikes – indeed, many buildings are surrounded by concrete or tarmac.
For these reasons, many other methods of earth resistance testing have been devised, but one that it particularly well suited for use in lightning protection installations is the stake-less method, which has the important advantage of requiring no test spikes. Instead, a clamp-type tester is used, which is simply clamped around the connection to the electrode under test, and the result read off the instrument display.
This method is quick and easy, but it does have certain limitations and, in order to understand these, it’s necessary to examine how stake-less testing works. The tester clamp actually has two cores, one of which is energised from a constant voltage ac source and injects the test current into the earth electrode system. The other core measures the induced current, which is inversely proportional to the resistance of the earth electrode system.
Note the reference to the resistance of the system rather than simply the resistance of the electrode under test. This is because the stake-less method is only applicable to earth systems with multiple electrodes, as electrodes other than the one under test are needed to complete the circuit, as is shown in Figure 1.
This figure shows a clamp test being carried out on a system that has three other earth electrodes, plus a system earth and a connection to a water pipe. It will be seen that the instrument actually measure the resistance of the electrode under test plus the resistance of all the other earth connections in parallel. In this case, the instrument gives a result of 12.99 Ω, whereas the true resistance of the electrode is 10 Ω.
A difference of this amount between the true electrode resistance and the measured value is however unimportant in most cases. All that is actually necessary in lightning protection installations is for the test to show that the electrode has a reasonably low resistance – the absolute value is of little concern. Stake-less testing is, therefore, well suited to these applications, especially as lightning protection installations invariably involve multiple earth electrodes.
Figure 2. shows a typical lightning protection installation. The electrodes are normally placed at each corner of the building with extra electrodes in between for larger buildings. The conductors used are usually copper tapes up to 50 mm wide, so it’s important when considering the purchase of a clamp tester that the model selected should be able to accommodate this size of tape.
As described earlier, to test an electrode, the instrument is simply clamped around the connection to it. Sometimes, however, this is more difficult than it may sound because the electrode is buried in a small pit. Fortunately, many lightning protection down tapes are fitted with removable links, often referred to as ‘jug handles’, which make ideal locations for using a stake-less clamp tester.
Another slightly less obvious advantage of stake-less testing is improved hygiene. Many links in lightning protection systems are near the ground in locations that accumulate waste and debris, but with a clamp tester it’s not necessary to touch the link.
Lightning protection systems on many factory buildings, especially in European countries, use lightning receptors mounted at regular intervals on the roof. These receptors are all interconnected, which decreases the series resistance of the parallel earth path, with the result that the value measured by the stake-less method is even closer to the true earth resistance of the electrode under test.
When testing earth electrodes in lightning protection systems, it is important to remember that there may be multiple connections to the system. Care should, therefore, be taken to ensure that the tester is clamped around the earth tape below the point where these connections are made, otherwise the resistance measured will be that of the electrode in parallel with any other paths to earth.
A final caution when measuring earth electrode tests is to ensure that the earth path really is included in the circuit being measured. This sounds obvious but, in cases where a metal structure is involved, it is possible that the circuit may be completed via this structure rather than via the earth. Always check carefully, therefore, that you really are measuring what you think you are measuring!
To ensure that a building lightning protection system will work properly when called upon to do so, it’s essential to test the earth connections used in the installation. Fortunately, as we’ve seen, stake-less testing with a clamp tester makes this a relatively straightforward task provided that a good quality instrument, such as Megger DET14C or DET24C, is used and the inherent limitations of the method is adopted are understood and taken into account.