Even the most sophisticated of building lightning protection installations will be ineffective without good earth connections. This can only be ensured by testing. Here, Megger’s Paul Swinerd looks at how testing can be carried out conveniently, and provides guidance on how to get dependable results

A key factor in providing effective lightning protection for buildings is low resistance earth connections. Therefore, to ensure that a lightning protection installation will perform satisfactorily, it is 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.

Traditional methods

The classic technique is the fall-of-potential method. This requires the test set to be connected to the electrode under test and to two temporary earth stakes. The first temporary stake (the current stake) injects a test current into the earth, and must be as far away as possible from the electrode under test. The second stake (the voltage stake) is driven into the earth at several locations between the electrode and the current stake. At each location a voltage reading is recorded.

Since the injected current is known, the voltage readings for the different locations of the voltage spike can be converted to resistance values. When these values are plotted against the voltage spike position, the graph has a plateau, and the plateau value is the resistance of the earth electrode under test. This method of testing gives good results, but it is neither fast nor convenient.

That’s why other methods of earth resistance testing have been devised, and one that is well suited for use in lightning protection installations is the stake-less method, which needs no test spikes. Instead, a clamp type tester is used, which is simply clamped around the connection to the electrode under test. This method is quick and easy, but it does have limitations. To understand these, it’s necessary to examine how stake-less testing works.

The tester clamp has two cores. One is energised from a constant voltage AC source and injects the test current into the earth system. The other core measures the induced current, from which the resistance of the system is calculated. Note the reference to the system rather than simply 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 shown in Figure 1.

In this figure it will be seen that the instrument actually measures the resistance of the electrode under test plus all the other earth connections in parallel. In this case, the instrument reads 12.99O, although resistance of the electrode is 10O. The difference is, however, usually unimportant. All that’s needed in lightning protection installations is to confirm that the electrode has a reasonably low resistance.

Figure 2 shows a typical lightning protection installation. The conductors used are usually copper tapes up to 50mm wide, so it’s important when purchasing a clamp tester to ensure that it can accommodate this size of tape.

As mentioned earlier, to test an electrode, the instrument is clamped around the connection to it. Sometimes this can be difficult because the electrode is buried in a small pit, but many lightning protection down tapes are fitted with removable links that make ideal locations for using a stake-less clamp tester.

Lightning protection systems on factory buildings often use lightning receptors mounted at regular intervals on the roof. These receptors are interconnected, which decreases the resistance of the parallel earth path, making the value measured by the stake-less method even closer to the true earth resistance of the electrode under test.

When testing earth electrodes in lightning protection systems, remember that there may be multiple connections to the system. For reliable results, always ensure that the tester is clamped around the earth tape below the point where these connections are made. A final caution 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.

To ensure that a building lightning protection system works properly, it’s necessary to test the earth connections. Fortunately, stake-less testing with a clamp tester makes this straightforward, provided that a good quality instrument is used and the inherent limitations of the method are understood.