Getting Down to Earth 35electrode system area. For example, if the area measures 100 by 100 ft, the diagonal equals about 140 ft. From the table, you run down the first column to 140 and read across. P should be 365 ft from the electrode and C should be 590 ft.Lazy SpikesThe latest designs of digital earth testers can operate with very high temporary spike resistances and still give reliable and accurate results. Because the current and voltage are measured separately, it enables electrode measurements to be carried out with test spike resistances up to 400 kΩ.The advantage of these instruments tolerating such high spike resistance is generally that tests can be performed quickly on a green field site because electrodes do not have to be inserted too far into the ground. However, in urban situations, tests can be carried out using street furniture such as sign posts, metal fences and bollards. Where this is not possible, results have been obtained by laying the temporary electrodes on a wet patch of concrete. Coiled metal chains or metallized ground mats, with water poured over them, make an even better electrode because they conform more intimately to the earth’s surface than does a rigid spike. This technique has led to measured values of “spike” of less than 10 kΩ, well inside the maximum value that will cause an error to the reading. With modern instruments, any problem with the temporary spikes will be indicated on the display to show that a reading may not be valid. A more suitable position for the spike may have to be used such as along the gap between paving stones, a crack in concrete, or in a nearby puddle. As long as warning indicators do not appear, sufficient contact has been made and a reliable test may be performed.
