Electrical earth testing can be a real challenge in built-up areas. Not only are there very few soft areas to deploy the test probes, but underground cabling and conductive structures are everywhere. These two factors massively affect the measured readings leading to measured values that bear no relation to the actual electrode tested. Therefore, what follows is the Earth Testing Procedure Greymatters adopts.
Follow the guidance in HSG47. The city or town’s infrastructure produces large amounts of in-ground noise and interference. Mitigation by reading buried service/cable records; observing signs of previous excavations; and use a cable avoidance tool (CAT) can make sense of where the structures are.
Buried conductive structures like cables and pipes provide a preferred route for the earth tester’s signal to travel along, instead of it passing through the surrounding geology. So, you end up measuring the effect of the cable, pipe or object. Not good!
Find a workable traverse for the earth tester’s leads, which isn’t obstructed by something above ground and also escapes the electrical influence of the electrode under test. When you’ve done that you can deploy the leads per the chosen method, for example, fall-of-potential, intersecting curves or slope test.
Lead deployment distances for the fall-of-potential methods need to escape the electrical influence of the electrode under test. Therefore, the lead deployment distances can be quite vast. In theory, up to 6-10 times the longest dimension of the electrode under test. In some cases, this is not practical; then a distance of no less than 2.5-times should be used. Therefore, if the electrode under test has a longest dimension of 50m, then one requires the current probe to be some 300+m out from the electrode, ideally.
For some densely populated urban environments, this is not practical, but we touch on what you can do if this happens further into this article.
Protect your leads! Some of the more curious passers-by like nothing more than to see you struggle for a laugh by messing with your test leads when you’ve turned your back. If on a construction site, your friendly 6-28 tonne dumper driver will obligingly drive his load over your delicate leads in a heartbeat and watches your tears fall. Yes, this really can and does happen, so be warned. 😉
Up to this point, the previous 3-steps have been about preparation and avoiding the introduction of errors into your measurements. Step 4 is about the execution of the earth test itself, which we cover in our online training courses.
Once all the measurements are taken; it often best practice to validate the readings before recovering your leads and heading to the next test location. Why? Most testers simple record a reading. They do not interpret or hint to a possible error in the data you’ve just spent a long time acquiring. So it’s good practice to do a rough sense-check for data integrity before leaving the location.
As one of my metalworking teachers used to say at college; “…measure twice, cut once!”
Returning to the site because the measurements were junk is no fun at all and costly. So, run some quick checks before leaving site means you can adapt a lead-deployment and tune out potential errors; or remedy a duff reading in minutes.
For the space-constrained earth tests. The final step is to post-process the data; build a model of the electrode, the soil, and the test equipment and simulate the physical earth tests carried out. This includes matching the lead deployments and distances.
The computations from the physical-to-virtual earth tests should show a good agreement between the two domains. Once in agreement, one can extrapolate the lead deployments in the “virtual-world” much further than is possible in the physical world. Thus, enabling the more ideal probe distances of x6 to x10-times the electrode to properly escape the electrical influence mentioned in step 2.
So, to summarise Step 6. The software model is calibrated to the physical measurements. Then because the software model is not constrained by physical obstacles; the model can extrapolate the lead distances to whatever distance is required in order to exceed the electrical influence of the electrode. This approach fixes the limitations of earth testing procedures in built-up areas and ensures reliable, validated earth tests that keep people and your reputation safe!
IEC EN 50522 states:
Measurements shall be carried out after construction, where necessary, to verify the adequacy of the design.
Design and installation of the earthing system shall allow measurements to be carried out periodically. Or following major changes affecting fundamental requirements, or even for continuity tests.”