50 www.megger.comIt is important for the operator to understand the limitations of the stakeless test method so that he/she does not misuse the instrument and get erroneous or misleading readings. A clamp-on tester is an important tool in the bag of a test technician and trending results will add value over time with regard to early warning of degrading ground resistance.Advantages of Stakeless TestingThe primary advantage of the clamp-on method is that it is quick and easy, as no probes have to be driven and the ground rod does not have to be disconnected from the system. One time saving example is when measuring a large industrial estate for lightning protection grounding, doing a fall-of-potential test on hundreds of ground points would take several days. The same testing performed using a clamp-on tester would take several hours. There are also examples like substation or cell-tower grounding where disconnection is not possible and the stakeless test is all that can be done. The clamp-on tester can be used to measure any loop resistance and application, such as a ground plate on a structure with multiple lightning conductors connected to it, as long as the user bears in mind the fact that it measures total loop resistance. Many clamp-on testers include a current clamp measurement aimed at testing current flowing in a ground loop, ranging from less than 1 mA to 35 A ac. A current measurement is a useful safety check and will identify grounds that should not be disconnected because they can cause a spark. A current measurement should be performed if 50 Hz or 60 Hz (or any other power frequency) is thought to be interfering with clamp-on ground resistance reading.Clamp-on testers operate at higher frequencies (typically between 1.5 kHz and 3.5 kHz) than 2-, 3-, and 4-pole testers (48Hz to 128 Hz) to reduce core size which improves access to ground tapes and cables and reduces tester weight. The downside is that this approach is less representative of a fault at power frequency than the traditional ground testing but the higher frequencies do reject inductive reactance which is advantageous for transmission tower grounds, lighting and similar sites.
