For many years, stakeless or clamp-on testing has been accepted as an established method of testing earth system resistance, and is now included in the German standard VDE 0100-600:2008 (appendix B3). This method has a number of real user benefits, in particular not having to disconnect the electrode under test, increased user safety and huge savings in time and aggravation. However, many users are failing to enjoy the benefits of stake-less testing, and that’s mainly down to two main issues. Firstly many potential users do not fully understand the testing technique, and avoid it; secondly many people have found some problems when testing, resulting in a number of concerns.
The first issue is easy to solve when you know where to go. You can find forums and micro-sites to help with many of these problems, or you could phone one of Megger’s regional technical support offices. Easy, if you have time to stop your work and do the research!
The second issue is not so easily fixed. Problems experienced in actual testing are often caused by the abilities – or inadequacies - of the test equipment. Let’s take a look at these concerns and see how we can eliminate or maybe get round them. For obvious reasons, most users have safety as their primary concern. In substation environments, the stakeless testing method may be the only method available to you. But what happens if you are holding the instrument when a massive fault produces a huge transient on the cable that you happen to be clamped around? This could also be a concern to users testing lightning protection; there can be a sudden lightning bolt that could kill you. The other and maybe less obvious hazard will present itself if, for whatever reason, the user has to disconnect one of the electrodes that is being tested. If there is too much current flowing down the electrode when it is disconnected, it will quickly turn into a hazardous voltage.