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How to choose a ground resistance tester

time:2020/8/9   source:华天电力  reading:269 time

A lot of research decades ago showed that ground resistance measurements should be made using a true direct current (0 Hz) source with inductive polarization (IP) capacity. In fact, the problems between AC and DC signals and polarization problems can be traced back at least to the original white paper published by Frank Winner on July 15, 1915, where he introduced the 4-point soil resistivity test method to the world. Mr. Frank Winner was forced to use AC because the IP technology for processing DC signals has not yet been invented. But even he knows that AC is not ideal.


Earth Resistance Tester.png


By the way, in addition to a DC meter using IP technology, you also need a meter that can generate hundreds of volts of signal strength and hundreds of watts of power. These types of meters usually require car batteries or small generators to get the power needed for proper testing.


Now, the reason why some companies produce ground resistance meters at these frequencies is because direct current (DC) measurement is difficult to do, and alternating current (AC) is easier and cheaper. If you notice in the literature, companies will often try to deceive you by claiming that the signal is a "pulse DC". This is a marketing tool that can avoid calling the signal a square wave AC signal.


There are various problems with AC signals when accurately measuring ground resistance. Many of these reasons are very complex and involve not only the ground but also the interface between the probe itself and the probe. Most importantly, you need a DC/IP tester.


In addition, the AC signal has a "crosstalk" problem between the test leads, which can cause measurement errors, especially when using coiled cables. Most importantly, overhead 50 Hz or 60 Hz power lines can significantly affect the accuracy of these cheaper measuring devices, because many of these AC ground resistance meters use power supplies that produce signals less than 0.1 watts. Generally, these cheaper AC testers can only inject 10 or 20 mA into the earth at a voltage of 20 to 50 volts, and the return signal is significantly reduced.


You may have seen the requirement for shielded test leads? Since the return signal is so incredibly small, usually less than one milliamp, it can be easily suppressed by nearby power lines. This is why cheaper ground testers use 94Hz, 105Hz, 111Hz and 128Hz AC frequency signals to avoid interference from stray 50/60 Hz AC currents.


We hope this helps you choose a ground resistance tester.


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