Numerous studies decades ago indicated that ground resistance measurements should be made using a true DC (0 Hz) source with induced polarization (IP) capacity. In fact, the problem of polarization between AC and DC signals dates back at least to the original white paper published by Frank Wenner on July 15, 1915, where he introduced the world to the 4-point soil resistivity test method. Mr Frank Wenner was forced to use AC because the IP technology for processing DC signals had not yet been invented. But even though he knew AC wasn't ideal.

By the way, in addition to A DC meter with IP technology, you need a meter that can produce hundreds of volts of signal strength and hundreds of watts of power. These types of meters usually require a car battery or a small generator to get the power needed for proper testing.

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

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

In addition, AC signals have "crosstalk" problems between test leads, which can lead to 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, as many of these AC ground resistance meters use power sources that produce signals of less than 0.1 watts. Typically, these cheaper AC testers could only inject 10 or 20 milliamps into the Earth at 20 to 50 volts, with a significant reduction in the return signal.

May you have seen the requirements for shielded test leads? Because the return signal is so incredibly small, usually less than a milliampere, it can easily be 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 50/60Hz AC currents.

We hope this helps you choose a ground resistance tester.