Re: Grounding Impedance Test
I going to make the assumption you are trying to measure the resistance to ground of either a ufer type ground or a ground rod/plate.
Only two things out there I know of.
1. Fall of potential meters: These require you set a couple of ground stakes out away from the one you want to measure, hook up the wires and press the button. They generally work okay for the intended purpose if one follows the directions. I have not seen a lab grade type instrument - just grunt grade.
2. Clamp-on measuring devices have gotten popular the last few years cause you clamp it around the GEC to the rod, press the button, and digital readout gives one an "exact" reading. Unfortunately they really don't read the resistance to ground of the rod. Rather they read the loop resistance. Say I have two ground rods set 20 feet apart with a #4 between them. Either at one of the rods, or tapped to the #4 is another #4 that goes to the panel. No other grounding connections to the system.
Okay, clamp the tester around the #4 between the two rods and push the button, and the meter reads 48 ohms. You just measured the series resistnce between the two rods. you don't know if one is 1 ohm and the other is 47 ohms or anywhere in between.
Now clamp on the GEC from the rods to the panel - what do you read? I think it will read infinite - no return path.
The only condition I know of where a clamp-on type will give a true reading is where there is great grounding system in place and one wants to measure the resistance of one rod. Clamp on to the GEC to that rod, and the reading is indicative of the resistnce to ground of that rod, cause the resistance to ground of the rest of the system is near zero.
My opinion on the usefullness of each:
The few times I needed to measure ground resistance, the spec called for less than 10 ohms for the system (rods, ring, ufer in the foundation). A clamp-on won't work, had to use fall of potential, with the aux measuring grounds set waaaay out. Yes it was less than 1 ohm
The only time I saw a clamp-on used was when an inspector wanted to see the individual resistance on the rods driven around the perimeter of a large installation. Likely the whole system is less than 10 ohms, probably less than 1 ohm. Yes, he could measure each rod fine. It looked like useless data to me, I asked the purpose, but never got any answer that made sense.
I did call one of the mfgs of a clamp-on type and wanted to discuss how to interpret the readings cause there was very little in the instruction book. I couldn't get to an engineer, but the sales sort I talked to assured me he was very well versed in the operation. I went through what we were seeing for readings, measuring the loop resistance, and the need for a reference to read to. His response was to get upset and suggest we hire an engineer. sigh
Just in case you haven't guessed, my opinion of clamp-ons is low.
carl