I am planning to do a grounding review at one of our power plants (and maybe several others eventually) and I am doing research on ground testers to buy or rent.  At this time I will be looking at connections and resistance of the grid to remote earth.  I may do soil resistivity testing at some point so would probably get a 4-probe unit.  My past experience has been with the Vibroground units, which are no longer made.  But in addition to the 3-probe and 4-probe solid-state equipment that I expected to see, there are some clamp-on units by AEMC.  Do any of you have experience with these types of units and would you provide feedback on them?

I have seen units by Biddle, AEMC, and Erico with price ranges from about $650 to $2000.  Once again, recommendations would be appreciated.  Stepping up from a 50 year old Vibroground with radio tubes to a new unit is like starting from scratch for me.

Thanks in advance for any advice.

The clamp-on AEMC ground tester may not be suitable for your application.  It is mainly designed for testing small electrodes like one or two rods while connected to a multi-grounded neutral.  It depends on the electrode resistance being much greater than the paralleled resistance of all grounds connected to the neutral. If your power plants have large grids, this wouldn't be applicable.

For very large grids, measuring the resistance with the fall-of-potential method requires getting the current probe very far from the grid.  This can be a problem.  

You might want to look into the EPRI Smart Ground Multimeter (SGM) that supposedly eliminates the problems with large grids.  I say supposedly because there is some dispute about the theory.  See "A PC Based Ground Impedance Measurement Instrument," A.P. Sakis Meliopoulos et al, IEEE Transactions on Power Delivery Vol. 8, No. 3, July 1993.  Dr. Meliopoulos' company Advance Grounding Concepts markets the instrument.  Make sure that you also read the discussion by F. Dawalabi.  Dr. Dawalabi does not recommend the SGM.  Both Dr. Meliopoulos and Dr. Dawalabi are both highly recognized grounding experts.  I recently discussed this with an engineer with Dr. Dawalibi's company, Safe Engineering Services, and found that he has not changed his mind since 1993.

Thanks for that info.  You are absolutely right about the difficulty in using the fall of potential method on a large grid.  I took measurements on the grid from a power plant we built in a downtown area.  Had to get our probes about a half mile from the plant to get meaningful readings.  Cars driving over the leads made for some funky results.

I'll read the Transactions document you referenced.  Thanks again.

Occasionally, one aspect of ground testing gets overlooked, and errors may be introduced.  For low-range measurements, the electrode under test should be terminated with separate potential and current leads; id est, a Kelvin connection.
  

There is another solution to the large grid problem.  We use the SED CDEGS grounding analysis program which has a fall-of-potential option.  With the ground grid as the main electrode, you can put in a return ground electrode at the point where you would put your current probe for fall-of-potential measurements.  The location does not have to be all the way out of the influence of the main grid, but the further the better.  Then define a potential profile between the point on the grid where you would connect the ground tester and the return electrode. The program will graph apparent resistance vs distance from the grid.

You then find the distance at which the apparent resistance equals the calculated grid resistance.  Put the potential probe at this distance.  A resistance measurement at this point will be pretty close to the correct resistance because the distance is relatively independent of the actual soil resistivity.  This method can be used with multi-layer soils.