Resistance to ground measurement


Thread Starter


1. Can anyone tell how resistance to ground measurement is performed ideally?

2. And how can it be performed if no specialised instrument is available? If I want to perform it by say current and voltage measurements, then is it possible? And which voltage measurement should I have? Let's say I want to measure resistance to ground measurement of a conductor surface...

Please pardon me if my questions are not proper. I want to know anything if you can on RTG measurement. I couldn't get anything relevent on the internet.

Bob Peterson

My understanding is the measurement is not all that simple. A better place to ask is somewhere like Mike Holt's Code Forum. It comes up there now and then.
It's been a long time (wrote a paper on this in 1975). If you don't have, or don't want to buy the specialized ground resistance tester here is what you need to do what is called the "Fall of Potential" method:
1. Identify the ground you want to test. Measure with a clamp on ammeter if there is any current flowing (if yes reduce that to below what you can measure - this may require a total shutdown).
2. Prepare a long cable (#14 is fine) some 200 to 500 feet away from any metal structures or underground metal, and connect it to a spike pounded into the ground (usually a foot will suffice). (I think I remember this correct, that you need to be at least 5 times the greatest distance measurement of the ground system you are trying to measure). Connect the wire to the distant ground and leave the other end of the wire isolated, treat it as if it is hot. With a voltmeter at highest range check the voltage between the ground to test and the distant ground. If there is any more than about 2 Volt you will need to shut the site down or find some other way to reduce that.
3. Connect a current generator from the ground under test and the distant ground and inject, say 1 to 5 Amp (that must be held steady throughout the test).
4. With a sensitive voltmeter connected to the ground under test and a short "roving ground rod" (connected with another long #14 wire), pound the roving rod in about every 1/10th of the distance between the ground under test and the distant ground. Measure and record the voltage at each point.
5. Plot a distance vs voltage graph with distance on the x axis. You should see more or less of an S shape. The inflection point from the middle flat part toward the higher curve is taken to be the characteristic resistance (obtained by Ohms law). Did I say be careful with distant electrode wiring?