What is a ground resistivity test, and how does one go about testing a system ground?
The most common way to test ground resistance is with a fall of potential tester some times called a earth megger.
It has 3 leads one goes to the ground rod under test,the second lead goes to a driven test electrode 60'from the rod under test and the 3rd driven test electrode is at 120'from the rod under test.All three of the points should be in a streight line. Now what happens is that a voltage is passed between the rod under test and the rod at 120'. The rod at 60' measures the voltage drop between the 60'point and the rod under test.The unit works on null balance an you adjust the 4 knobs until the zero center meter reads zero.The figure at the 12:00 on the knobs is the resistance of the ground.
A word of caution a new ground rod will have a higher resistance than one that has been in the ground for a couple of days.Let the earth heal around the rod before testing you will get a better reading.
Service Upgrade $2000
Ground Testing $800
Be done with it, install the two rods!
Along with Gfretwells link here is another one to check out.
http://www.tradeport.on.ca/ApplicationNotes/aemc.pdf If this isn't a performance test required for a job specification, I would do as LK sugests and drive the second rod.
Roger
Here in Jersey the public, and commercial pools, need a performance test, and inspection, there are companies that are set up for just that work.
That's funny. I think the best electrode in the world is probably a gunite pool. You just can't call it that.
I have worked on several projects where they did performance testing of the grounding system. Mostly radio tower/equipment shelter installations and toll booths. The radio engineers design the tower systems and we had Bert Lavin (former CMP kinda guy)working on the tool booths. They turn out to be a lightning magnet around here.
To add to hypress description the center lead (Potential lead) should be placed at a distance that is 52% of the distance of the furthest lead (Current lead), record the value and move the center rod to 62%, record, then 72%, record. These values should be plotted to show a relativly flat line to verify your readings. If the plot is not flat your readings are being affected by the sphere of infulence of the ground rod under test and the current lead and the current lead should be moved further away from the rod under test and the entire procedure repeated until your 3 point plot is relativly flat.
Another method that is becoming more accepted is the clamp on method, simple and fast, but the test equipment is more expensive.
thanks for the feedback so far, FYI this is a commercial building, concrete encased grounding grid I believe, since the structure sits on a pier. Also 12.4kv coming in, with 4 2500kva transformers in the room.
You want to see less than 5 ohms for that type of system per NETA MTS