Earth Resistance

[Mike6061]

A colleague and I were having an argument regarding earth resistance. He argued that between any (2) earth electrodes the resistance of the earth resistance would be (depending on the soil)1000s of OHMs. I disagree. I argued that most of the resistance would occur around the areas immediately adjacent to the grounding electrodes. As you proceed further out you would be looking at a much larger cross sectional area and the resistance would level off. So although a small cross sectional area of the earth may have a very high resistance, taken as a whole the earth resistance is much smaller due to the multiple parallel paths in the soil through which current can flow. This argument was a result of me trying to demonstrate that between the utility transformer (which is bonded and grounded) and the service of a building (also bonded and grounded) you will have measureable current flow through the ground in the case of a 3-phase load imbalance or ground fualt condition. Although the neutral is acting as the EGC and will carry most of the current, current takes all available paths and the ground will act as such a path.
So the question is
1) Am I correct
2) Taken as a bulk, does the earth provide a decent (not neccessarily low impedence) return path for fault current.

 

[jghrist]

You are correct about most of the resistance occurring close to the rod. The resistance between the transformer rod and the service rod will be close to the sum of the resistances to remote earth. This will usually be over 25 ohms. 25 ohms is much larger than the typical neutral impedance, so not much of the current will flow in the earth.

 

[dpc]

The answer to #2, I would say is no. The earth is generally not a very good conductor. This is why an energized overhead conductor can fall onto the ground and not cause enough ground current to cause a relay to operate or fuse to clear.

You can never rely on the earth alone to pass enough fault current to operate the overcurrent protection. If you're lucky it may, but many were not so lucky and some are no longer with us.

 

[opmgr1]

The IEEE green book will give a good account of what dpc and jghrist described above.

 

[WhyDoYouAsk]

http://www.aemc.com/techinfo/appnotes/Ground_Resistance_Testers/App-Ground-SoilResistivity.pdf

 

[Ceast]

More info here:

http://us.fluke.com/usen/products/Fluke+1625+1623.htm