Recommended Practice for Grounding of Industrial and Commercial Power Systems

[lucy27]

In the IEEE 142-2007 Recommended Practice for Grounding of Industrial and Commercial Power Systems (Green Book) states:
“System grounding is the intentional connection to ground of a phase or neutral conductor
for the purpose of:
a) Controlling the voltage with respect to earth, or ground, within predictable limits”

My questions were:
1) What does point a) mean?
2) Does earth value (resistivity I assume) changes?

 

[jghrist]

Point a) means that by connecting one conductor of a system to ground, the voltages of the other conductors to ground are limited by the system to predictable values.

I don't know what you mean by earth value. Earth resistivity will change with soil moisture content.

 

[lucy27]

jghrist

thank you for your answer. You answered my question about earth value, I now understand that it will change with soil moisture. It means it is not the same at every point.

 

[cuky2000]

From the practical perspective, the ground (earthing) point is at zero potential (or close) with respect to the voltage source and provides a consistent and repeatable reference point.
Although there are changes in the soil resistance, from the practical point of view those changes might not be determinant to consider the ground as a zero reference point. The engineer need to figure out how to make the grounding system to work for each application using guidelines such as the IEEE Std 80, IEEE Std 142 and other recognized industry practices.

The following is an excerpt from the NEC Article 250.1 that reflect the issue associated with the concept as ground zero reference: