Recommended Practice for Grounding of Industrial and Commercial Power Systems

[lucy27]

thread798-174983

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?

 

[itsmoked]

1) A separately derived source of power is created with most transformers used in all commercial power systems. A Separately Derived Source, while having a specific voltage between the various derived legs, has no particular voltage relationship between its legs and anything else in the universe.

That means it can ultimately have any voltage potential between its legs and the earth. How? Static charge from blowing wind, inductive coupling of nearby circuits, lightning, capacitive coupling to nearby circuits, etc. A high potential difference raises the possibility of an unexpected and dangerous current flow. Typically this can be because the insulation system designed for the voltages of the transformer's output can be exceeded pretty easily. Grounding literally puts-a-stake-in-the-ground on the ultimate potential limits seen by the system's insulation and any the world around it. This is why ALL systems must have a ground reference of some sort.

The common Delta power system has no obvious potential centric aspect so to meet (1) above one phase can be grounded or often the middle of one phase is grounded. This constrains all the rest of the phases to be at a particular potential above earth and no higher.


2) Yes, earth resistance can change and is different everywhere. When the ground plane of a power distribution system is contemplated the earth grounding system is designed after earth resistivity in the area is studied. Wetter ground is more conductive than dry ground where a lot of troubles can haunt installing an effective ground system.

The greater world at-large has a very low overall resistance. This can be noted by the practice of places like South Africa where a hundred mile high power transmission system often uses the earth as one of the two conductors.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[MJCronin]

This is a bad place for this question ... Suggest that you re-post on an electrical Power and Distribution forum ...


MJCronin
Sr. Process Engineer

 

[lucy27]

MJCronin

I'm terribly sorry, I am new to forums and posting questions. I just realized this is a Chemical process engineer forum.
My appologies to you and thanks to itsmoked for such detailed explanation.

 

[itsmoked]

I've answered it so there's no need to ask it again anywhere.

Keith Cress
kcress -

http://www.flaminsystems.com