×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Earth Resistance

Earth Resistance

Earth Resistance

(OP)
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.

RE: Earth Resistance

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.

RE: Earth Resistance

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.

 

RE: Earth Resistance

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

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources