High line-ground current when ground has a high resistance?

[shespuzzling]

Can someone explain why, if you have a line to ground fault (real ground, as in earth, as in a power line snapping and falling into a tree or something), you get very high fault currents if ground is actually a very bad conductor of electricity? How is it an infinite source and sink of electrons but has a high resistance? Thanks in advance for your help.

 

[ActionDave]

Basically the earth is a very good conductor, you just need a large enough connection to get a hold of all the parallel paths. Check out this link.......http://forums.mikeholt.com/showthread.php?t=116358

 

[GoldDigger]

The problem most people have is learning to deal with a large 3 dimensional block of material with a specific bulk resistivity instead of two or one dimensional resistors.
I cannot do the concept justice in one small list, but this may help you start to wrap your brain around it:
Imagine a long rectangular bar of material with a square cross section
The end to end resistance (with a square of metal on each end as the electrode contact.)
The resistance is proportional to the length of the bar but is inversely proportional to the side of the square.
Now double the linear size of the bar. It gets twice as long but also twice as wide and the overall resistance is cut in half.
Now imagine such a bar 100 miles long but also tens of miles wide or larger. The resistance between two points 100 miles apart on the Earth's surface will be close to zero.
Oops, we assumed a square electrode on each end. We really have a very small rod or plate at each end. That is where all of the measured resistance and voltage drop happens.
We can keep the same two electrodes and double the distance between them and not increase the resistance at all. Because of that we can say that the resistance of the earth itself, above and beyond the local electrode resistance is zero.
I hope that gives you some introduction to what is going on.

 

[ActionDave]

The problem most people have is learning to deal with a large 3 dimensional block of material with a specific bulk resistivity instead of two or one dimensional resistors.
I cannot do the concept justice in one small list, but this may help you start to wrap your brain around it:....

You just summed up what took four weeks and four hundred posts about four years ago on this forum. The link I posted is still a pretty good read though.

 

[GoldDigger]

You just summed up what took four weeks and four hundred posts about four years ago on this forum. The link I posted is still a pretty good read though.

The link you posted is indeed a great learning tool.
I especially like the illustrations, although the Hubbell video seems to have been lost. I even tried searching on the current Hubbell site and could not locate it. If anyone finds it, please post it here!
And of course the reference back to the earlier 81 post thread is nice.

 

[shespuzzling]

Thank you all!!! This information is very helpful. Now I've just got to try to let it all 'sink' in

 

[kwired]

Also consider that when a 120 volts to ground line drops the amount of current that may flow to earth is very low, but if a 7200 volts to ground distribution line drops the voltage is 60 times higher and will push more current through the same resistance.

 

[GoldDigger]

Also consider that when a 120 volts to ground line drops the amount of current that may flow to earth is very low, but if a 7200 volts to ground distribution line drops the voltage is 60 times higher and will push more current through the same resistance.

And the distance at which a dangerous touch or step potential exists is correspondingly larger.

 

[big john]

A big factor is also the rule of parallel resistors: A small cross section of earth may have a comparatively high resistance, however, every time you add another cross section to the circuit, you're left with a total resistance that is lower than the either of those cross sections.

You add enough points of contact with the earth--enough cross sections--and now you have a huge array of parallel paths and a cumulative total resistance that is extremely low. This is what you're trying to accomplish with substation grounding and can be confirmed with electrode testing: You ensure you have enough conductor in contact with the earth that the resistance is low enough that it will effectively clear a high-voltage fault.

 

[bob]

Dirt is not always a good conductor. I saw a 7200 volt phase conductor fall into a ditch filled with water. The water was boiling and
the ground relay, set at 250 amps, never tripped the sub breaker. The sub was not very far away from this location. I tested a ground
resistance in a very dry area of red clay an got a reading of 900 ohms. After driving 5 -10 ft. rods, the resistance dropped to 50 ohms.