Grid voltage profile around corners & sides 2

[Power0020]

The surface voltage profile in IEEE 80 examples shows that the voltage decreases at the last conductor near to the edge, however, these are the conductors with most leakage current into soil. I couldn't figure out the reason for this decrease of surface voltage towards the edges.

Any clue?

 

[jghrist]

The current flows from the grid wire to remote earth, causing a voltage drop in the earth. The more the current, the more the voltage drop from the grid wire voltage to the earth surface voltage. The grid wire itself is at 100% of GPR and the surface voltage is lower than the grid voltage. The touch potential is the difference between the grid voltage (where your hand is touching a grounded structure) and the earth voltage (where your feet are). So, although the surface voltage at the edge is lower than in the middle, the touch voltage is higher.

 

[Mbrooke]

Isn't the goal to gradually decrease the effect of the ground grid then suddenly cutting it off which prevents a high voltage difference at the edge?

 

[Power0020]

Thanks jghrist. This is useful.

Just to add here:

the drop between the edge conductor and earth surface is greater than that for the middle since more leakage current is encountered. This is well understood, but, how the surface voltage is affected by currents not going to the surface? in other words that the current will be dissipated into ground, something usually deep depends on soil resistivitiy.

Mbrooke, I couldn't get what you mean, but I don't think that sudden cutting of GPR effect is possible since soil conducts current in a pattern similar to spherical waves.

 

[Mbrooke]

This, starting at 45:00 in:

https://www.youtube.com/watch?v=pAs_FmdxXhQ

People are no different existing or entering a substation during a fault.

 

[stevenal]

Keep in mind the zero reference for the surface voltage is remote earth. The surface potential must continually decline as you move away if no other grounded metal is encountered. Absolute surface voltage is not so important. It is the difference in surface potential to GPR (touch potential) that counts, as well as the differences in surface potential (step potential).

The IEEE 80 example assumes an evenly spaced grid layout. The dip in surface potential and the resulting increase in touch potential near the edge argue for tightening the grid spacing as you move toward the fence.