Fall of Potential - Ground Testing Advice

[Elec81]

Hi,

I have a question regarding fall of potential ground testing. I'm familiar with the common FOP methods, including the slope method. However, my question relates to test electrode measurement when the surrounding terrain is not flat. Example, I have a substation grounding system at the bottom of very steep land embankments on a number of sides, around 50 feet high. In this case, initial connections are made to the grounding system at the bottom of the embankment, with the remote current probe and intermediate potential probe on the land at the top of the embankment. I appreciate the measurements are normally just a linear distance. However, how should the distance from the grounding system to the remote current and potential probe be measured in my case? Is this simply measured as a straight line (from a plan view of the site), or should I be measuring up the 'hypotenuse' of the embankment?

If possible, please quote any sources of information in your response.

Thanks in advance.

 

[sheimah]

hi, am sorry the way am going to answer your question but i need you to understand something

normal approach (on Flat Terrain)

On flat ground, the distance is simple — you measure the linear (horizontal) distance between the ground system, the potential probe (P2), and the current probe (C2). For example:
Distance from the grounding system to P2 (potential) = 60m.
Distance from the grounding system to C2 (current) = 120m.
The distance is usually measured along the ground's surface (not the hypotenuse of any slope) since that's how the current flows in the earth.

Your Situation (Embankment / Sloped Terrain)
Here, the substation is at the bottom of a 50-foot-high embankment, while the probes are placed on top of this slope.

The total path is now like a triangle, with:
Base (horizontal distance on plan view)
Height (vertical distance, i.e., the 50-foot embankment)
Hypotenuse (the actual distance if you "walk" from the bottom to the top)

Measure the ground surface distance (hypotenuse), not the horizontal distance (base). This is because the actual path of the current in the soil follows the earth's surface, not a straight line through the air.

if the Substation is at the base of the 50-foot embankment.
You place the potential probe 60m away (as measured along the surface of the slope).
you place the current probe 120m away (again, measure along the surface of the slope).
Use a measuring wheel or tape measure to follow the path along the ground's surface.

note
If you measure only the horizontal distance, you’ll underestimate the true distance.
Example: For a 60m horizontal distance with a 50-ft (15m) rise, the actual surface distance (hypotenuse) is:
Hypotenuse=/(60m)2+(15m)2 =/3600+225 =/3825≈61.85m
If you only measure 60m (horizontal), but the actual current path is 61.85m, you will introduce error in your test

NB
Be sure to place the probes far enough away (typically 3 to 5 times the diagonal size of the ground system).
If possible, avoid placing probes in areas where there are steep changes in elevation (but if you have no choice, use the ground surface distance).
If you have access to a GPS, a Total Station, or a laser distance meter, you can get the exact surface distance.

source
IEEE Std 81-2012: "Guide for Measuring Earth Resistivity, Ground Impedance, and Earth Surface Potentials of a Ground System." This standard describes FOP testing on uneven terrain and suggests using surface distance.