Substation GPR Drop-off Gradient Protection

[RSChinn]

When we examine the GPR gradient at a substation, there is a very steep drop-off at the edge of the grid. Has anyone protected people against this steep gradient by using commercially available insulation such as using rubberized "nugget chips" or a rubber sidewalk? Extending horizontal grounding conductors beyond the existing grid simply extends the drop-off edge for the GPR, doesn't it?

 

[7anoter4]

The potential drop from the Substation Ground Grid fence –if the grounding grid is extended 3ft outside – will be equal with Touch Potential.
The Earth Voltage at any point (x) outside the Grounding System can be evaluated using the expression:
Vex=2*Vg/PI*arcSin(r/(r+dx) where Vg is GPR ;r=equivalent radius of the grounding grid area; dx =distance from the Grounding Grid.
So, for a further 3 feet from the fence the potential drop will be less than Step Potential and from here the potential drop for each 3 ft will drop dramatically.
As example, for 10000 sq.ft area and 3 KV GPR the first potential drop up to 6 ft from the fence will be approximately 600 V, further 240 V and so on.
My opinion is you don't need isolation unless pipes or rails enter the Substation perimeter.
Regards

 

[jghrist]

GPR is the grid voltage with respect to remote earth. It is a single value. I think what you are referring to is the surface voltage gradient. The gradient in V/m is the same as the step-voltage.

As Zanoter indicates, this is normally not a problem unless you have pipes or rails that locally distort the gradient or if there is something grounded within reach (making the step-voltage into a touch-voltage). The step-voltage along the perimeter does need to be checked, however, and if necessary, an insulating surface covering like crushed rock can be extended beyond the grid area.