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Grounding Profile - How far outside the fence should we go

Grounding Profile - How far outside the fence should we go

Grounding Profile - How far outside the fence should we go

(OP)
How far outside the fence should one go modeling their substation grounding profile?

Is 5ft safe enough to view the step and touch potentials?



  

RE: Grounding Profile - How far outside the fence should we go

How far out does the gate swing?  How far away from that can someone be standing and touching the gate?

RE: Grounding Profile - How far outside the fence should we go

(OP)
gate opens inwards.

My concern was mostly in regards to the step potential.

Judging from your questions as well it seems like in most of our modeling we pay alot more attention to touch and how far one can go and still touch the fence or gate etc, but what happens to step when the surface rock layer is no longer there i.e. when one is standing on the natural soil after the surface rock ends?
 

RE: Grounding Profile - How far outside the fence should we go

The step-voltage is normally not a problem; it is usually lower than the touch voltage and the allowable is higher.  Five feet outside the perimeter grid wire should be enough to determine if the step-voltage is too high.  It will decrease with distance from the last grid wire.  You can check against the allowable without crushed rock surfacing.
 

RE: Grounding Profile - How far outside the fence should we go

(OP)
jghrist, is that what is typically done? i.e. calculating step voltage without crushed rock layer outside the fence?

Is there a quick way of calculating the actual step potential however many ft outside the fence?

I am using the following equation for my step voltage threshold calculations

Estep50 =(1000 + 6Cs x ρs)0.116 ⁄ ts

where Cs = Surface Layer derating factor
      ρs = surface layer resistivity
      ts = shock duration
 

RE: Grounding Profile - How far outside the fence should we go

Quote:

jghrist, is that what is typically done? i.e. calculating step voltage without crushed rock layer outside the fence?
Yes.

Quote:

Is there a quick way of calculating the actual step potential however many ft outside the fence?
If you are using modelling software, this should be available.  Otherwise, no.  The step-voltage at the edge will be the worst case unless you have structures like pipelines or cables that can transfer potential outside the grid.

In the case of no crushed rock surfacing, Cs = 1 and ρs = ρ, the resistivity of the earth.
 

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