Grounding grid and structures

[lume7006]

Hello,

I hope you may help us again:

Does anybody have modeled the metallic structures that are connected to the grounding grid?
Do the structures have a real effect in reducing the grid resistance or the benefit is only creating an homogeneous potential?

I will thank your comments about this.

 

[alehman]

I think more information is needed. Are you referring to building structures? What are you trying to determine exactly?

 

[lume7006]

Hello,

Yes, you are right, we are refering to building structures that are "connected" to the grounding grid.

What we want to determine is:
why are they "connected" to the grounding grid?
in order to reduce the GPR, reducing the grid resistance?

We had previously done some grid designsignoring the building structures, however, in this case, we have to include them and we want to understand what we are expecting.

I thank you in advance

 

[jghrist]

The building structures are connected to the ground grid to equalize the voltage between equipment and the building structure. Exposed structural metal frames that may become energized are required to be grounded by the NEC.

If the building frame is connected to the rebar of concrete foundations, the foundations can reduce grid resistance and will affect the step- and touch-voltages. The building itself will not affect the grid resistance, but may affect touch-voltage by providing a grounded point for someone to touch.

 

[lume7006]

Thank you for your answer!

Final point:

Has anybody modeled the foundations as a part of the grounding grid in an specialised software?

What are the expected value of resistance of the building structure itself, thinking it is steel?

Best Regards

 

[jghrist]

I haven't modelled substation foundations nor specified that the rebar be bonded to make them a proper ufer ground. They will help not hurt, so they act to make the design somewhat conservative.

The resistance of the building steel frame, unless it is a pretty large building, would be negligible.