substation perimeter block wall

[stevenal]

A concrete block wall makes up a portion of a distribution substation perimeter. The wall's purpose might be just to reduce the visual impact on one side, or it may be part of a control building that intersects the fence. In either case, the outside of the wall can be contacted by neighbors or by the general public without entry of the station. The ground grid extends several feet beyond the substation perimeter, including the block wall portion. The wall is reinforced with vertical an horizontal rebar which is not bonded together or bonded to the ground grid. Do you see a hazard here? Thanks.

 

[jghrist]

You could treat the wall as a grounded object when checking touch voltage in the grounding design. Essentially, it would treated the same as a fence. Some software (CDEGS) could model the wall as a separate object not connected to the grid, which would be equipotential because of the rebar. This probably would result in a lower touch potential; if so, treating it as a grounded object would be conservative.

There may be a hazard if there are overhead lines that could fall on the wall and energize it without providing a low impedance path for fault current. Barbed wire on top, bonded to the ground grid, would solve that possible hazard.

 

[cuky2000]

Jghrist, I do concur with you regarding considering the masonry wall as a grounded object when checking touch voltage in the grounding design.
However, I am not clear regarding the comment about line fall. I believe that Will be a hazard situation regardless the wall ground impedance. The protection system should detect as a fault a broken line and instant later after hit the wall or the earth.
Often a substation fence includes an isolation panel to limit the transfer potential of neighbor property fence.

 

[stevenal]

Thanks. To me, the ideal would be to weld all the rebar together and tie it into the grid. This would ensure the copper underfoot would be at the same potential as the rebar beyond your hand, likewise ensuring both parts of the body are at the same potential. Treating the wall as a grounded object would seem to argue all this bonding isn't needed. Do I understand correctly?