Step Voltage-Earth rods around the building

[abrahamJP]

Hi,

I have a building with three 11/6.6kv, 5MVA transformers with MCC's as loads.Proposed Grounding system is a single row of ground rods at every 8 meter around the periphery of a building interconncted by a cable (of suitable earth fault current carrying capacity).

I want to put this information in a power system analysis software like "CYMGND'.But software is asking for something like a "earth grid configuration".But in my situation,
it is not like a continuous grid/mat since building foundation is coming in between them.

How can I calculate the step and touch voltage?If I take mean value of soil resistivity and concrete resistivity & consider it like a continous mat-is it o.k??

thanx in advance

Abraham

 

[ScottyUK]

The IEEE 80 method for a square / rectangular grid and two conductors in each direction (i.e. defining the perimeter) plus whatever number of rods you include should give you a reasonable estimate if you know soil conditions. It's a simplistic calculation compared to the dedicated software but also fairly conservative. Half an hour with a spreadsheet should get you there. You can probably do the same in the software - two X conductors and two Y conductors define the perimeter of the ring.

 

[jghrist]

The earth grid configuration is the perimeter wire and the ground rods. The building, if it has a reinforced concrete floor slab would ideally be modelled as an area of low resistivity within the grid. I'm not familiar with CYMGRD, but it probably does not allow modelling the buildings lab like that. The slab will essentially be an equipotential surface and you can ignore step and touch voltages in the building. Your concern will be outside the building. Leaving the building slab out of the model would be conservative.

Keep in mind that touch and step voltages are caused by fault current flowing through the earth. If the source of the fault current is the secondary of a grounded wye transformer winding where the neutral is connected to the grid, then the fault current doesn't flow through the earth. Touch and step voltages only exist for faults with a source remote from the grid.