Step and Touch Voltage Calculations for Grounding

[timm333]

I am trying to figure out that as per NEC/IEEE: when do we need to calculate the step/touch voltages for grounding. For an industrial plant, the incoming 25kV is stepped down to 4.16kV. Step/touch voltages are calculated for 25kV substation. However there are also 4.16kV motors and transformers inside the industrial plant.

Is it required to calculate the step/touch voltage for 4.16kV motors and 4.16kV transformers inside industrial plant? Thanks

 

[Julius Right]

The current producing EPR -connected with touch/step voltage-it has to pass through the ground [Earth] not through grounding grid-where specific resistance is very low. So, if the motor and inside transformer are located above the grounding grid no touch/step check is required.

 

[timm333]

There is no ground grid directly below the 4.16kV motors. There is a ground ring of bare copper conductor which encircles the area where the 4.16kV motors are installed, and the body of the motors is connected to this ground ring.

Is this encircling ground ring enough, or should there be ground grid directly below the 4.16kV motors?

 

[Julius Right]

The touch/step voltage are potential differences from different points on Earth.
If the short circuit current from high voltage grounding fault may produce such elevated voltages up to considered points, outside the grounding grid, you have to extend the grounding grid accordingly

 

[ron]

Practically, if the equipment is enclosed, we don't provide a ground grid or provide step / touch potential calculations.