Reduction in Step & touch potential
Reduction in Step & touch potential
(OP)
Hi,
I am not sure, if I am using a smart approach while trying to accomodate step and touch potential as it does not seems to be economical.
My GPR is high approx. 18 KV. but local authority wants us to satisfy Step and touch potential issues. I have reduced the size of meshes to half, I gues they have reduced from
6ft by 6ft to 1.5 ftby 1.5ft in a 80 ft by 38 ft area.
Just wondering, is there any other way to deal witha situation like this.
I would appreciate a word.
Thanks
I am not sure, if I am using a smart approach while trying to accomodate step and touch potential as it does not seems to be economical.
My GPR is high approx. 18 KV. but local authority wants us to satisfy Step and touch potential issues. I have reduced the size of meshes to half, I gues they have reduced from
6ft by 6ft to 1.5 ftby 1.5ft in a 80 ft by 38 ft area.
Just wondering, is there any other way to deal witha situation like this.
I would appreciate a word.
Thanks






RE: Reduction in Step & touch potential
What is the system voltage? You can't get more GPR than the line-to-ground voltage of the system. You can get enough fault current such that the fault current times the grid resistance will exceed line-to-ground voltage, but if you do, this means that less than the total fault current will be flowing through the earth.
RE: Reduction in Step & touch potential
I managed both but just thought, is there any other way get these issues resolved in small substations ?
One more point, you can shed some light on:
1. I am getting a bit increase in step potential by reducing the size of meshes. Is this normal ?
2. While designing the grid, I am not considering any concrete structure foundation modeling. That means during a fault situation my touch and step on the swgr building could be an issue. I will be connecting rebars to the grid but below the foundation there will only granular material and no grid.
From your experience, concrete foundation of structures can pose a problem?
Regards,
RE: Reduction in Step & touch potential
It is not really reasonable to assume that all of the fault current goes through the soil. If this were really the case, say if there were no sky wire, the fault current would be lower because the zero-sequence impedance would be higher. I don't know how you can convince the local authority of this. Maybe you can convince him by referencing IEEE std 80-2000 Chapter 15.
Another option would be to include the effect of substation grid resistance in the calculation of fault current. See IEEE std 80-2000, Section 15.4. I don't know how this could be accomplished if you were given the available fault current or if you calculated it with power system analysis software.
The concrete foundation should not be a problem, but if it is a slab foundation as opposed to deep piers, and is poured on relatively high resistivity granular material, it won't help much either. The foundation with its bonded rebar will all be essentially at the potential of the grid. You have to maintain low enough touch-potentials around the building perimeter to allow standing outside the building and touching the building. Inside the building, everything will be pretty close to the grid potential, so touch- and step-voltages will not be a problem. You could model the rebar and concrete in detail to prove this, but I personally do not think it is necessary. There could be a problem with transferred potentials if you have communication cables in the building. This is a GPR problem, however, not a step- and touch-potential problem, and could be solved with proper isolation.
RE: Reduction in Step & touch potential
I used WinIGs and my fault current is after considering a power flow analysis option which considers grid impendance as the fault impedance. My fault impedance is 24 ohms.
I guess you cleared most of my doubts. I am just wondering, if I have to theoretically picture increasing the depth of grid, how it can effect the step and touch potential ?
If you could just drop few words, that would be helpful.
Thanks
yes one more things Jghrist, I am not sure, if you recall but an year ago, I used to have a different username. You answered all my questions on ground grid and today if I could design ground grids. I owe it to you for this expertise. So thanks.
RE: Reduction in Step & touch potential
Try varying the grid depth in your WinIGS model.
Some of the current from the grid goes up and then out. This causes a vertical voltage drop. With large meshes, increasing the depth will flatten out the voltage profile, and the voltage at the middle of the mesh will be closer to that directly above the wire. Even though the voltage drop directly above the wire will be lower for shallower grids, this is offset by the increased flattening of the voltage profile. With small meshes, the voltage above the mesh is pretty uniform anyway even with shallow grids, so the vertical component of the voltage drop will be more important.
Consider an extreme case of meshes smaller than your foot and at the surface. In this case, there will be no touch voltage; you are effectively covering the entire substation with a switch operating platform or mesh. Your feet will be at the same voltage as the grid (because you will be standing on the grid) and the equipment you can touch will be at the same voltage at the grid because it is bonded to it.
RE: Reduction in Step & touch potential
In my opinion the IEEE80 is not universal applicable.
Other theory as Optimum Design is more suitable for your application:
"Optimum design of substation grounding in a two layer earth structure"
By F. DAWALIBI & D.MUKHEDKAR-MONTREAL CANADA 1974
Also using 10-20 vertical grounding rods of 3/4" diameter and 20-50 feet long
may reduce the grounding grid resistance .
The material supplied by Erico
http://www.erico.com/products/GEM.asp
ERICO's Ground Enhancing Material (GEM) is a superior conductive material
that improves grounding effectiveness, especially in areas of poor conductivity
such as rocky ground, areas of moisture variation, and sandy soils.
I hope this may help
Regards
RE: Reduction in Step & touch potential