Insulated Ground Grid and Touch Voltage

[DistCoop]

Hello,

I'm trying to wrap my head around something but can't quite get there. Suppose I have two separate ground grids in close proximity that are connected to each other by conductor. One of these grids is bare conductor, and one is insulated (both have bare ground rods). If I'm the area with the insulated grid and a fault occurs, a GPR will exist on the grid because there is a path through the ground rods (or because the fault is on a grounded structure). There is a different potential on the surface, so an employee working on something could experience a potential difference across him.

I'm debating with someone who suggests that because there is insulation, the fault current cannot diffuse into the soil and no potential will exist. I can see how this may effect step potential, but I don't see that touch potential will be influenced to a significant degree (unless the fault was a conductor falling on the ground vs falling on a structure, perhaps).

Any thoughts?

 

[cuky2000]

Here are some thoughts in this subject:
Touch and step potential will be influenced in the adjacent yard with insulated conductor during a ground fault event because the following:
Any metallic or any conductive object (equipment, steel structure, fence, etc.) above grade will be virtually at the same potential than the grid conductor. However the potential at the yard surface will vary because the following
• Current will be injected into the ground through the bare rods will travel through the earth impedance creating voltage drops along the yard surface.
• Coupling connection through the earth between adjacent yards will also create voltage difference along the current path.
• Do to the insulated conductor (no bare grid) the voltage in the surface could vary widely.
From a practical point of view, the following should be expected:
1. Touch potential: voltage difference between grounded object (equipment grounded) and surface voltage ~3 ft. (1m) around the equipment could exceed the allowable voltage.
2. Step Potential: voltage difference every ~3 ft. (1m) could exceed the allowable voltage

PS: Will be difficult to model insulated conductor using commercial software.

 

[jghrist]

As cuky said, metallic objects will be grounded and at the grid potential. Current will flow through the earth from the bare conductor grid and through the ground rods in the insulated conductor grid, causing a voltage drop through the earth and lower then grid potential on the earth's surface in the insulated conductor part of the substation. There will be much more touch voltage on the insulated conductor part because of more voltage drop in the earth.

 

[7anoter4]

From IEEE 80/2000 Annex F, F2.4. Ground rods only Table F1 electrode R5 [9 rods] Touch voltage 40.8 %[from GPR] and compared with Table F2 Grid and grounding rods SR5 Touch voltage 21%.