Equipotential zone

[rezaa]

To etablish an equipotential zone, a ground mat should be installed under the operation area of equipment and then connctect the mat to main grid. My question is that where the mat should be installed on the surface of the soil or below the sufrface of the soil. What are the difference between this two practices?

 

[stevenal]

If you want equipotential from equipment to mat (or from worker's hand to feet) then the bond you want is from mat to equipment. This bond should be fully visible so connections can be inspected prior to use. The equipment is then bonded to the grid to provide a safer path for fault current. To ensure workers know where to stand, mat needs to be visible, above ground.

 

[rezaa]

By installing below the sufrface of the soil, we can reduce the touch voltage level.

 

[stevenal]

If you know the answer, why did you ask? I disagree, though. The most effective way to ensure the voltage across any resistor is zero is to parallel the resistor with a zero impedance path.

The other part of the story is step potential. On soil, your feet will be at two different potentials. Worse yet, is when one foot is above the mat and the other off.

 

[jghrist]

Installing a grid below the surface can reduce the touch voltage from the soil in the middle of a large mesh (open space in the grid). I assumed, however, that the OP referred to a closely spaced ground mat specifically for installation where an equipment operator would stand, such as at a switch operating handle. If this is the case, there is no significant soil area in the middle of a mesh, and stevenal's initial response is correct. If the operators feet are touching the ground mat conductor, there is no touch voltage when the ground mat is bonded to the equipment. It is a zero impedance path. Burying the ground mat would add soil resistance to the path.

 

[rezaa]

This gradient control mat is for an air breaker switch for a 138 kV transmission line. I was told that the main reason for installing below the surface soil is to keep away from stealing those coppers.

 

[jghrist]

If you're worried about theft, I'd consider using a galvanized steel switch operating platform, placed on grade.

 

[rezaa]

A galvanized steel seems to have higher resistivity with respect to copper.

 

[jghrist]

Resistivity is not a problem. The resistance of the ground mat is inconsequential. You might also want to use galvanized wire for a visible connection from the platform to the switch handle if theft might be a problem. Copper could be used for below grade connections to the pole ground.

If theft is a problem, I would worry about a buried copper ground mat. If any thieves know you are burying a few pounds of copper near the surface, they may disappear and the switch operator may not know he has no protection. If you want to bury the mat, I'd suggest using copperweld mesh instead of copper, and connect it to the equipment with copperweld wire. Another alternative would be a portable ground mat to lay on the ground only when operating the switch.