Earthing

Ground Potential Rise (GPR):
The maximum electrical potential that a substation grounding grid may attain relative to a distant grounding point assumed to be at
the potential of remote earth.
This voltage, GPR, is equal to the maximum grid current times the grid resistance.
Step voltage: The difference in surface potential experienced by a person bridging a distance of 1 m with the feet without contacting
any grounded object:
Estep = (RB + 2Rf ) ⋅ IB
Estep50= (1000 + 6Cs ⋅ ρs)*0.116/ts
Estep70= (1000 + 6Cs ⋅ ρs)*0.157/ts
Touch voltage: The potential difference between the ground potential rise (GPR) and the surface potential at the point where a
person is standing while at the same time having a hand in contact with a grounded structure:
Estep70 =(1000 + 1.5Cs ⋅ ρs)*0.157/ts
Estep50=(1000 + 1.5Cs ⋅ ρs)*0.116/ts
Mesh voltage: The maximum touch voltage within a mesh of a ground grid.
Metal-to-metal touch voltage: The difference in potential between metallic objects or structures within the substation site that may
be bridged by direct hand-to-hand or hand-to-feet contact.
Transferred voltage: A special case of the touch voltage where a voltage is transferred into or out of
the substation from or to a remote point external to the substation site.
Soil treatment to lower resistivity

It is often impossible to achieve the desired reduction in ground resistance by adding more grid conductors or ground rods. An
alternate solution is to effectively increase the diameter of the electrode by modifying the soil surrounding the electrode. The inner
shell of soil closest to the electrode normally comprises the bulk of the electrode ground resistance to remote earth. This phenomenon
is often utilized to an advantage, as follows:
a) Use of sodium chloride, magnesium, and copper sulfates, or calcium chloride, to increase the
conductivity of the soil immediately surrounding an electrode. State or federal authorities may not
permit using this method because of possible leaching to surrounding areas. Further, the salt treatment
must be renewed periodically.
b) Use of bentonite, a natural clay containing the mineral montmorillionite, which was formed by volcanic action years ago. It is
noncorrosive, stable, and has a resistivity of 2.5 Ω·m at 300% moisture. The low resistivity results mainly from an electrolytic
process between water, Na2O (soda), K2O (potash), CaO (lime), MgO (magnesia), and other mineral salts that ionize forming a
strong electrolyte with pH ranging from 8 to 10. This electrolyte will not gradually leach out, as it is part of the clay itself. Provided
with a sufficient amount of water, it swells up to 13 times its dry volume and will adhere to nearly any surface it touches. Due to its
hygroscopic nature, it acts as a drying agent drawing any available moisture from the surrounding environment. Bentonite needs
water to obtain
and maintain its beneficial characteristics. Its initial moisture content is obtained at installation when the slurry is prepared. Once
installed, bentonite relies on the presence of ground moisture to maintain its characteristics. Most soils have sufficient ground
moisture so that drying out is not a concern. The hygroscopic nature of bentonite will take advantage of the available water to
maintain its as
installed condition. If exposed to direct sunlight, it tends to seal itself off, preventing the drying process from penetrating deeper. It
may not function well in a very dry environment, because it may shrink away from the electrode, increasing the electrode resistance
(Jones [B90]).
c) Chemical-type electrodes consist of a copper tube filled with a salt. Holes in the tube allow moisture to enter, dissolve the salts,
and allow the salt solution to leach into the ground. These electrodes are installed in an augured hole and typically back- filled with
soil treatment.
d) Ground enhancement materials, some with a resistivity of less than 0.12 Ω·m (about 5% of the resistivity of bentonite), are
typically placed around the rod in an augured hole or around grounding conductors in a trench, in either a dry form or premixed in a
slurry. Some of these enhancement materials are permanent and will not leach any chemicals into the ground. Other available ground
enhancement materials are mixed with local soil in varying amounts and will slowly leach into the surrounding soil, lowering the
earth resistivity.

Earthing

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Ground Potential Rise (GPR):

Soil treatment to lower resistivity

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