SGI Earthing Design – A Concept Note

An earthing system basically provides a drain path to all kinds of faults that may develop

in an electrical system comprising of power circuits, control circuits, EMP and statics.

These faults may be caused by surges or transients from indirect lightning strikes, or

spikes, or harmonics, or faults emanating from within your own electrical system. The

nature of these faults is varied with respect to frequency, amplitude and time. Therefore,

the earthing system of any installation needs to be thoughtfully designed to handle all

such faults effectively.

Generally an earthing installation consists of an elaborate system of earth grid comprising

of earth rods or electrodes, tapes or strips, and pipes that are connected to the electrical

earth terminal. The entire grid acts as a carrier of such faults. However, the grid by itself

is not capable of discharging theses fault currents into the ground. It is the electrical

resistance property of the surrounding ground that helps to absorb and dissipate the fault

currents from the carrier grid it to the earth mass. Hence, while designing an earth grid,

one should consider the following points in great detail.

1. The grid should be capable of conducting such fault currents for a given time

duration taking current density absorption capacity for continuous and

instantaneous duty of the soil.

2. The neighboring groundmass should be capable of absorbing such fault currents

and dissipating it to the larger earth mass taking dangerous potentials developed,

into consideration.

IEEE 80-2000, IEEE 665, IEEE 142, IEEE 81-993, BS 7430, IS 3043 are the standards

which provide detailed procedures for designing such a grid.

If the soil resistivity is high, the penetration of the charges into the earth mass would be

poor. The concentration of charges in the vicinity of the earth grid will thus be high and

create high voltage. Further, in a high resistive soil, the concentration of charges dips

steeply as you move away from the earth grid. This results in a steep drop in voltage. The

presence of high voltage in the vicinity of the grid & its neighboring areas is a matter of

concern and a serious hazard to the safety of human life and livestock. Therefore, while

designing the earth grid and its various components, we need to be cautious of the

following parameters:

Step Voltage: It is the difference in voltage between the right & the left foot

when a person is standing in the vicinity of the grid.

Touch Voltage: It is the difference in voltage between the point at which a man

stands & the object he touches, which is connected to the grid.

Mesh Voltage: Maximum touch voltage within a mesh of a ground grid.

Grid Potential Rise: The maximum potential a grid may attain relative to a

distant grounding point, which is at the potential of the remote earth.

Human heart fibrillates if more than 0.116 ampere current flows through it. Thus any

earth grid should necessarily have the following parameters.

The earth grid should have the minimum cross section to carry the maximum fault

current for particular time duration without fusing.

The superficial area of the conductor in touch with the soil should be more than

continuous or instantaneous duty ratings based on soil resistivity and time of

clearance of fault.

The actual step potential should be lower than the allowable step potential.

The actual mesh potential should be lower than the allowable touch potential.

The grid potential rise should be as low as design permits.

It is also observed that lower soil resistivity reduces the earth grid size. The grid thus

becomes more compact and economical. In situations where soil resistivity is high, or in

installations where the grid resistance is desired to be very low, it is always desirable to

treat the soil with artificial treatment compounds in order to bring down the soil

resistivity.

Correction of Old Earth Grid :

In an old live substation , it is not possible to take a shutdown for days and relay the earth

grid. This old earth grid can be enhanced by creating a peripheral grid of the desired

length around the existing grid. The length of the conductor required , should be

calculated as per IEEE 80 2000. The total conductor required should be arranged in

multiple tiers such that desired length as per IEEE 80 2000 is achieved. The multi tier

conductors have to be linked to each other at a distance of 20 mtrs. The parallel length of

conductors should be minimum 1-metre apart. The parallel conductor rings should be

alternately treated with artificial compounds (TEREC+ & Earthcon combination) as per

clause 14.5.d , IEEE 80 2000 to achieve permanency of the earthing system. This

ingenious and modern way of peripheral grid correction is termed as Tier-earth. The

design is tailor made for each soil type and has to be handled by authorized agency.

As ELV systems in sub stations is prone to GPR (Grid Potential Rise) at time of fault and

it is impossible to develop a full proof surge protection system through existing power ,

data, control lines ,it is advisable to create a separate electronic earth at a distance from

the tier earth in accordance to IEEE 142. An earthing system with minimum reflection

factor in which the fault current travels progressively into soils of higher resistivity has to

be created such that the resistance of SIGMA Earth is below 0.5 ohms. The calculation

of SIGMA Earth depends on volume dissipation of earth using plate conductor of micro-

ohm meter, TEREC+ of < 0.12 ohm meter, earthcon of <2.5 ohm meter, soil < 25 ohm

meter and virgin surrounding soil. The design is tailor made for each soil type and has to

be handled by authorized agency. The usage of these patented products ensures quicker

transfer of fault current to surrounding soil within a given time.

Modern concepts of New Earth Grid design:

A new earth grid has the flexibility to be designed with Terec+ to achieve permanency.

The substation earth grid design has to be as per IEEE 80 2000 with desired spacing such

that the application of permanent artificial treatment (Terec+) as per clause 14.5.d gives

the desired reduction in soil resistivity. The choice of combination of artificial compound

has to be advised by a specialist. The earth grid for body and lightning can be connected

to each other using an earth coil.

The electronic earth needs to be separate as thousands of control, data and power cables

cannot be practically fitted with SPDs. The electronic earth has to be separate from main

earth pit as per IEEE 142. An earthing system with minimum reflection factor in which

the fault current travels progressively into soils of higher resistivity has to be created such

that the resistance of SIGMA Earth is below 0.5 ohms. The calculation of SIGMA Earth

depends on volume dissipation of earth using plate conductor of micro-ohm meter,

TEREC+ of < 0.12 ohm meter, earthcon of <2.5 ohm meter, soil < 25 ohm meter and

virgin surrounding soil. The design is tailor made for each soil type and has to be handled

by authorized agency.

Advantages of modern concepts of earth grid :

1. There is a limited choice in terms of grid location, usually in un-fertile/

wastelands wherein there maybe pockets of high resistance underground and

inadequate land mass available for construction.

2. Usage of micro-chip and control cards necessitates more careful handling of

earthing for modern transformers and control panels.

3. In case of a new grid the adoption of new technology is simple for design and

installation compared to old grid.

4. The practical issues in proper installation of salt& charcoal /watering both during

construction and also in periodic maintenance are removed. The issues of proper

compaction, uniform dispersion and water retention are a significant improvement

over conventional techniques used over the years.

5. The grid resistance of every point in the grid and earth pits attached to the grid is

identical

6. The actual step potential and touch potential across the grid is constant due to

identical resistivity of the artificial treatment compound around the conductor.

7. GPR, actual step potential , actual touch potential are below permissible limits.

8. The grid resistance and dangerous potentials calculated at the time of design will

remain constant permanently even with varying moisture conditions in the infinite

earth mass.

9. The SIGMA earth provides a constant resistance to the protection devices and a

prefect reference earth to the control system permanently

10. New grids will require comparatively lesser investments and ROI will be far

sooner and the best in class.