Modern Practices of Earthing Sonjib Banerjee (B.E) Technical Director DM

Member IEEE, NFPA Contact: Sonjib.banerjee@gmail.com

Synopsis:

Before we get to Modern practices of earthing, this paper revisits certain basic concepts. Important concerns of the industry are addressed with energy as perspective. Polar method to increase the computational accuracy of soil resistivity is explained. The variation of grid parameter along with variation of soil resistivity in horizontal plane is detailed. Usage of permanent Earthing compound as per IEEE 80 2000 clause 14.5 d is explained. Caution required while handling high frequency signal during earthing is elaborated. Science of mobile earth is hinted. Earth command and control center is conceptualized. Protection from LEMP and GIC is investigated.

What is Alternating Current?

It is commonly understood; current is movement of charge or holes. In Alternating Current, the charge actually does not travel at all. It only vibrates in its mean position. The positive cycle marks displacement of charge from the mean position towards the load and the negative cycle signifies its displacement towards the source. Across a Crossection area, the vibration begins with one charge and increases to the maximum number of charges

signifying amplitude of the sinusoidal AC waveform.

What is Tension?

The extent of movement of the particle from the mean position is due to prevailing Tension (similar to force) exerted during positive or negative cycle. More the tension, more the displacement of the particle from the mean position and vice versa. The charge oscillates, hence the net displacement of the charge across the Crossection area of conductor is ZERO.

What is Frequency?

The number of times the charge oscillates across its mean position is frequency.

‐ ve Cycle + ve Cycle

Low Tension High Tension

What MOVES?

It is the ENERGY that is transferred. The Energy is transferred thru vibrating charges across a Crossection to the next adjoining Crossection. Hence current is the movement of the disturbance and not the charge. The Energy transferred is proportional to

• the number charges vibrating,

• the displacement of the charges from the mean position and

• the number of vibrations per second

Series Circuit

In a series circuit, the Number of charges vibrating across the Crossection of the conductor and frequency never changes. The current is constant. The extent of displacement of the charge from the mean position changes, with every passing of impedances. This in normal words is called voltage drop.

Parallel Circuit

In a parallel circuit, the extent of displacement of charge from its mean position across the Crossection of the conductor, and frequency does not change. The tension is constant. The number of charges vibrating in each parallel path changes and is inversely proportional to the impedance offered by that path.

IMPEDANCE

HIGH IMPEDANCE

LOW IMPEDANCE

What is earth?

Earth is a huge mass with enormous amount of charges. When energy is applied to earth, the energy spreads, the number of charges vibrating increases, the displacement of charge from the mean position progressively reduces. Finally the Tension becomes infinitesimal. In a bird’s eye view, the charges appear static. The energy is transferred thru huge amount of charges diluting tension.

In normal condition charges in EARTH are stable with hardly any tension.

What is Load?

Energy can neither be created nor be destroyed. The energy transferred in the conductor can be diverted impeding the charge vibration.

• The loss of energy in a conductor due to reluctance of charge to vibrate causes Impedance due to resistance.

• The loss of energy due to inertia to maintaining the number of charges vibrating (current) thru a time domain in a loop causes impedance due to inductance.

• The loss of energy thru a dielectric for maintaining the displacement of charge

from its mean position (voltage) across a time domain causes impedance due to capacitance.

• The impedances are deliberately used to hinder the energy transfer in the conductor and convert this energy into usable form. These impedances are termed load.

What is Fault?

Any electrical signal can be characterized by

• Amplitude (number of charges vibrating),

• Frequency (number of oscillations across mean position per second),

• Voltage (displacement of charge from its mean position),

• Symmetry,

• Shape of wave.

All the above definitions have minimum and maximum acceptable limits. Any signal falling outside acceptable limits is termed a fault. Fault can be classified as continuous and instantaneous. Few examples of fault are as follows

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After 6400C water turns to vapor almost instantaneously. In case the copper or GI conductor is allowed to rise to Tm suggested in Table 1 IEEE 80 2000, the moisture round the conductor would escape converting the soil in the immediate vicinity of conductor into an insulator. The Earth grid would fail.

Respite comes from the fact that there are multiple paths thru which the Grid Current

divides. When an above ground earth conductor is connected to the grid, in worst case scenario IG can divide in minimum 2 paths at the point of entry. Crossection area of underground grid conductor can thus be calculated taking the fault current as IG/2, and Tm as 95

0C. Necessary corrections can be also made for ambient temperature Ta around the conductor which is appx. 600mm under the ground .

What is Soil Resistivity?

When the energy is dissipated into the ground mass, the tension dilutes 3 dimensionally. The energy faces variable resistance in every infinitesimal cone having height of 1 mt. The resultant resistance encountered by the energy to spread 1mt in all directions from point of

injection is termed as soil resistivity. There are many proven ways of collecting the data of soil resistivity. The Polar method enhances the accuracy of computation to find the net soil resistivity from a point on a given surface.

The steps followed in the polar method are as follows.

• Take the soil resistivity by any method as prescribed in IEEE 81 1983 in as many directions as possible

• Interpolate the readings to 7.50

• Draw a Polar curve connecting each point

• Compute the area of the Polar curve

• Draw a circle of an area equal to the polar curve

• The Radius of the circle is the average resistivity

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Variation of Soil resistivity on horizontal plane

In a large Area involving Substation, Switchyard or Generating Station the soil resistivity in a horizontal plane varies. In case the variance of Soil resistivity is huge, then an earth grid with

same spacing between conductors is dangerous. We shall look at the above case study.

Problem

The Soil resistivity is seen to vary between 480Ωm to 1018Ωm in a span of 175m X 150m. If we have a grid with equal conductor spacing, then the energy would dissipate more easily in the low soil resistivity area and hardly in high soil resistivity area. This unequal energy

distribution completely distorts the empirical values of tensions at each point. The actual step potential and actual mesh potential is different from calculated value. It safety is fully compromised.

Solution

The layout is divided into multiple zones such that the range of soil resistivity variation in each region is within 30%. Separate Earth grid is designed for each zone. The spacing between conductors in each zone is different in this case.

A second split factor is introduced to cater to the current distribution in each component grid. Many other grid parameters can also be varied in this case to achieve safe potentials.

Result

• Actual potential are close to empirical potentials is achieved.

• Grid is cost efficient.

A Particle Earth grid is shown below.

20 Earth Pits

20 Earth Pits

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How to tackle variation of Resistivity with Moisture, Salt and temperature?

IEEE 80 2000 clause 14.5 d fully explains the above problem. The abstract of the clause is mentioned below:

‘’Ground enhancement materials some with a resistivity of less than 0.12Ωm (about 5%of the resistivity of Bentonite), are typically placed around the rod in an augured hole or around

ground conductor in a trench in either a dry form or premixed in slurry. Some of these enhancement materials are permanent and will not leach any chemicals into the ground.’’

What is IEEE 80 2000 14.5 d material

The ingredients comprise of the following compounds in a specific grain size and purity.

• Ionic compounds ensures free ions

• Dispersion compounds stop the reversal of ions to salt

• Hygroscopic compounds can draw water from the surrounding soil

• Super absorption compound to store and maintain ground humidity

• Expansion compound for internal compaction

• Diffusion compound for growing conductive roots in the microscopic pores of surrounding soil.

Application

An example of application is shown below to lower soil resistivity and above all achieve permanency.

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How to achieve very low resistance for reference Earth or Electronic earth in High soil resistive area?

IEEE 142 1991 Chapter 4 beautifully explains the extent of resistance experienced by the charge as the distance from the conductor increases. These numbers are vital to create sigma Earth where the reflection of the Energy towards the conductor is minimized. This involves some detailed calculation using onion peal method of

applying artificial treatment compound to enable free flow of displacement current (so called). The dissipated energy is thus quickly diluted, immediately reducing the tension around the vicinity of the pit. Discussing the detailed construction and mathematics of sigma Earth is outside the preview of this paper.

Application‐

The drawing shown above is only representative. The Sigma Earth needs detailed calculation looking into Soil resistivity and space

available. In case of the above case, the Sigma Earth is giving 0.2 ohms in a soil resistivity of 630Ωm.

Does Efficiency of earthing depend on Frequency?

The Resistivity and Permittivity of soil depends on frequency. Watch the graph carefully

In Order to take care of Low frequency and High frequency signals in the same earth pit, one needs to design it very carefully. High frequency signals like lightning do not enter deep earth. It needs aid of capacitance of the pit. As the current moves into the Dielectric soil, the

Displacement current is kept high by applying Plates or mesh at the upper end of the earth pit. Stray rods can be used along the plate to increase the dissipation area. Detailed calculation and application techniques are available for specific solutions.

Application‐ IAF Critical Command Center (Undisclosed location)

This pit Earths VHF, UHF signal along with –ve terminal of DC . The 22mt deep bore single earth gives 0.17 Ω continuously for last 3 years in a harsh soil resistivity of 830Ωm rocky soil.

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How to Tackle Ground Induced Currents and EMP?

Earthing is the most effective way of handling surges produced during a high altitude nuclear blast of Solar flare. Three Types of EMP are produced:

• E1 Pulse – very fast component of nuclear EMP. It is too fast for ordinary lightning protectors and destroys computers and communications equipment.

• E2 Pulse – many similarities to pulses produced by lightning. Least dangerous type of EMP because of the widespread use of lightning protection.

• E3 Pulse – much slower pulse caused by the Earth’s magnetic field being pushed out of the way by the nuclear explosion or solar storm followed by the field being restored to its natural place. This process can produce geo‐magnetically induced currents in long electrical conductors (like power lines) which can damage or destroy power line transformers.

The most effective way of handling these surges in the long transmission lines is to create islands and ground the surge effectively. Earth

command and control center, static command control center and Ground switch with auto re‐closure can protect the power system.

USA has over 2000 large transformers, which are supposed to be effected drastically taking out the whole power grid in case of High altitude nuclear detonation or Solar flare exceeding 5000Nt. The world’s capacity to manufacture such transformers is only 142 per year.

• “In a report regarding threat to United States, the EMP Commission’s assessed that functional collapse of the electrical power system region within the primary area of assault is virtually certain”

• “Should the electrical power system be lost for any substantial period of time … the consequences are likely to be catastrophic … machines will stop; transportation and communication will be severely restricted; heating, cooling and lighting will cease; food and water supplies will be interrupted; and many people may die”

The cheapest, viable and most effective solution lies in earthing techniques.

Bibilica

IEEE 80‐2000 IEEE guide for safety in AC Substation Grounding

IEEE 142‐1991 IEEE recommended Practices for grounding of industrial and commercial power stations

IEEE 665‐1995 IEEE guide for generating station Grounding

IEEE 81‐1983 IEEE guide for measuring earth resistivity, Ground Impedance, and Earth surface Potentials of a ground system.

Report of the commission to assess the threat to United States from Electromagnetic Pulse (EMP) Attack

Ground for grounding a circuit‐to‐system handbook by Elay B. Joffe and Kai‐Sang Lock IEEE Press

AN‐345 (Application Note) Grounding for Low and High frequency circuits by Paul Brokaw and Jeff Barrow