GROUNDING SYSTEMS

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• GROUNDING SYSTEMS
• Part 1

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• The objective of a grounding system are
• 1. To provide safety to personnel during normal
  and fault conditions by limiting step and touch
  potential.
• 2. To assure correct operation of
  electrical/electronic devices.
• 3. To prevent damage to electrical/electronic
  apparatus.
• 4. To dissipate lightning strokes.
• 5. To stabilize voltage during transient
  conditions and to minimize the probability of
  flashover during transients.
• 6. To divert stray RF energy from sensitive
  audio, video, control, and computer equipment.

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• A safe grounding design has two objectives
• To provide means to carry electric currents into
  the earth under normal and fault
  conditions without exceeding any operating and
  equipment limits or adversely affecting
  continuity of service.
• 2. To assure that a person in the vicinity of
  grounded facilities is not exposed to the
  danger of critical electric shock.

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• The PRIMARY goal of the grounding
• system throughout any facilities is
• SAFETY.
• Why ground at all?
• PERSONNEL SAFETY FIRST
• EQUIPMENT PROTECTION SECOND

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What are the three main typesof grounding?

• The three main types are
• EQUIPMENT GROUNDING (SAFETY)
• SYSTEM GROUNDING
• LIGHTNING/SURGE GROUNDING

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• Soil Characteristics
• Soil type. Soil resistivity varies widely
  depending on soil type, from as low as 1 Ohmmeter
  for moist loamy topsoil to almost 10,000
  Ohm-meters for surface limestone.
• Moisture content is one of the controlling
  factors in earth resistance because electrical
  conduction in soil is essentially electrolytic.

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DEFINITIONS

• EATRH

• Earth electrode

• Earth Electrode Resistance

• Earth fault current

• Earthing grid

• Earthing conductor

• Earthing system

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Cable(Earthing conductor)
Clamp
Test link
Rod(Earthing electrode)
Rod coupler
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Classification of low voltage systems

• TN system

• TT system

• IT system

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TN systems
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Factors involved in effective earthing

• Soil resistivity

• Effect of shape on electrode resistance

• Plate

• Rod

• Horizontal strip or round conductor electrodes

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Recommended values of earth resistance
Recommended earth resistance(ohm) system
0.5-1 Light current
5 Low voltage
2.5 Medium voltage
0.5 High voltage
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Substation earthing system

• Step Touch voltage

• Grounding grids

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• Step and touch voltages

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• Step potential
• Step potential is the voltage
  between the feet of a person standing near an
  energized grounded object.
• It is equal to the difference in voltage,
  given by the voltage distribution curve, between
  two points at different distances from the
  electrode.
• A person could be at risk of injury during a
  fault simply by standing near the grounding point.

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• Touch potential
• Touch potential is the voltage between the
  energized object and the feet of a person in
  contact with the object.
• It is equal to the difference in voltage
  between the energized object and a point some
  distance away.
• The touch potential could be nearly the full
  voltage across the grounded object if that object
  is grounded at a point remote from the place
  where the person is in contact with it.

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• Driven rods

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• Resistance of driven rods
• The Ground Resistance (R) of a single rod, of
  diameter (d) and driven length (i) driven
  vertically into the soil of resistivity (?), can
  be calculated as follows
• where     ?        Soil Resistivity in m
•      l       Buried Length of
  the electrode in m
•       d          Diameter of the
  electrode in m
• The rod is assumed as carrying current uniformly
  along its rod.
• Examples
• (a) 20mm rod of 3m length and Soil resistivity 50
  O-m .....R16.1 O
• (b) 25mm rod of 2m length and Soil resistivity 30
  O-m .....R13.0 O

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Earth resistance shells surrounding a vertical
earth electrode
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• The resistance of a single rod is not
  sufficiently low.
• A number of rods are connected in parallel.
• They should be driven far apart as possible to
  minimize the overlap among their areas of
  influence.
• It is necessary to determine the net reduction in
  the total resistance by connecting rods in
  parallel.
• The rod is replaced by a hemispherical electrode
  having the same resistance.

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• Rod Electrodes in Parallel
• If the desired ground resistance cannot be
  achieved with one ground electrode, the overall
  resistance can be reduced by connecting a number
  of electrodes in parallel.
• These are called arrays of rod electrodes.
• The combined resistance is a function of the
  number and configuration of electrodes, the
  separation between them, their dimensions and
  soil resistivity.
• Rods in parallel should be spaced at least twice
  their length to utilize the full benefit of the
  additional rods.

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• If the separation of the electrodes is much
  larger than their lengths and only a few
  electrodes are in parallel, then the resultant
  ground resistance can be calculated using the
  ordinary equation for resistances in parallel.
• In practice, the effective ground resistance will
  usually be higher than this.
• Typically, a 4 spike array may provide an
  improvement of about 2.5 to 3 times.
• An 8 spike array will typically give an
  improvement of may be 5 to 6 times.

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• The multiple driven rod electrode
• The driven rod is an economical and simple means
  of making an earth connection but its resistance
  is not sufficiently low.
• A number of rods are connected in parallel.
• They should be driven far apart as possible to
  minimize the overlap among their areas of
  influence.
• It is necessary to determine the net reduction in
  the total resistance by connecting rods in
  parallel.
• The rod is replaced by a hemispherical electrode
  having the same resistance.

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• The method consists of assuming that each
  equivalent hemisphere carries the same charge.
• Calculate the average potential of the group of
  rods.
• From this and the total charge the capacity and
  the resistance can be calculated.

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Two ground electrodes
Equivalent hemisphere
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Earth clamping 1
AT-090H
AT-090H
Earth clamping 2
AT-093J
AT-089J
AT-087J
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Conductors
Bare copper tape
Tinned copper tape
PVC covered copper tape
Aluminium tape
PVC covered aluminium tape
Stranded copper cable
PVC covered stranded copper cable
PVC covered round cable
Round cable
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AT-010H
AT-011K
AT-012K
Bonding bars
AT-020H
AT-051F
AT-054J
AT-050F
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• METHODS OF DECREASING GROUND RESISTANCE
• Decreasing the ground resistance of a grounding
  system in high resistivity soil is often a
  formidable task.
• Recently, some new methods have been proposed to
  decrease ground resistance.

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• 1-Chemical Rods
• Chemical rods are electrodes with holes along
  their length, filled with mineral salts.
• The specially formulated mineral salts are evenly
  distributed along the entire length of the
  electrode.
• The rod absorbs moisture from both air and soil.
• Continuous conditioning of a large area insures
  an ultra-low-resistance ground which is more
  effective than a conventional electrode.

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• If the conductive salts are running low, the rod
  can be recharged with a refill kit.
• These rods are available in vertical and
  horizontal configurations.
• They may be used in rocky soils, freezing
  climates, dry deserts, or tropical rain forests.
• They provide stable protection for many years.

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• Disadvantages are
• Chemicals concentrated around electrodes will
  cause corrosion
• Chemicals leach through the soil and dissipate
• Scheduled replacement may be required
• May be prohibited because they may contaminate
  the water table

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• 2- Grounding Augmentation Fill (GAF)
• About 95 of the grounding resistance of a given
  electrode is determined by the character of the
  soil within a hemisphere whose radius is 1.1
  times the length of the rod.
• It is obvious that replacing all or part of that
  soil with a highly conductive backfill will
  facilitate the achievement of a low-resistance
  ground connection.
• The greater the percentage of soil replaced, the
  lower the ultimate grounding resistance.

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The critical soil cylinder within an interfacing
hemisphere
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• The amount of the backfill material required is
  determined in most cases by the Interfacing
  Volume and Critical Cylinder principles.
• A ground electrode establishes a connection to
  earth by affecting only a certain volume of
  earth, called the Interfacing Volume (IV).
• For practical purposes for a ground rod the
  entire connection to earth is contained within an
  IV whose radius is 2.5 times the length of the
  rod.

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• Most of the earth connection takes place in a
  cylinder close to the electrode, called the
  Critical Cylinder.
• A study of the influence of soil within the IV
  demonstrates that six inches of soil along any
  radial makes up 52 per cent of the connection to
  earth a 12 inches makes up 68 percent of the
  connection.

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• Beyond a diameter of 24 inches there is very
  little improvement for much larger diameters.
• Therefore, the recommended diameter for the
  Critical Cylinder is between 12 and 24 inches,
  and the calculated amount of the required
  backfill material is based on that diameter and
  the length of the ground rod.

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• 3- Cracks with Low Resistivity Materials (LRM)
• This method requires 3 steps
• Drilling deep holes in the ground, developing
  cracks in the soil by means of explosions in the
  holes, filling the holes with low resistivity
  materials (LRM) under pressure.
• Most of the cracks around the vertical conductors
  will be filled with LRM, and a complex network of
  low resistivity tree like cracks linked to the
  substation grid is formed.

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• Field tests show that the optimum span between
  vertical conductors is in the range of 1.5-2
  times the length of the vertical conductor.
• This method is effective in reducing ground
  resistances in rocky areas.

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• Soil Treatment Alternatives
• Ground enhancement material
• Cement-like compound
• Non-corrosive
• Extremely conductive
• Installed around the electrode
• Easy installation
• Permanent

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• Conductive Cement
• Concrete has a resistivity range of 30 to 90
  Ohm-meters.
• Since it is hygroscopic by nature it will tend to
  absorb moisture when available and keep it up to
  30 days, thus maintaining a resistivity lower
  than the surrounding soil.
• However, during a long dry season concrete will
  dry out with a subsequent rise in resistivity.
• Also, if a substantial amount of fault or
  lightning current is injected into a concrete
  encased electrode, the moisture in the concrete
  may become steam, dramatically increasing in
  volume and placing a substantial stress on the
  concrete.

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• Installing an EARTHLINK 101 earthling strip is
  simple

Dig a trench and lay in the wire.
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Pour EARTHLINK 101 conductive cement, using the
handy applicator bag, and shovel in a thin
protective layer of soil.
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Backfill the remaining soil using a front-end
loader and restore the surface to grade.