Reliance Energy Center, Santacruz, Mumbai – 400 055, Tel – (022) 3009 9999 1 of 27 EARTHING OF ELECTRICAL SYSTEM Introduction Definition of Earthing and Grounding: • Grounding implies connection of power system neutral to ground (earth). e.g. neutral grounding / system grounding. In grounding current carrying parts are connected to ground. • Earthing implies the connection of non current carrying parts to ground e.g metallic enclosures. Another term for earthing is equipment grounding. Earthing is done for human and equipment safety. Human Element • Electric 'shock' is possible only when the human body bridges two objects of unequal potential. Current flows when potential difference exists between hand and feet (touch potential), or between feet (step potential). TRANSFORMER GENERATOR NG NG EARTHING
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Reliance Energy Center, Santacruz, Mumbai – 400 055, Tel – (022) 3009 9999
1 of 27
EARTHING OF ELECTRICAL SYSTEM
Introduction Definition of Earthing and Grounding:
• Grounding implies connection of power system neutral to ground (earth). e.g.
neutral grounding / system grounding. In grounding current carrying parts are
connected to ground.
• Earthing implies the connection of non current carrying parts to ground e.g
metallic enclosures. Another term for earthing is equipment grounding.
Earthing is done for human and equipment safety.
Human Element
• Electric 'shock' is possible only when the human body bridges two objects of
unequal potential. Current flows when potential difference exists between
hand and feet (touch potential), or between feet (step potential).
TRANSFORMER
GENERATOR
NG NG
EARTHING
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• Maximum tolerable current for a human body is approximately 160 mA for
one second duration (i.e. if current through body exceeds about 160 mA, for
more than a second, almost certain death, due to ventricular fibrillation or
heart attack).
• Allowable body current ΙB (Ampere) as per IEEE Standard 80 is as given
below:
ΙB = 0.116 / √TS, for a body weight of 50 Kg
ΙB = 0.157 / √TS, for a body weight of 70 Kg
where TS is the duration of current exposure (fault clearance time).
• For various exposure times, the withstand currents of human body are as
follows:
TS ΙB (50 Kg) ΙB (70 Kg)
0.2 sec
259 mA 351 mA
0.5 sec 164 mA 222 mA
1 sec 116 mA 157 mA
Thus human body can withstand higher current for shorter time duration. The
advantage high-speed protection is evident from human safety point of view.
• Average value of human body resistance RB is approximately 8000 Ω to 9000
Ω (under dry conditions). For standards purposes, RB is taken as 1000Ω as
per IEEE Std 80. Use of lower RB value results in conservative values for
allowable touch and step potentials.
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Soil Resistivity
Resistivity ( ρ) of element:
• R = ρ L / A ⇒ ρ = R A / L ⇒ Ω M2 / M = Ω M ; where ρ - Resistivity of
element, R – Resistance of element, L – Length, A – Area,
• Earth is a not a good conductor. In fact it is one of the worst equipment-
grounding conductors. The comparison of resistivity is given below:
Material Resistivity
Earth 100 ΩM
GI 10-7 ΩM
Copper 1.7 x 10-8 ΩM
• Soil resistivity ( ρ in Ω M ) is value of resistance in Ω of 1M cube. Soil
resistivity for different types of soil is given below:
Type of Earth Wet Soil Moist Soil Dry Soil Bed Rock
Resistivity (Ω - M ) 10 100 1,000 10,000
Effect of moisture on soil resistivity:
• Soil resistivity ρ rapidly increases for moisture content of less than 10 % of
soil weight. There is marginal decrease in soil resistivity for moisture content
exceeding 25% of soil weight.
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Effect of salt on soil resistivity:
• Minute amount of salt causes sharp decrease in soil resistivity provided there
is moisture content of say 10%. Salt when added to dry soil gives hardly any
improvement in the resistivity value.
• Substances used for improving soil resistivity are sodium chloride (common
salt), copper sulphate, calcium chloride and magnesium sulphate. To account
for corrosion, electrode size is increased from calculated value.
Corrosion intensity:
• Soil classification based on corrosion intensity is given below:
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ρ of virgin soil Ω M <25 25 to 50 50 to 100 ρ > 100
Corrosion Intensity Severe Moderate Mild Very Mild
• Alternative substances used for earthing are coke or wood charcoal and
Bentonite. Coke is less corrosive compared to salt. Bentonite is a natural clay
and non-corrosive and needs watering periodically.
• Performance over time for earth resistivity is shown below:
After treatment there is initial decrease in ρ. However there is gradual
increase in soil resistivity ρ with time as the salt is washed away by continual
water seepage. Hence, re-treatment is typically required to be carried out
once in 3 years.
Effect of Temperature on Soil Resistivity:
• Soil resistivity decreases with increase in temperature. In summer ρ is less
and in winter it is more. However, effect of temperature on ρ is not serious
until freezing point is approached. Near 0°C, ρ abruptly rises to a very high
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value. It is preferable to place electrodes well below ground level. Surface
temperature may be freezing but below 1M, soil temperature will be higher.
Soil resistivity measurement:
• Soil resistivity measurement setup is as shown below. AC supply source is
preferred for the set up as compared to the DC supply source.
• Resistivity measured for spacing ‘A’ represents apparent soil resistivity to
depth of 'A'. Measurements are made with different spacings. Rapidly
increasing value of ρ with spacing 'A' indicates underlying stratum is rock and
it is difficult to install earth electrodes to great depths.
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Resistance to earth
• Resistance between metal of electrode in question and general mass of earth
is known as earth resistance. It is resistance between specific electrode and
imaginary electrode of zero resistance placed at infinity. 90% of resistance is
contributed by earth within 5 meters distance.
Resistance vs Distance:
• Resistance to earth of hemi-spherical electrode is shown below:
dX
A
X
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R = ρ L / A; RX = ∫ ρ dX / 2 Π X2 Resistance area of tube or rod electrode:
• Resistance area is the region of earth that immediately surrounds the
electrode and contributes to practical value of resistance.
• Current flows away from electrode in all directions and through a series of
shells of earth of continuously increasing cross section. At sufficient distance
from earth electrode, shells approach hemi-spherical shape. Hence results
given earlier for hemi-spherical electrode are also nearly valid here.
Resistance of electrode to earth here also is predominantly influenced by
earth with in the vicinity of electrode. This justifies artificial treatment of soil in
the immediate neighborhood of soil to achieve low resistance between
electrode and earth.
Resistance of Driven Rod or Pipe Electrode: L : depth of Driven Rod in met
Φ : diameter of Driven Rod in met
R = (ρ / 2 Π L ) [ LN (8L /Φ x 2.7183 )] R ≅ (ρ / 2 Π L ) [ LN (4L / Φ )]
L
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Length / diameter of rod vs Resistance is given below:
The diameter of the rod has minor influence on the resistance. Length of the rod has
major impact. For lengths more than 3M, resistance is almost the same even if
diameter is increased by four times.
Resistance of Horizontal Wire (Strip) Electrode: If excavation is difficult beyond a meter due to underlying rock, strip electrode is the
alternative. The earth electrode is as shown below:
RESISTANCE OF ROD ELECTRODE
0
50
100
150
200
250
300
0 1 2 3 4 5 6 7 8 9 10LENGTH OF ROD, MET
RES
ISTA
NC
E, O
HM
S
2.5 CM
10 CM
DIAMETER : 2.5, 5, 7.5, 10 CM
RHO - 100 OHM-MET
ROD LENGTH: 6M
R10 = 15.3 ΩR2.5 = 16.4 Ω φ ⇑ 300%
R⇓ 7%
RESISTANCE OF ROD ELECTRODE
0
50
100
150
200
250
300
0 1 2 3 4 5 6 7 8 9 10LENGTH OF ROD, MET
RES
ISTA
NC
E, O
HM
S
2.5 CM
10 CM
DIAMETER : 2.5, 5, 7.5, 10 CM
RHO - 100 OHM-MET
ROD LENGTH: 6M
R10 = 15.3 ΩR2.5 = 16.4 Ω φ ⇑ 300%
R⇓ 7%
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