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Chapter 5: Earthing
and Bonding
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The TT system has two, separate ground rods.
The neutral is connected to its ground rod at the
service entrance.
The protective conductor is connected to its own
ground rod, remote from the neutral ground rod.
In some cases, the ground rod may be the steel
frame of the building. In any case, there is no
direct copper connection between the enclosure
and the supply system.
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The IT system also has two, separate ground rods.
The neutral is connected through an impedance to its ground rod at
the service entrance.
The protective conductor is connected to its own ground rod, remote
from the neutral ground rod. In some cases, the ground rod may be
the steel frame of the building. In any case, there is no direct copper
connection between the enclosure and the supply system.
One characteristic of the IT system is that the system is tolerant of a
fault to ground. That is, a fault to ground does not operate the circuit
breaker, so the system remains operational. (An alarm identifies the
fault to ground, but the system continues to operate.)
As in the TT system, there is no direct copper connection between
the enclosure and the supply system.
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The TN-S system has a single ground rod.
At the service entrance, the neutral conductor is connected to the ground
rod.
The protective earth conductor is connected to the neutral at the service
entrance.
The “S” in the designation means that the protective earth conductor is a
separate system conductor.
Unlike the TT and IT systems, in the TN system the equipment and man are
grounded through different paths. If the current through the different paths is
different, then a potential difference will occur between the equipment and
the man, and current will pass through the man.
To minimize the potential difference due to the difference between the
equipment and the man, it is imperative to keep the equipment ground
circuit resistance as low as practicable.
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The TN-C system has a single ground rod.
The neutral conductor is connected to the ground rod
located at the service entrance.
The protective earth conductor is connected to the
neutral in the equipment.
There is no separate protective conductor.
The “C” in the designation means that the protective
earth conductor is combined with the neutral conductor.
The TN-C system is used for electric dryers, electric
ranges, and electric water heaters in the United States.
How Does Grounding Provide Protection
Against Electric Shock or a TN-S system?
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If the equipment resistance is 0.1 ohm as required by the
various standards, what is the voltage at accessible
grounded parts for various fault currents?
The analysis is for a 120-V, 15-A, 3% system voltage
drop circuit.
The current is an arbitrary 150 amperes (10 times the
circuit-breaker rating).
This current will clearly operate the circuit-breaker in a
relatively short time.
The accessible part voltage is 33 volts .
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If the equipment resistance is 0.1 ohm as required by the various
standards, what is the voltage at accessible grounded parts in the
event of a short-circuit?
Clearly, the voltage will not be less than 30 volts? How high is the
voltage?
The analysis is for a 120-V, 15-A, 3% system voltage drop circuit.
The current is limited only by the source resistances and the 0.1ohm resistance of the equipment. (This current will clearly operate
the circuit breaker in a relatively short time.)
Using similar calculations as for the 150-ampere fault current, the
accessible part voltage is 77.7 volts volts .
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For a TN-S system, grounding does not provide an
equipotential environment due to the finite resistances of
the equipment grounding circuit.
However, equipment grounding through its protective
conductor does serve to limit the voltage for low fault
currents.
For higher fault currents, another scheme provides
protection against electric shock: limited duration of the
current through the body by means of automatic
disconnection of the supply (operation of the circuitbreaker).
Grounding design
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A grounding system should be installed in a manner that
will limit the effect of ground potential gradients to such
voltage and current level that will not endanger the
safety of people or equipment under normal and fault
conditions, as well as assure continuity of service
Substation usually have ground grid system with ground
mat as extra protection.
Grid system has the form of horizontally buried grid
conductor, supplemented by a number of vertical ground
rods connected to the grid.
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Some of the reasons for using the combined system of vertical rods
and horizontal conductors are as follow:
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Single rod by itself is inadequate in providing a safe grounding system.
When several electrodes are connected together and to all equipment
neutrals, frames and structures that are to be grounded an excellent
grounding system is developed
 Grid is installed in a shallow depth usually 0.3-0.5 m below grade,
sufficient long ground rod will stabilize the performance of the combined
system. Resistivity of upper layer soil vary with seasons, while the
resistivity oflower soil layers remains constant.
 Rods penetrating the lower resistivity soil are far more effective in
dissipating fault current whenever a two or multilayer soil is encountered
and the upper soil layer has higher resistivity compared to lower layer.
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Design in difficult condition
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In area soil resistivity is high or the substation space is at
premium, it is not possible to use grid system. Some solution
to design grounding in this area is:
1.
2.
3.
4.
Connection of remote ground grid and adjacent grounding
facilities
Use of deep driven ground rods and drilled ground wells in
combination with a chemical treatment of soil or use of bentonite
clays for backfilling.
Use of counterpoise wire mat. Copper clad steel wires of AWG
No 6 size, arrange in 0.6mx0.6m grid pattern, installed 0.050.15m depth, then the main grounding grid 0.3m-0.5m.
A nearly low resistivity material can be used as extra grid.
Example: clay deposit, part of large structure such as concrete
mass of hydroelectric dam
Earth electrode & earth conductor
Earth chamber
Earth clip
Earth clip bar and test point
Selection of Conductors and Joints
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Basic requirement
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1.
2.
3.
Each element in grounding system including grid conductors,
joints, connecting leads and all primary grounding electrodes
should be design for the expected design life of installation,
the element will be:
Have sufficient conductivity
Resist fusing and mechanical deterioration under most
adverse combination of a fault current magnitude and duration
Be mechanically reliable and rugged to a high degree,
especially on location exposed to corrosion and physical
abuse.
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Choice of material related to corrosion problems
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Copper
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Aluminum
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High conductivity
Resist to underground corrosion – cathodic with respect to other metals
Disadvantage - Form a galvanic cell with buried steel structure, pipes, and
any of lead-based alloy that may present in cable shealth
Used less frequently
Corrode in certain soils – layer of corrode material is nonconductive for all
practical grounding purpose
Gradual corrosion due to alternating currents
Steel
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Eliminate most of the adverse effects of copper
Use application of galvanized or corrosion-resistant steel in combination of
cathodic protection to extend life
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Minimum size
 Selection
of minimum size of material used for
grounding depend on ambient temperature,
maximum allowable temperature and
magnitude of fault current.
 To determine the minimum size of conductor
cable can be determine from the following
formula
Amm
2
tc r  r .10 4
TCAP
I
  Tm  Ta 

ln 1  
  K 0  Ta 
where
I  rms current in A
A  conductor cross section in mm 2
Tm  maximum allowable temperatu re in  C
Ta  ambient te mperature in  C
Tr  reference temperatu re for amterial constants in  C
 0  thermal coefficien t of resistivit y at 0  C
 r  thermal coefficien t of resistivit y at reference temperatu re Tr
 r  the resistivit y of the ground conductor at reference temperatu re Tr in μ/cm 3
K0  1/ 0
tc  time of current flow in s
TCAP  thermal capacity factor from table 1 in J/cm 3 /  C
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Example: Calculate the minimum size of copper-clad steel core ground
conductor required to withstand 5kA rms short circuit current in 1 second at
ambient temperature 40 ۫C ?
Amm
2
tc r  r .10 4
TCAP
I
  Tm  Ta 

ln 1  
  K 0  Ta 
 5000
 30.45
57.16
1.5397
Selection of joints
 Connection
must able to withstand
mechanical stresses without any significant
deterioration due to corrosion metal fatigue
and electromagnetic forces for many years
 Another factor must be considered are
connectivity, thermal capacity, mechanical
strength and reliability
 Type of connection: exothermic welds, brazed
joints and pressure type
Soil characteristics
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Most soil behave as both as conductor or resistance and as
dielectric
Effect of current magnitude- soil resistivity effected by current
flowing from electrode to soil. Thermal characteristic of soil and
moisture content of soil will determine is a current of given
magnitude and duration will cause significant drying and thus
increase the effective soil resistivity.
Effect of moisture temperature and chemical content – resistivity of
soil rise abruptly whenever the moisture contents accounts for less
then 15% of soil weight. Amount of soil depend on the grain size,
compactness. Effect of soil resistivity negligible for temperatures
above temperatures the freezing point. At 0 ºC the water start to
freeze and the resistivity increase rapidly. The composition and
amount of soluble salts, acids or alkali may consider affecting soil
resistivity.
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Use of crushed-stone layer
 Gravel
or crushed rock covering usually about
0.08 – 0.15m in depth are very useful
retarding evaporation of moisture and thus
limiting the drying of topsoil layers during dry
weather periods.
 Crushed rock with high resistance value
reduce shock current
What is a good earthing
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2.
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4.
Low electrical resistance measured in
ohms
Good corrosion resistance
Ability to carry high currents repeatedly
Ability to perform above function for 30
years or more
Method of providing good earthing
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2.
3.
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5.
6.
Deep driven earth rods
Parallel driven earth rods
Buried conductor (wire or tape etc)
Buried earth plate or mats
Underground metal pipe system
Steel reinforcing rods and/ or wires for
concrete
Soil resistivity measurement
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Why measure?
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Data obtained from measurement used to make sub-surface
geophysical surveys as an aid identifying ore locations, depth to
bedrock and other geological phenomena
Resistivity has direct impact on the degree of corrosion in
underground pipelines. Decrease in resistivity relates to an
increase in corrosion activity – protective treatment will be used
Affects the design of grounding system – advisable to locate
area at lowest soil resistivity in order to achieve the most
economical grounding installation
There are two method to measure soil resistivity: 4-point
method and 2-point method.
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4-point measurement
 Also known as Wenner method
 Requires the insertion four equally spaced and in-line electrodes
into the test area.
 Known current from constant current generator is passed
between the outer electrodes, then potential drop (a function of
resistance) is measured across the two inner electrodes
4AR

1
2A

2A
A2  4 B 2
4 A2  4 B 2
where : A  distance between th e electrodes in centimeter s
B  electrode depth in centimeter s
if A  20B, the formula becomes :
  2AR (with A in cm)
Solution
4AR

1
2A

2A
A  4B
4 A2  4 B 2
A  20B, the formula becomes :
  2AR (with A in cm), if the meter reading is 15, R  15
 2 (450cm)(15)
 42390cm
2
2
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For a single ground electrode, Professor H. R. Dwight of the
Massachusetts Institute of Technology develop a formula to estimate
earth rod resistance to earth:
   4L  
R
ln    1

2L   r  
where,
R  resistance in ohms of the ground rod to the earth(or soil)
L  grounding electrode length
r  grounding electrode radius
  average resistivit y in  - cm (can be obtained by 4 point method)
Field measurement of constructed
grounding system
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Two point method (Ammeter-Voltage
Method)
 This
method measure the total resistance of
the unknown and an auxiliary ground.
 Auxiliary ground resistance assume to be
negligible in comparison with resistance of the
unknown ground
 This method subject to large errors
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Three point method
 Involve
use of two test electrodes with their
resistances designated as r2 and r3 and with the
electrode to be measured designated as r1.
 The resistance between each pair of electrodes is
measured and designated as r12,r13 and r23 where r12
= r1 + r2. Solving the simultaneous equations,
(r12 )  (r23 )  (r13 )
r1 
2
 Therefore,
by measuring the resistance of each pair
of ground electrodes in series and substituting these
values in equation above, the value of r1 can be
determined.
 If the two test electrodes have substantially higher
resistance than the electrode under test, the errors on
the individual measurements will greatly magnified in
final results.
 Spacing between the three electrodes should be
more than 10 m
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Ratio method
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This method compares the resistance of the electrode under test
to that of a known resistance, generally the same electrode
configuration as in the fall-of-potential.
Fall-of-potential method
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Consist of passing a current through the station ground via a
ground electrode C remote from the station, and measuring the
voltage between the station ground and the remote from the
station at P
Term “remote” – very large electrode spacing where earth
current density approaches zero.
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References
 IEEE
Guide for Safety in AC Substation
Grounding (ANSI/IEEE Std 80-1986) –
Institute of Electrical and Electronics
Engineers.
 J. Philip Simmons, Electrical Grounding and
Bonding, Thomson Delmar Learning, 2005