Tes p-122.03-r0

PAGE NO. 2 OF 34TEP122.03R0/MAA
TRANSMISSION ENGINEERING STANDARD TES-P-122.03, Rev. 0
Date of Approval: October 17, 2006
TABLE OF CONTENTS
1.0 SCOPE
2.0 TYPES AND LOADINGS
2.1 Types of Conductor and Overhead Ground wire
2.2 Conductor and Overhead Ground Wire Loadings
3.0 DESIGN SPANS
3.1 Basic Design Span
3.2 Wind Span or Horizontal Span
3.3 Weight Span or Vertical Span
3.4 Average Span and Efficiency Factor
3.5 Ruling Span or Equivalent Span
4.0 CONDUCTOR SELECTION
4.1 Main Considerations
4.2 Thermal Ampacity Consideration
4.3 Bundled Conductors
4.4 Corona and Conductor Surface Gradient
5.0 CORONA PERFORMANCE
5.1 Factors Affecting Corona
5.2 Disruptive Critical Voltage
5.3 Corona Loss
5.4 Radio Interference
5.5 Television Interference
6.0 OVERHEAD GROUND WIRE SELECTION
7.0 SAG AND TENSION CALCULATIONS
7.1 Computer Programs
7.2 Sag and Tension Tables
7.3 Stringing Sag Tables

8.0 CONDUCTOR AND GROUND WIRE SAGGING
8.1 General
8.2 Temperature Check
8.3 Sagging Methods
9.0 BIBLIOGRAPHY
FIGURE TE-2203-0100-00 Optimum or Basic Span
FIGURE TE-2203-0200-00 Wind and Weight Span
FIGURE TE-2203-0300-00 Ruling Span
APPENDIX-1 Typical Sag and Tension Table
APPENDIX-2 Typical Stringing Sag Table

1.0 SCOPE
This standard defines the important considerations to be taken into account while
selecting and evaluating the electrical and mechanical properties of conductors and
overhead ground wires to be installed on the overhead transmission line system of
Saudi Electricity Company, (SEC), Saudi Arabia.
2. TYPES AND LOADINGS
2.1 Types of Conductor and Overhead Ground Wire
There are several types of aluminum conductors but the types listed hereunder
are widely used in SEC transmission system because of their mechanical
strength, wide spread manufacturing capacity and cost effectiveness.
ACSR Aluminum Conductor Steel Reinforced
ACSR/AW Aluminum Conductor, Aluminum-Clad Steel Reinforced
ACAR Aluminum Conductor Alloy Reinforced
AAAC All Aluminum Alloy Conductor
AACSR/AW Aluminum Alloy, Aluminum-Clad Steel Reinforced
Following types and sizes of conductor and overhead ground wire have been
standardized and are extensively used on the overhead transmission line system
of SEC:
2.1.1 Conductors
ACSR/AW, 4/0 AWG, 6/1 Stranding, 211.6 kcmil (107.2 mm²), code
word “PENGUIN”
ACSR/AW, 30/7 Stranding, 336.4 kcmil (170.5 mm²), code word
“ORIOLE”
ACSR/AW, 26/7 Stranding, 795 kcmil (402.8 mm²), code word
“DRAKE”
ACSR, 30/7 Stranding, 477 kcmil (241.7 mm²), code word “HAWK”
ACSR, 26/7 Stranding, 636 kcmil (322.3 mm²), code word
“GROSBEAK”
ACSR, 54/7 Stranding, 795 kcmil (402.8 mm²), code word
“CONDOR”

ACAR, 18/19 Stranding, 1080 kcmil (547.4 mm²)
AAAC, 61 Stranding, 1000 kcmil (500 mm²)
ACSR and ACSR/AW conductor shall mainly be used for
transmission line voltages up to 230 kV level in the Inland areas and
Coastal areas respectively whereas ACAR/AAAC conductors shall be
used on 380 kV transmission lines. ACSR “Condor” and ACSR/AW
“Drake” may also be used on 380kV lines if determined to be
economical and provided the structures are designed accordingly.
2.1.2 Overhead Ground Wire
7 Strand, 9.525 mm (3/8 inch) diameter, 48kN strength, Galvanized
Steel
7 No. 8 AWG, 9.78 mm diameter, 70kN strength, Aluminum Clad
Steel
Steel
Steel
7 No. 5 AWG, 13.86 mm diameter, ACSR/AW or AACSR/AW type,
70kN tensile strength and 13 kA fault current rating for 0.3 second at
50ºC initial temperature
19 No. 8 AWG, 16.31 mm diameter, ACSR/AW or AACSR/AW
type, 120kN tensile strength and 24 kA fault current rating for 0.3
second at 50ºC initial temperature
For corrosion resistance requirements, ground wire shall be of
Aluminum Clad Steel or ACSR/AW or AACSR/AW unless otherwise
specifically required in the project SOW/TS.
2.1.3 Composite Optical Fiber Ground Wire
Optical Fiber Ground Wire (OPGW), 13 kA fault current rating for
0.3 second at 50ºC initial temperature
Optical Fiber Ground Wire (OPGW), 24 kA fault current rating for
0.3 second at 50ºC initial temperature
2.2 Conductor and Overhead Ground Wire Loadings
2.2.1 Loadings shall include the wind and temperature conditions for
which:

a. Clearance to structure or ground shall be ascertained,
b. The supporting structures shall be designed,
c. Vibration of the conductors are studied,
d. Maximum stress levels in the conductors are investigated.
2.2.2 Conductor Limiting Conditions for AAAC/ACAR Type Conductors
For new as well as expansion/modification of existing transmission
lines, the maximum tension of the line conductor shall not exceed the
limits specified for the following conditions:
a. Final tension with no wind at an every day temperature shall not
exceed 17% of ultimate strength.
b. Final tension with 1064 N/m² wind on bare conductor at an
every day temperature shall not exceed 50% of ultimate
strength.
c. Initial tension with 430 N/m² wind on bare conductor at a
temperature of minus one (-1)ºC shall not exceed 30% of
ultimate strength.
For the purpose of calculating final tensions and sag values, the effect
of conductor creep shall be taken at 20ºC.
2.2.3 Conductor Limiting Conditions for ACSR Type Conductors
For transmission lines to be designed with new family of towers using
ACSR or ACSR/AW type conductors, the maximum tension of the
line conductor shall not exceed the limits specified for the following
conditions:
strength.
ultimate strength.

2.2.4 Conductor Limiting Conditions (Existing Steel Latticed Type
Structures)
lines to be built with existing family of towers using ACSR or
ACSR/AW type conductors, the limiting conditions shall be as stated
in Clause 2.2.2 or 2.2.3 as the case may be depending upon the design
of structures.
2.2.5 Conductor Limiting Conditions (New/Existing Steel Latticed Type
Structures)
lines to be built with existing/new family of latticed type steel
structures using ACAR or AAAC type conductors, the limiting
conditions shall be as stated in Clause 2.2.2.
2.2.6 Conductor Limiting Conditions (Existing and New Wood/Steel Pole
Structures)
lines, the maximum tension of the line conductor shall not exceed the
limits as stated in Clause 2.2.2.
2.2.7 Overhead Ground Wire and Optical Fiber Ground Wire Limiting
Conditions
strength.
ultimate strength.
2.2.8 Every day temperature for various SEC Operating Areas to be
considered in the aforementioned paragraphs shall be as in Table 03-1
below:

Table 03-1: Every Day Temperature
SEC Operating Area
Every Day
Temperature, o
C
Central 25
Eastern 27
Western 30
Southern 25 & 30
2.2.9 The maximum design temperature of conductor producing maximum
sag for determining minimum vertical ground clearances shall be as
in Table 03-2 below:
Table 03-2: Maximum Design Temperature
SEC Operating Area
Maximum Design
Temperature, o
C
Central 80
Eastern
85 (ACAR)
93 (ACSR/AW)
Western 80
Southern
85 (AAAC)
93 (ACSR)
2.2.10 The design engineer responsible for preparing the base design or
detailed design shall verify and ensure that the design of existing
family of structures is suitable for the conductor and ground wire
limiting conditions being applied for a particular location. If required,
necessary modifications/reinforcements may be performed.
2.2.11 Sag and tension values (initial and final) for conductor, overhead
ground wire and optical fiber ground wire shall be calculated for the
following temperatures at no wind condition:
a. ACSR/AW Conductor: -1ºC, 10ºC, 16ºC, 20ºC, 25ºC, 27ºC,
30ºC, 35ºC, 40ºC, 50ºC, 60ºC, 70ºC, 75ºC, 80ºC, 85ºC, and
93ºC
b. ACSR Conductor: -1ºC, 10ºC, 16ºC, 20ºC, 25ºC, 27ºC 30ºC,
35ºC, 40ºC, 50ºC, 60ºC, 70ºC, 75ºC, 80ºC, 85ºC, and 93ºC
c. ACAR Conductor: -1ºC, 10ºC, 16ºC, 20ºC, 25ºC 27ºC, 30ºC,
35ºC, 40ºC, 50ºC, 60ºC, 70ºC, 75ºC, 80ºC, and 85ºC
d. AAAC Conductor: -1ºC, 10ºC, 16ºC, 20ºC, 25ºC, 27ºC 30ºC,
35ºC, 40ºC, 50ºC, 60ºC, 70ºC, 75ºC, 80ºC and 85ºC

e. Overhead Ground Wire and OPGW: -1ºC, 10ºC, 16ºC, 20ºC,
25ºC, 27ºC, 30ºC, 35ºC, 40ºC, 50ºC, and 60ºC
3.0 DESIGN SPANS
3.1 Basic Design Span
The basic span is the optimum span (most economical span), which gives the
minimum cost for a particular transmission line on a given line route and is the
horizontal distance between centers of adjacent structures on a level ground.
The optimum spacing of structures and their height is a financial exercise.
Detailed cost study is required in order to determine an optimum span for a
particular transmission line along a given route. With short spans and low
structures the total number of structures and associated fittings will be large to
cover a certain route but less steel will be required. On the other hand, if long
spans are used then the conductor sag between structure points becomes greater
and fewer stronger, higher structures and fittings, but with correspondingly
more steel, would be required to ensure correct clearances. The construction
cost associated with variable number of structures for a given route will also be
an important consideration.
Given the mechanical loading conditions, phase conductors and ground wire
types an evaluation of the basic span may be made as follows:
3.1.1 Assume an arbitrary length in a flat area and estimate the number of
angle/section structures based on past experience. Select a basic span
and work out the number of suspension structures by dividing the
length by basic span and deducting the number of angle structures.
3.1.2 Work out the costs for supply and installation for conductors and
ground wires.
3.1.3 Select and calculate the number of insulators and other fittings, the
selection of which will depend upon mechanical loading and
pollution levels.
3.1.4 Determine the structure (latticed type) weight by using P.J. Ryle
formula:
Approximate weight MtHK1 (Eq.03-1)
Approximate base width MtK2 (Eq.03-2)

Where H is the overall structure height and Mt is the ultimate
overturning moment (OTM) at the base of the structure. This shall be
the largest corresponding to the highest loading condition and taken
as sum of transverse and longitudinal moments due to conductor
tension, structure and conductor wind loads. For convenience the
OTM due to wind on the structure as proportion of all other loads
may be taken as 25% for suspension structures, 10% for small angle
structures and 8% for medium and heavy angle type structures. The
values of K1 and K2 shall be taken as 0.008 and 0.30 respectively.
With knowledge of suspension and tension structure weights, supply
and installation cost may be estimated.
3.1.5 Determine the foundation cost. This will depend on the soil properties
and compression and uplift loads which can be worked out from OTM.
3.1.6 Summation of the costs involved will give an indication of the total
cost. By varying the span length (with its influence on sag of
conductor and associated quantities), cost versus span may be
evaluated and plotted. From the curve the span giving the minimum
cost shall be selected as the basic span and is the optimum span for
the transmission line under consideration. A typical plot showing
transmission line cost versus span length is illustrated in Figure TE-
2203-0100-00. These curves are, in practice very flat at the bottom
and experience shows that a span selected slightly greater than the
minimum derived from such analysis gives an over all optimum
choice.
Unless otherwise determined from a detailed span optimization study
following basic spans shall be considered for structure spotting and
the design of transmission lines with new family of latticed steel
structures:
69kV through 230 kV Transmission Lines: 300 to 350 m
380 kV Transmission Lines: 400 m
Transmission lines to be designed with existing family of latticed
steel structures shall have the basic span same as was used for the
design of structures. Whereas for steel tubular structures the basic or
ruling span shall be as specified in the relevant TMSS.

3.2 Wind Span or Horizontal Span
The wind span is half the sum of the adjacent span lengths supported on any
one structure as shown in Figure TE-2203-0200-00. This is often termed as
horizontal span. The wind span shall normally be taken as slightly higher than
the basic span. For flat area this shall be taken as 1.05 times the basic span, and
for highly uneven ground/hilly or broken area it will be appropriate to consider
1.10 times the basic span, to allow some flexibility in structure spotting.
3.3 Weight Span or Vertical Span
The weight span is the distance between the lowest points on adjacent sag
curves on either side of the structure as shown in Figure TE-2203-0200-00. It
represents the equivalent length or weight of conductor supported at any one
structure at any time. For design purposes, it is the value under worst loading
conditions (minimum temperature in still air), which gives the greatest value.
In a level terrain, weight span shall be taken as 1.2 to 1.25 times the basic span
and in uneven terrain; it shall be up to 1.6 to 1.8 times the basic span. For hilly
area this shall be up to twice the basic span to make structure spotting more
flexible.
The ratio of weight span to wind span is important as the insulators on lightly
loaded structures may be deflected excessively thus impairing electrical
clearances. A ratio of weight span to wind span of approximately 0.7 is often
considered acceptable. The design engineer must be aware of maximum weight
span and such ratios.
3.4 Average Span and Efficiency Factor
The average span shall be determined when structures are spotted on the
profile drawings. The total length of the line divided by the number of total
structures will give the average span length.
The efficiency factor or span utilization factor is defined as the average span
divided by the basic span and shall always be less than one (1) due to physical
obstructions on the line route.
Efficiency of 90% and above can easily be achieved in a flat terrain.
3.5 Ruling Span or Equivalent Span
The basic design span is also called the design ruling span, as it is the optimum
span resulting in lowest line cost. The design of the transmission line and
average structure heights shall be based on this span length.

Ideally each span on the line route should be equal to the design ruling span
and consequently equal structure heights at all locations, otherwise line would
not remain economical. Practically this requirement cannot be met due to
uneven ground and varying nature of land features along the line route.
Therefore in actual practice shorter as well as longer span shall be encountered
and thus conductor tensions will be different in each span (higher in longer
spans and lower in shorter spans). This condition will make the insulator
strings at suspension structures to become out of plumb. Additionally there will
be considerable differential tensions on the structures, which are not desirable
and therefore should be avoided as far as possible.
The above situation can be corrected by using the concept of an equivalent
span, which is normally termed as ruling span. It is an imaginary or fictitious
single span length in which tension variations due to load or temperature
changes are nearly the same as in the actual spans in a section between dead
end structures. The mathematical treatment to obtain the equivalent span is
based on the parabolic theory and there is no similar concept using Catenary’s
equations.
The equivalent span shall be used for determination of sag in spans for which
the tension in any section will be equal to that, which would apply to a single
span equal to the equivalent span. The mathematical relationship for equivalent
span or ruling span is given below:
Ruling Span =
2
1
21
33
2
3
1
.........
.........
⎥
⎥
⎥
⎦
⎤
⎢
⎢
⎢
⎣
⎡
++
++
n
n
LLL
LLL
(Eq.03-3)
Where L1, L2, L3,………Ln are the actual span lengths between two dead end
structures as shown in Figure TE-2203-0300-00. Generally, as the number of
actual spans increase, the ruling span approaches the average span.
On a line where all spans are equal, the ruling span will be the same as the line
spans. Where spans vary in length, the ruling span will be between the shortest
and the longest span lengths on the line, but will be mainly determined by the
longer spans.
The purpose of the ruling span in design and construction is to provide a
uniform span length, which is representative of the various lengths of spans
between dead ends so that sags and clearances can be calculated for structure
spotting and conductor stringing.
The actual ruling span should be reasonably close to the design-ruling span,
which is used for spotting of the structures, otherwise there might be
significant differences between the predicted conductor tensions and
clearances, and the actual values.

4.0 CONDUCTOR SELECTION CONSIDERATIONS
4.1 Main Considerations
The selection of the most appropriate conductor size at a particular voltage
level shall take into account both the technical and economic criteria listed
below:
4.1.1 The maximum power transfer capability must be in accordance with
the system requirement.
4.1.2 The conductor cross-sectional area should be such as to minimize the
initial capital cost and capitalized cost of losses.
4.1.3 The conductor should conform to the standard sizes already used
elsewhere on the network in order to minimize spares holding and
introduce a level of standardization.
4.1.4 The conductor thermal capacity shall be adequate.
4.1.5 The conductor diameter or bundle size must meet recognized
international standards for radio interference and corona discharge.
4.1.6 The conductor must be suitable for the environmental conditions.
From material point of view the choice between ACSR and AAAC/ACAR is
not so obvious. However, at larger conductor sizes the AAAC/ACAR option
becomes more attractive because of significant strength/weight ratio and for
some constructions it gives smaller sags and/or lower tower heights. Moreover,
AAAC/ACAR is slightly easier to join than ACSR conductor.
Copper has a very high corrosion resistance and is able to withstand desert
conditions under sand blasting but is very expensive, not normally used on
high voltage transmission lines. Aluminum conductors have also good
corrosion behavior essentially resulting from the formation of an un-disturbed
protective surface oxide layer, which prevents further corrosion attack. ACSR
is known to suffer from bimetallic corrosion, which is noticeable as an increase
in conductor diameter due to corrosion products in the steel core called “bulge
corrosion”. For this reason high temperature grease has been used in the past to
solve the corrosion problems. This grease prevents the onset of any galvanic
corrosion between galvanized steel core and the outer aluminum wire.
Nowadays Alumo Weld steel core (AW) is being used in ACSR conductors for
installation in the aggressive environment. AAAC/ACAR will obviously have
superior corrosion resistance than un-greased ACSR conductors.

The conclusions of international research on corrosion show that:
Pure aluminum have the best corrosion resistance under majority of the
environmental conditions.
Smooth body conductors are the most corrosion resistant especially if inner
layers are greased.
Small diameter wires are most susceptible to corrosion damage and to
failures. Thus for a given conductor area it is preferable to have fewer
larger diameter strands.
The overall corrosion performance of aluminum alloy conductors depends
upon the type of alloy used.
For very aggressive environments the following order of preference has been
suggested:
Aluminum conductor fully greased
Aluminum conductor with alumo weld core fully greased
ACSR fully greased
Aluminum alloy conductor fully greased
Aluminum conductor with alumo-weld core ungreased.
ACSR with greased core.
4.2 Thermal Ampacity Consideration
In practice, power transfer capacity of a transmission line will be limited over
long distances by the conductor natural impedance (voltage regulation) as well
as conductor thermal capacity. Therefore this factor must also be considered
when sizing a phase conductor. The conductor should be able to carry the
maximum expected long-term load current without overheating.
The size and number of conductors shall be selected based on the required
power transfer. The conductor thermal current rating shall be calculated with
the help of heat balance equation given below or any suitable computer
program. Current ratings depend on many factors such as wind and sun
conditions and maximum temperature of the conductor and can easily be found
in manufacturer’s catalogues.
Heat generated (I²R losses) = Heat lost by convection (watts/km) + Heat lost
by radiation (watts/km) - Heat gained by solar radiation (watts/km)

src
2
HHHRI −+= (Eq.03-4)
( )[ ] ( ) ( )[ ] SdttsdEvdtRI sc αθπθθα −+−+++=++
44448.0
20
2
273273)(3871
Where,
I = current rating, amperes
R20 = resistance at 20ºC, Ohms/km
α = temperature coefficient of resistance per ºC
t = ambient temperature, ºC
θ = temperature rise, ºC
αs = solar absorption coefficient, depends upon outward condition of
conductor and varies between 0.6 for new bright and shiny to
0.9 for black condition or old conductor. Average value of 0.80
may be taken for design purposes.
S = intensity of solar radiation, watts/m²
D = conductor diameter, mm
V = wind velocity normal to the conductor, m/s (usually 0.6 m/s is
taken)
EC = emissivity of conductor, differs with conductor surface
brightness. Typical values are 0.3 for new conductor and 0.9 for
old conductor. Average value of 0.6 may be taken.
S = Stefan-Boltzman’s constant, 5.7x10-8 watts/m²
π = constant (22/7) = 3.1416
In practice, heat balance is highly complex but for calculation purposes the
above equation is adequate. To have an approximate current carrying
capability, the heat gained by solar radiation may be neglected. However, to
include this effect and for precise calculations, the method and procedure
described in IEEE Std. 738 may be followed.
Thermal rating of the transmission lines is dependent on the maximum
allowable temperature of the overhead line conductors which further is limited
by the maxmium allowable sag (determined from specified ground clearance
requirements) and the loss of conductor strength at higher operating
temperature.

For SEC transmission lines following operating temperatures have been
adopted in the design and System Planning Criteria for various types of
conductors.
Table 03-3: Conductor Operating Temperatures
Conductor
Normal
Operation, o
C
Emergency
Operation, o
C
ACSR “Condor” 80o
C 90o
C
AAAC(1000kcmil) 80o
C 90o
C
ACAR (1080kcmil) 85o
C 95o
C
ACSR “Hawk/Grosbeak” 93o
C 125o
C
ACSR/AW “Dake”
and other ACSR/AW
type conductors
93o
C 125o
C
Emergency operation shall be limited to 10 hours or less per year.
4.3 Bundled Conductors
Bundled conductors are economical for use on EHV transmission lines i.e., 230
kV and above. However, bundled conductors (2-bundle) may be used on high
voltage transmission lines below 230kV (i.e., 69kV, 110kV, 115kV and
132kV) primarily to increase the power transfer capability at the same voltage
level. The advantages of bundled conductors are:
4.3.1 Reduced Inductive Reactance
4.3.2 Reduced Voltage Gradient
4.3.3 Higher corona extinction voltage level with corresponding reduced
corona power loss
4.3.4 Higher power transfer capacity per unit mass of conductors
For SEC transmission system 2-bundled conductor per phase has been standardized
for use up to 230 kV level, whereas 4-bundled configuration is adopted for 380 kV
lines.
4.4 Corona and Conductor Surface Gradient
High voltage gradients surrounding conductors (above about 18 kV/cm) will
lead to a breakdown of air in the vicinity of conductor surface known as corona
discharge. The effect is more pronounced at higher altitudes. Generally the
breakdown strength of air is approximately 31 kV/cm (peak) or 22 kV/cm
(r.m.s). This is a useful guide for selection of a conductor diameter or
conductor bundle arrangement equivalent diameter.

Corona discharge and radio interference noise generated cause problems with
the reception of radio communication signals and also affect the performance
of power line signals. At higher voltages and certainly at 380 kV and above
interferences due to corona effect is the dominating factor in determining the
physical size of the conductor rather than conductor thermal rating. Increasing
the conductor diameter may be necessary in order to reduce the surface
gradient to acceptable limits. Obviously the size would be limited with regard
to practical size, strength and handling capacity. Therefore bundling of
conductors is used to obtain an effective increase in overall conductor
diameter.
The surface voltage gradient may be determined from Gauss’s theorem:
r
Q
Vg
02πε
= (Eq.03-5)
Where,
Vg = surface gradient, volts/m
Q = surface charge per unit length (coulomb/m)
r = conductor radius, cm
εo = permitivity of free space (1/(36. π.109
)
In practical form, this equation may be expressed as
,
)r/D(logr
U
V
e
p
g = kV/cm
Where,
Vg = surface gradient, kV/cm
UP = phase to ground voltage, kV
r = conductor diameter, cm
D = distance between phases for single phase line or equivalent
spacing for three phase line, cm
D=(D12D23D31)1/3

5.0 CORONA PERFORMANCE
Corona is a luminous partial discharge, which takes place at the surface of a
transmission line conductor when electrical stress, that is, the electric field intensity
(or the surface potential gradient), of a conductor exceeds the breakdown strength of
the surrounding air. Corona on transmission lines causes power loss, radio and
television interference, and audible noise in the vicinity of the line. At higher operating
voltages such as above 230 kV level, the corona factor becomes extremely important
for the design of transmission line.
Corona on transmission lines therefore should be avoided for reducing the interference
levels and energy losses associated with it.
5.1 Factors Affecting Corona
At a given voltage level, the factors affecting corona include line configuration,
conductor type, conductor surface condition and weather. In a horizontal
configuration, the field near the middle conductor is larger than the field near
the outer conductor. Therefore, the disruptive critical voltage is lower for
middle conductor, causing larger corona loss than the ones for the two outer
conductors. If the conductors are not spaced equally, the corona losses are not
equal.
The conductor height above ground also affects the corona loss that is, the
greater the height, the smaller the corona losses.
Corona loss is also proportional to the frequency of the voltage and thus
increases at higher frequencies.
The irregularity of the conductor surface in terms of scratches, raised strands,
die burrs, die grease, and particle of dust and dirt that clog the conductor can
significantly increase the corona loss. For smoother conductor surface the
disruptive voltage becomes higher. The size of conductors and their spacing
have considerable effect on corona loss. The larger the conductor diameter, the
less likelihood of corona loss. Therefore, the use of larger diameter conductor
or use of bundled conductors increases the effective diameter by reducing the
electric stress at the conductor surface.
5.2 Disruptive Critical Voltage
The breakdown strength of air varies with atmospheric conditions. It is directly
proportional to the density of the air. The air density factor is defined as:
t+
=
273
p9211.3
δ (Eq.03-6)
Where,
P = barometric pressure in cm of mercury and

t = ambient temperature in ºC
Foul weather conditions (e.g., rain, snow, hoarfrost, sleet and fog) all lower the
critical voltage and increase the corona. Rain affects corona loss more than any
other factor. Heavy winds have no effect on the critical voltage or on the
corona loss, but presence of smoke lowers the critical voltage and increases the
corona. Corona in fair weather may be negligible up to a voltage close to the
critical disruptive voltage for a particular conductor. Above this voltage, the
impacts of corona increase very quickly.
The transmission lines shall be designed to operate just below the disruptive
critical voltage in fair weather so that corona only takes place during adverse
atmospheric conditions. Therefore the calculated disruptive critical voltage will
be an indication of the corona performance of the line. However, a high value
of the disruptive critical voltage is not the only criterion of satisfactory corona
performance. The sensitivity of conductor to foul weather should also be
considered (e.g., corona increases more slowly on stranded conductors than on
smooth conductors). Due to the numerous factors involved, the precise
calculation of peak value of corona is extremely difficult, if not impossible.
However, the minimum voltage at which the ionization occurs in the fair
weather is called the disruptive critical voltage and can be determined from the
following equation:
)/ln(
0
0
rDr
V
E = (Eq.03-7)
Where,
Eo = value of electric stress (or critical gradient) at which disruption
starts, kV/cm
Vo = disruptive critical voltage to neutral, kV (rms)
r = radius of conductor, cm
D = spacing between two conductors, cm
Since, in fair weather, the value of Eo of air is 21.1 kV/cm (rms),
Vo=21.1 r ln (D/r), kV (Eq.03-8)
Which is correct for normal atmospheric pressure and temperature (76 cm Hg
at 25ºC). For other conditions,
Vo=21.1 δ mo r ln (D/r), kV (Eq.03-9)
Where δ is the air density factor and mo is the conductor surface irregularity
(0.87 to 0.9 for weathered conductors with more than seven strands).

There is no visible corona at the disruptive critical voltage. In the event that the
potential difference (or critical gradient) is further increased, a point is reached
at which a weak luminous glow of violet color can be seen to surround the
conductor. The voltage value at this point is called the visual critical voltage
and is given by equation:
( )rDn
r
rmV vv /1
3.0
11.21 ⎟⎟
⎠
⎞
⎜⎜
⎝
⎛
+=
δ
δ (Eq.03-10)
Where,
Vv = visual critical voltage, kV (rms)
mv = irregularity factor for visible corona (0.7 to 0.75 for local visible
corona and 0.8 to 0.85 for general visible corona on weathered
stranded conductors).
The voltage equations given above are valid for fair weather conditions and
shall be multiplied by a factor of 0.8 to obtain the values for wet weather
conditions. For a three-phase horizontal conductor configuration, the calculated
disruptive critical voltage shall be multiplied by 0.96 and 1.06 for the middle
conductor and for the two outer conductors, respectively.
5.3 Corona Loss
Corona loss on a conductor is a function of the voltage gradient at its surface.
The effect of reduced conductor spacing and lowered height is to increase the
corona loss as a function of increased gradient. On transmission lines using a
flat conductor configuration, the gradient at the middle phase conductor is
higher than on the outer conductor. This results in corona being more prevalent
on the middle phase conductor.
Corona loss on a satisfactory line is primarily caused by rain. The corona loss
at certain points on a transmission line can reach high values during bad storm
conditions. However, such conditions are not likely to occur simultaneously all
along a line.
A transmission line should be operated at a voltage level below the voltage at
which the loss begins to increase rapidly under fair weather conditions.
Operation at or above this point can result in uneconomical corona loss. Fair
weather corona loss per phase or conductor can be calculated from the
following equation developed by Peek:
( )( ) ( ) 52
0
2
1
c 10VVD/r25f
241
P −
−+
δ
= kW/km (Eq.03-11)
Where:

f = frequency in hertz
V = line to ground operating voltage in kV
Vo = disruptive critical voltage in kV
The wet weather corona loss can be calculated from the above equation by
multiplying Vo by 0.80. This Peek’s equation gives correct results if frequency
is between 25 to 120 Hz, the conductor radius is greater than 0.25 cm and the
ratio of V to Vo is greater than 1.8.
From the above equation it can be observed that the larger the radius of the
conductor, the larger the power loss and larger the spacing between conductors,
the smaller the power loss. Similarly for a given voltage, the larger the
conductor size, the larger the disruptive critical voltage and therefore the
smaller the power loss.
5.4 Radio Interference
The radio interference (also called radio influence) is a noise type that occurs
in the AM radio reception, including the standard broadcast band from 0.5 to
1.6 MHz. It does not take place in the FM band.
Radio noise (i.e., electromagnetic interference) from overhead transmission
lines occurs due to partial electrical discharges (i.e., corona) or due to complete
electrical discharges across small gaps (i.e., gap discharges, specifically
sparking). The gap-type radio noise sources take place in insulators, at tie wires
between hardware parts, in defective electrical apparatus and on transmission
lines themselves.
Radio noise is a general term defined as any unwanted disturbance within the
radio frequency band. The corona discharge process produces pulse of current
and voltage on the line conductors. The frequency of such pulses is so large
that it can include a significant portion of radio frequency band, which extends
from 3 kHz to 30,000 MHz. Therefore the term radio noise is a general term
that includes the terms radio interference and television interference. Radio
noise (RI or TVI) is usually expressed in millivolts per meter or in decibels
above 1μV/m.
The radio interference properties of a transmission line conductor are specified
by radio influence voltage (RIV) generated on the conductor surface. This term
refers to the magnitude of the line-to-ground voltage that exists on a device
such as power line or station apparatus at any specified frequency below 30
MHz. The threshold of RIV coincides with the appearance of visual corona. At
this visual corona voltage, the RIV is negligibly small, but with the initial
appearance of corona, RIV level increases quickly, reaching very high values
for small increases above the visual corona voltages. The rate of increase in RI
is affected by conductor surface and diameter, being higher for smoother
conductors and large diameter conductors. The corona and RI problems can be

reduced by the correct choice of conductor size and the use of conductor
bundling, often made necessary for other design requirements.
As conductor age, RI levels tend to decrease. Since corona is mainly a function
of potential gradients at the conductors and RI is associated with corona, the RI
as well as corona will increase with higher voltage, other things being equal.
The RI level also depends on the line layout, including number and location of
phase and ground conductors, and the line length.
There are numerous methods for measuring and calculating RI levels of
transmission lines. The approximate value of RI can be determined from the
following empirical formula:
RI = 50+K (Em-16.95)+17.3686 ln (d/3.93)+Fn+13.8949 ln (20/D)+Ffw
(Eq.03-12)
Where,
RI = radio noise in decibels above 1μV/m at 1 MHz
K = 3 for 750 kV voltage class
= 3.5 for others, (gradient limits 15-19 kV/cm)
Em = maximum electric field at conductor (gradient) in kV/cm (rms)
d = sub conductor diameter in cm
Fn = -4 dB for single conductor
Fn = 4.3422 ln (n/4) for n >1, n=number of sub-conductors in a
bundle
D = radial distance from conductor to antenna in meters
= (h²+R²)1/2
h = line height in meters
R = lateral distance from antenna to the nearest phase in meters
Ffw = 17 for foul weather
Ffw = 0 for fair weather
Alternatively, the RI of a transmission line can also be determined from a
method adopted by the Bonneville Power Administration (BPA, USA). The
method relates the RI of any given line to that of a RI (under the same

meteorological conditions) for which the RI is known through measurements.
Therefore, the RI of a given line can be determined from:
RI=RIo+120 Log10 (g/go)+40 Log10 (d/do)+20 Log10 (h Do²/ho D²)
(Eq.03-13)
Where,
RIo = radio interference of reference line
G = average maximum (bundle) gradient, kV/cm
d = sub-conductor diameter, mm
h = line height, meters
D = direct (radial) distance from conductor to antenna, meters
Acceptable noise levels depend upon the quality of service required and is
described in terms of an acceptable signal-to-noise signal plus noise-to-noise
ratio. Some reception classifications are given in Table 03-4 below:
Table 03-4: Signal Reception Classifications
Signal-to-Noise
Ratio (dB)
Reception Quality
32 Entirely satisfactory
27 Very good, background unobtrusive
22
Fairly satisfactory, background
evident
16
Background very evident, speech
easily understood
6 Difficulty in understanding speech
0 Noise swamps speech
Thus if a signal has a field strength of, say, 60 dB above 1μV/m and a fairly
satisfactory reception is required then noise from the adjacent overhead
transmission line should not exceed 60-22=38 dB above 1μV/m.
Generally, for EHV transmission lines the RI level should be less than 38 to 42
dB above 1μV/m for acceptable performance under fair weather conditions.
Under wet weather conditions this level may be up to 70 dB.
The RI levels for a given transmission line can also be determined with the
help of base case method described in Transmission Line Reference Book: 345
kV and above, EPRI, USA.

5.5 Television Interference
In general, power line radio noise sources interfering television reception are
due to non-corona sources. Such power line interference in the VHF (30-300
MHz) and UHF (300-3000 MHz) bands is almost always caused by sparking.
Like RI, TVI is categorized as fair weather TVI and foul weather TVI. Since
the sparks are usually shorted out during rain, sparking is considered to be fair
weather problem rather a foul weather one. The foul weather TVI is basically
from a water droplet corona on the bottom side of the conductors, and
therefore, it does not require source locating. If the RI of a transmission line is
known, its foul weather TVI can be determined from the following:
( )
( )
2.3
h/151
H/R1
fLog20RI1TV 2
2
10 +
⎥
⎥
⎦
⎤
⎢
⎢
⎣
⎡
⎭
⎬
⎫
⎩
⎨
⎧
+
+
−= (Eq.03-14)
Where,
TVI = television interference, dB above 1μV/m at a frequency f in
MHz
RI = radio interference, dB above 1μV/m at a 1 MHz and at standard
reference location of 15 m laterally from the outermost phase
F = frequency, MHz
R = lateral distance from antenna to nearest phase, m
H = height of the closest phase, m
Alternatively, the foul weather TVI of a transmission line may be determined
from the following method adopted by Bonneville Power Administration
(BPA), USA.
TVI=TVIo+120 Log10 (g/go)+40 Log10 (d/do)+20 Log10 (Do/D) (Eq.03-15)
Where,
TVIo is television interference of a reference line and other parameters are as
defined earlier.
6.0 OVERHEAD GROUND WIRE SELECTION
6.1 In the process of selecting conductors and overhead ground wires, special
consideration shall be given to sag-matching the overhead ground wire to the
conductor. Other factors are corrosion resistance and conductivity.

6.2 The structural properties of overhead ground wires are adversely affected by
the corrosive atmosphere. For transmission line protection from direct
lightning strokes, high-strength galvanized steel wire shall be used unless it is
determined inadequate for its current carrying capacity or its susceptibility to
corrosion in any particular application.
6.3 Alumoweld (Aluminum Clad Steel), ACSR, AACSR/AW or ACSR/AW type
overhead ground wires shall be used when it is required for its higher current
carrying capacity near large power sources, such as generating plant and bulk
supply points, or if it is found necessary in the coastal and contaminated areas
where wet salt spray is present in the atmosphere.
6.4 The primary function of the overhead ground wire is to protect a transmission
line from damage and outages caused by lightning. The overhead ground wire
shall be suitably grounded, located to provide adequate clearance to the phase
conductors at the structures and throughout the span for all operating
conditions.
6.5 To avoid unnecessary high mechanical stresses in the overhead ground wire,
supporting structures and guys, the overhead ground wire shall not be strung
with any more tension than is necessary to attain an overhead ground wire sag
of not more than 80 % of the conductor sag at an every day temperature (25ºC,
27ºC, 30ºC, whichever is applicable in a particular operating area), no wind,
final condition, to prevent midspan flashover due to a lightning stroke
terminating on the overhead ground wire or switching surge on the phase
conductors.
6.6 When fault current levels are low, minimum ground wire size is usually
dictated by lightning considerations rather than power frequency fault level.
However, when fault current levels exceed about 20 kA, ground wire current
can exceed current carrying capabilities. When this occurs, larger sizes are
necessary to prevent ground wire damage.
6.7 The ground wire fault current magnitudes are affected by various system
parameters of which structure footing resistance and fault location are the
major ones. When a ground fault occurs on a system, the fault current returns
from the fault point via ground wires as well as earth.
6.8 Ground wire currents are highest in spans adjacent to the fault location. In
subsequent spans, ground wire currents decrease as one moves away from the
fault location because more and more current is shunted through structures-to-
ground. Ground wire current, like the fault current, varies as a function of fault
location.
6.9 Fault current distribution studies have shown that in case of faults close to a
station major portion of the fault current is carried by ground wires and most of
the current flows in the ground wires on the station side of the fault. Whereas
for faults remote from a station, the ground wire current is significantly
reduced. Therefore the required ground wire capabilities out along the line may

be significantly reduced from that necessary near a station. Thus it may be
economical to utilize two different ground wire sizes on one line.
6.10 Based on fault current distribution studies following approach shall be adopted
for selection and design of ground wire sizes for overhead transmission line
system.
6.10.1 Unless otherwise specified in the Project SOW/TS the fault current
level shall be considered as 63 kA and 40 kA for 230kV/380kV and
69kV/110kV/115kV/132kV systems respectively.
6.10.2 Maximum allowable temperature for ground wires during fault
conditions shall not exceed the limits given in Table 03-5 below:
Table 03-5: Maximum Allowable Temperature
Type of Ground Wire Allowable Temperature, ºC
EHS/HS Galvanized Steel 400
Aluminum-Clad Steel 400
ACSR/AW & AACSR/AW 200
OPGW (with Aluminum-
Clad Steel Strands)
300
6.10.3 Ground wire sizes shall be selected out of the following standardized
sizes:
OPGW 13 kA and 24kA (minimum)
OGW 13 kA and 24kA (minimum)
The fault current ratings shall be based on an ambient temperature of
50ºC, 20 cycles fault current duration and shall be calculated as per
method presented in Reference 5.
The size of conventional (OGW) and OPGW type overhead ground
wires shall be such that fault current is distributed evenly between the
two ground wires. This requirement may be met if OGW and OPGW
have equal dc resistance at 20ºC with a tolerance of plus or minus five
percent (+/- 5%). Alternatively this requirement may be met if the dc
resistance at 20ºC of the two ground wires is such that the actual flow
of fault current in each ground wire does not exceed its rated fault
current value.
6.10.4 On the basis of short circuit levels established for SEC system and the
approach described in preceding paragraphs, the recommended
ground wire fault current ratings for various transmission line
voltages are given in Table 03-6:

Table 03-6: Recommended Sizes of Ground Wires
Line
Voltage
(kV)
Fault
Current
(kA)
Number
of Ground
Wires
OPGW
Rating
(kA)
OGW
Rating
(kA)
Application
24 24 Within 12 km from Substation
380 63 2
13 13 Beyond 12 km from Substation
24 24 Within 6 km from Substation
230 63 2
24 24 Within 6 km from Substation132
115
110
69
40 1
24 24 Within 2 km from Substation132
115
110
69
40 2
7.0 SAG AND TENSION CALCULATIONS
7.1 Computer Programs
Several computer programs are available in the industry for calculating
initial/final sags and tensions, stringing sags and performing other calculations
for various types of transmission line conductors and ground wires. All sag
tension calculations shall be carried out with the help of some standard
computer program, which shall be able to:
7.1.1 Check all limiting conditions simultaneously and follow the
governing parameter
7.1.2 Account for creep
7.1.3 Use average tension values (not at the support or midway)
7.2 Sag and Tension Tables
Sag and tension tables shall include initial and final sag and tension values
corresponding to various temperatures along with ruling span length and
information on conductor and applicable stress strain chart number. Sag and
tension tables shall be prepared for all set of possible ruling spans between the
maximum and minimum ruling spans likely to be encountered on the
transmission line route. For example, if design ruling span is 400 m, the actual
ruling spans may range from 320 m to 450 m with intermediate ruling spans as
330 m, 340 m, 350 m, 360 m, 370 m, 380 m, 390 m, 400 m, 410 m, 420 m, 430
m and 440 m.

Temperature values to be considered for preparing sag tension tables shall be
as in Table 03-7:
Table 03-7: Recommended Temperatures for Sag-Tension
Conductor/Ground Wire
Tension Condition
Temperature, ºC
Minimum Temperature, Initial -1
Maximum Temperature, Final
60ºC (OGW/OPGW)
85ºC (ACAR & AAAC)
93ºC (ACSR & ACSR/AW)
Every Day Temperature, Final 25ºC, 27ºC or 30ºC (as
applicable)
Effect of Creep, Final 20ºC
Intermediate Temperatures as given in Clause 2.2.11
A typical sag and tension table prepared for a ruling span of 400 m and ACAR
type conductor is attached as an Appendix-1 for illustration purposes.
7.3 Stringing Sag Tables
Stringing sag tables show the amount of initial sag required for each individual
span between a set of deadends. The stringing sag tables shall be prepared from
the ruling span sag and tension calculations. The stringing sag tables shall be
used for sagging the individual spans within the set of deadends on which the
ruling span is based on.
Stringing sag tables shall be prepared with the help of computer program that is
used for sag and tension calculations. The stinging sag shall be calculated with
the help of following formula:
( )
( )2
2
RS
IS
SD = (Eq.03-16)
D = the amount of initial sag in an individual span at the same
temperature as “S”, in meters.
S = the amount of initial sag in the ruling span at a given
temperature, in meters (from sag tension tables).
IS = individual span length, in meters.
RS = ruling span length, in meters.

The stringing sag tables shall be developed for a range of individual span
lengths which should be 20 meters lower and 20 meters higher than the
individual ruling span in the set of ruling spans described in Clause 7.2 above.
For example for a ruling span length of 400 m, the individual spans shall be
taken as from 380 m to 420 m with 2 m increments. This way, all possible
individual span lengths corresponding to a particular ruling span shall be
covered. The temperatures for stringing sag tables shall be considered from
10ºC to 60ºC with 2ºC increments.
A typical stringing sag table prepared for a ruling span of 400 m and individual
spans from 380 m to 420 m and ACAR type conductor is attached as an
Appendix-2 for illustration purposes.
8.0 CONDUCTOR AND OVERHEAD GROUND WIRE SAGGING
8.1 General
The conductor and overhead ground wire shall be installed in accordance with
the information shown in the stringing sag data. In order to ensure that the
conductors installed in the field will produce approximately the same values of
sag and tension at the various loads and temperature as those used in the design
of the transmission line, certain checks should be made at the same time the
conductors are installed. Two most important of these field checks are the
temperature of the conductors at the time of installation and the sag of
conductors in the span being checked.
8.2 Temperature Check
The temperature of the conductors shall be determined by taking a short
section of conductor to be sagged and removing of enough strands to allow a
thermometer to be inserted into the center of conductor. The section of
conductor and thermometer are then suspended from the structure, in full sun
near the sag man position, at least half hour prior to sagging the span of
conductors. The sag man and the inspector can monitor the temperature during
the sagging operation in order to determine the correct value of sag to be used
in the span.
8.3 Sagging Methods
There are various methods for sagging the transmission line conductors, the
most accurate one is the transit-target method and shall be used unless
otherwise specifically mentioned elsewhere. The details of this sagging method
are given in TCS-P-122.07.

9.0 BIBLIOGRAPHY
9.1 Transmission Line Design Manual, Holland H. Farr, United States
9.2 Design Manual For High Voltage Transmission Lines, REA- Bulletin 62-1,
United States Department of Agriculture
9.3 Transmission and Distribution Electrical Engineering, by Colin Bayliss,
Butterworth Heinemann
9.4 Electrical Power Transmission System Engineering, Analysis and Design, by
Turan Gonen, John Wiley & Sons
9.5 IEEE paper published in PAS-103, No.3, “Minimum Shield Wire Size-Fault
Current Considerations”.

APPENDIX-1
SAMPLE SAG AND TENSION CALCULATIONS (TYPICAL)
ALUMINUM CONDUCTOR ALLOY REINFORCED (17 % EDS)
CONDUCTOR 1080.6 KCMIL 18/19 STRANDING ACAR
AREA= 547.5472 SQ. MM.
DATA FROM CHART NO. 1-1206
METRIC UNITS
SPAN= 400.0 MTRS
CREEP IS A FACTOR *DESIGN CONDITION
DESIGN POINTS FINAL INITIAL
TEMP ICE WIND K WEIGHT SAG TENSION SAG TENSION
Cº CM KGSM KG/M KG/M MTRS KG MTRS KG
-1. .00 43.99 .00 2.017 13.70 2963. 12.73 3185.
27. .00 108.98 .00 3.639 16.03 4580. 15.44 4751.
10. .00 .00 .00 1.510 13.75 2212. 12.67 2398.
16. .00 .00 .00 1.510 14.02 2169. 12.95 2346.
20. .00 .00 .00 1.510 14.20 2142. 13.14 2313.
27. .00 .00 .00 1.510 14.51 2097.* 13.46 2258.
30. .00 .00 .00 1.510 14.64 2079. 13.60 2236.
40. .00 .00 .00 1.510 15.07 2020. 14.05 2165.
50. .00 .00 .00 1.510 15.49 1966. 14.49 2100.
60. .00 .00 .00 1.510 15.90 1916. 14.92 2040.
70. .00 .00 .00 1.510 16.31 1869. 15.34 1985.
75. .00 .00 .00 1.510 16.51 1847. 15.55 1959.
80 .00 .00 .00 1.510 16.70 1826. 15.75 1934.
85. .00 .00 .00 1.510 16.90 1805. 15.96 1909.

APPENDIX-2
SAMPLE STRINGING SAG CALCULATIONS (TYPICAL)
ALUMINUM ALLOY REINFORCED CONDUCTOR (17 % EDS)
CONDUCTOR 1080.6 KCMIL 18/19 STRANDING ACAR
STRINGING SAG AT TEMP DEG C (INITIAL)
RULING SPAN = 400 MTRS
TEMP ºC 10. 16. 18. 20. 22. 24. 26. 28. 30.
SPAN
380.0 11.43 11.69 11.77 11.86 11.94 12.02 12.11 12.19 12.27
382.0 11.55 11.81 11.89 11.98 12.06 12.15 12.23 12.32 12.40
384.0 11.67 11.93 12.02 12.11 12.19 12.28 12.36 12.45 12.53
386.0 11.80 12.06 12.15 12.23 12.32 12.41 12.49 12.58 12.66
388.0 11.92 12.18 12.27 12.36 12.45 12.53 12.62 12.71 12.79
390.0 12.04 12.31 12.40 12.49 12.58 12.66 12.75 12.84 12.93
392.0 12.17 12.44 12.53 12.62 12.71 12.80 12.88 12.97 13.06
394.0 12.29 12.56 12.66 12.75 12.84 12.92 13.01 13.10 13.19
396.0 12.42 12.69 12.78 12.88 12.97 13.06 13.15 13.24 13.33
398.0 12.54 12.82 12.91 13.01 13.10 13.19 13.28 13.37 13.46
400.0 12.67 12.95 13.04 13.14 13.23 13.32 13.41 13.51 13.60
402.0 12.80 13.08 13.18 13.27 13.36 13.46 13.55 13.64 13.73
404.0 12.92 13.21 13.31 13.40 13.50 13.59 13.68 13.78 13.87
406.0 13.05 13.34 13.44 13.54 13.63 13.73 13.82 13.92 14.01
408.0 13.18 13.47 13.57 13.67 13.76 13.86 13.96 14.05 14.15
410.0 13.31 13.61 13.71 13.80 13.90 14.00 14.09 14.19 14.29
412.0 13.44 13.74 13.84 13.94 14.04 14.14 14.23 14.33 14.43
414.0 13.57 13.87 13.97 14.07 14.17 14.27 14.37 14.47 14.57
416.0 13.70 14.01 14.11 14.21 14.31 14.41 14.51 14.61 14.71
418.0 13.84 14.15 14.25 14.35 14.45 14.55 14.65 14.75 14.85
420.0 13.97 14.28 14.38 14.49 14.59 14.69 14.79 14.89 14.99

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