Unit 3

Mechanical Design
5 of Overhead Lines

Which of the following statements is

Objective Questions & Answers incorrect?
for Online Examination ~ As the temperature rises the tension
in the transmission line decreases
0.1 Between two supports, due to sag the
conductor takes the form of [I] As temperature rises the sag in
GJ catenary ~ triangle
transmission lines red uces

EJ ellipse @] semi-circle ~ Tension and sag in transmission lines

are complementary to each other
[Ans.: a]
0.2 The effect of ice deposition on conductor I]] None of the above [Ans. : cl
is
Wooden poles for supporting
GJ increased skin effect
transmission lines are used for voltages
lliJ reduced corona losses up to
o increased weight ~ 22 kV [B 11 kV
[I] reduced sag ~ 400 V @] 66 kV
The effect of wind pressure is more
Maximum permissible span for wooden
predominant on
poles is
~ transmission lines
~ 10 meter
lliJ neutral wires
o insulator
~ 20 meters 200 meters
[Ans.:b]
[I] supporting towers [Ans. : d] For a 66 kV line having span of
200 meters between towers the
Which of the following statement is approximate sag will be
correct?
~ 0.02 m
~ Wind pressure reduces corona effects
~ 2m
lliJ Ice on conductors improves power [Ans.: cl
factor In the above case if the span is doubled,
o Wind pressure is taken to act in a the sag will be
direction at right angles to that for ~ 2m 4m
ice ~ 1m 8m
Q] Wind pressure and ice together [Ans.:d]
improve regulation of power The sag of a transmission line is least
transmitted. [Ans.: cl affected by
~ self weight of conductors
 [EJ temperature of surrounding air
Q.13 Galloping is frequency
~ current through conductor amplitude vibration of
GO ice deposited on conductor
[Ans.: c1
~ high, large
Q.11 The sag of the conductors of a
transmission line is 1.5 m when the span [I]
none of these
[Ans. :al
is 100 m. Now if the height of supporting
For transnusslOn line conductors, as the
towers is increased by 20 %, the sag will
tempera ture increases sag and
tension Q
~ increase by 20 %
~ decreases, increases
[EJ increase by 10 %
[EJ increases, decreases
~ reduced by 20 %
~ increases, increases
GO remain unchanged
GO decreases, decreases
Q.12 For overhead transmission line when the
Q.15 plays an important role in the
conductors are supported between two
design of overhead transmission lines. Q.
supports, the variation of tension T with
temperature is represented by ~ Sag ~ Insulation
thickness
~ Charging current
GO Tower material

The shape of the conductor connected

between the two supports at same level is
called Q.2
~ sag-amplitude curve
[EJ sag-span curve
~ sag template
o string chart

The tension at any point on the

conductor connected between the two
supports at same level acts
~ in vertical direction
[EJ at 45° to normal
~ at 60° to normal
o in tangential direction

Q.18 component of the tension is Q.23

constant throughout the length of the
wire.
Curve A CurveB Vertical Horizontal
Curve C Curve D Tangential None of these
[Ans.: c1
[Ans.: bl Q.24
 frequency An overhead line has a span of 250 m. ~ 70 ~ 82
vibration of The tension in the line is 1500 kg while ~ 53.45 [ill 63.88 [Ans.: dl
the weight per unit length is 0.75 kg/m
then the maximum sag in the conductor Q.25 An overhead line uses the conductor of
is m. size 0.92 cm 2 and density 8.87 gm/ cm 3
~ 6.58 then its weight per meter is _
~ 13.71
GJ 3.9062 [ill 4.95 [Ans. : c] ~ 0.652 ~ 0.816
~ 0.784 [ill 1.1481 [Ans.: bl
An overhead line has a span of 300 m.
The tension in the line is 2500 kg while Q.26 An overhead line uses the conductor of
the weight per unit length is 1.1 kg/m size 1.29 cm 2 and density 8.9 gm/ cm 3
then the maximum sag in the conductor then its weight per meter is _
is m.
~ 0.652 ~ 0.816
~ 22.25 ~ 5.17 ~ 0.784 [ill 1.1481 [Ans.: dl
GJ 3.9062 [ill 4.95 [Ans.: dl
Refer data given below for Q.27 and Q.28
An overhead line has a span of 450 m.
An overhead line conductor is supported
Insulation The tension in the line is 1200 kg while
by the two towers having height 90 m each.
thickness the weight per unit length is 0.65 kg/m
The span is 250 m. The tension in the line is
then the maximum sag in the conductor
is m. 1490 kg while the weight per unit length is
1.1481 kg/m.
~ 13.71 ~ 2.045
GJ 3.9062 [ill 4.95 [Ans. : al Q.27 The maximum
m.
sag in the conductor is

An overhead line has a span of 150 m. ~ 3.124 ~ 6.0198

The tension in the line is 1100 kg while
the weight per unit length is 0.8 kg/m
~ 4.89 [ill 8.63 [Ans.: bl
then the maximum sag in the conductor Q.28 The clearance at a point midway between
is m. the tower is m.
~ 13.71 ~ 2.045 ~ 83.98 ~ 78.27
GJ 3.9062 [ill 4.95 [Ans. : bl ~ 90 [ill 63.88 [Ans.: al
Refer data given below for Q.23 and Q.24 For a conductor used for overhead lines
An overhead line conductor is supported the factor of safety is 4 while the ultimate
by the two towers having height 70 m each. strength is 3000 kg then its tension is
__ kg.
The span is 300 m. The tension in the line is
1500 kg while the weight per unit length is ~ 1500 750
0.816 kg/m. ~ 2000 2500

the tension is Q.23 The maximum sag in the conductor is For a conductor used for overhead lines
e length of the m. the factor of safety is 2 while the ultimate
~ 6.12 ~ 5.45 strength is 2000 kg then its tension is
Horizontal GJ 2.14 [ill 8.63 [Ans. : al
__ kg.

None of these ~ 1000 ~ 750

Q.24 The clearance at a point midway between
[Ans. :bl
the tower is m.
~ 2000 [ill 2500
0.31 For a conductor used for overhead lines p x (d + t)
the tension is 1200 kg while its ultimate P x (d + 2t)
strength is 2400 kg then its factor of
P
safety is (d + 2t)
~ 1 ~ 2
[illp
[£] 3 [ill 4 [Ans.: bl (d + t)

For a conductor used for overhead lines If conductor is subjected to both

the tension is 1000 kg while its ultimate loading (Wi) as well as wind presS
strength is 3000 kg then its factor of (w w) then the total weight acting on
safety is conductor is given by __ .
~l ~ 2 ~ ~(Wi)2 + (ww)2
[£]3 [ill 4
[E] ~(wJ2+(w+ww)2
For a conductor with fixed ultimate
strength if its tension increases then the [£] ~(W+Wi)2+(ww)2
factor of safety
[ill none of these [Ans. : l
~ remains same
In case of ice loading as well as wit
[£] increases becomes infinite
pressure acting on the conductor, the s.
[Ans.:bl
direction is at an angle e measured wi
For a conductor with fixed ultimate
respect to vertical. The e is given 1
strength factor of safety is
~ proportional to square root of
-1
tension t an ---Ww
w+wi
proportional to square of tension
-1 Ww
t an --
proportional to tension wi

proportional to reciprocal of tension tan-1 w+ Ww

[Ans. :dl wi
For ice loading, the weight of ice per unit
-1 w+ wi
length is given by ----' if d is original tan ---
Ww
conductor diameter and t is the thickness
of ice coating. For a conductor acted upon by ic
~ density of ice x n t (d + t) loading and wind pressure, the slant sa
is given by __ .
[E] density of ice x (d:\)
~ wtL
8T
l
[£] density of ice x
nt
(d
+t
)

The effect of wind is considered in terms For a conductor acted upon by ice
of wind force per unit length given by loading and wind pressure, the vertical
__ where p is the wind pressure, d is sag is given by __ .
the diameter of the conductor and t is the ~ (w+wi) ~ Wt L2
thickness of ice coating. 8TL2 8T
 (w+ Wi) L2 Q.46 If a conductor has breaking stress of
8T 5588 kg/em 2 and area of the conductor
[Ans.:dl is 0.95cm 2 while its factor of safety is 4
If w is the weight of conductor and is then its working tension is __ kg.
acted upon by wind force of w w then the ~ 21234.4 ~ 1469.73
total weight is given by __ . [EJ 1327.15 [ill 1397 [Ans. : cl
~ ~w2+w~
Refer following data for the Q.47 to Q.52.
[£] W
A transmission line has a span of 250 m.
The diameter of the conductor is 1.4 ern and
its weight is 1 kg/m. The working stress is
1050 kg/cm2, The thickness of ice coating is
Q.42 A conductor has weight 1.691 kg/m and
subjected to wind pressure of 1.5 kg/m 1.25 ern and wind pressure is 40 kg / cm 2.
then its total weight to be considered for The ice density is 1 grn/ em 3.
sag calculation is __ kg. Q.47 The tension in the conductor is __ kg.
~ 1.691 ~ 1.5 1616.35
~ ~ 2524.78
[£] 2.2604 [ill 2.5365 [Ans.: cl [EJ 1050 [ill 1875.49
[Ans.: al
Q.43 The maximum tension which a conductor
Q.48 Wind force per unit length is
can withstand without mechanical failure
kg/m.
is called
~ 2.2 40
electrostatic stress ~
breaking stress 0 1.56
[ill 3.25 [Ans.: cl

breakeven stress The force due to ice coating is __ kg/m

allowable stress ~ 2.56 ~
Q.44 The factor of safety is _
o 3.27 [ill 2.2

~ ratio of working tension to ultimate Q.50 The total weight of the conductor
strength including effect of ice and wind is
kg/m.
[E] ratio of maximum sag to ultimate
strength ~ 2.2 W 1.04
[£] ratio of working tension to slant sag o 3.6 [ill 2.5685 [Ans. : dl
[[] ratio of ultimate strength to working Q.51 The slant sag is __ m.
tension [Ans. : dl
~ 12.4145 ~ 10.5278
If a conductor has breaking stress of
4465 kg/cm2and area of the conductor is
o 6.5798 [ill 8.1457 [Ans.: al

1.29 cm 2 then the ultimate strength of The vertical sag is __ m.

that conductor is __ kg. ~ 5.2478 ~ 7.5689
[U 3461.24 ~ 5759.85 o 9.8627 [ill 2.4781 [Ans.: cl
[E] 6624.89 [ill 4465 [Ans. : b 1
Refer following data for the Q.53 to Q.57.
A transmission line conductor has an
effective diameter of 91.5 rnm and wei ht
0.85 kg/m. It is subjected to wind force of ~ x= 5 n2 + 6n + 1
39 kg/ m 2 and ice coating of thickness [E] x= n2 + 3n + 1
12.7 mm. The factor of safety is 2. The [£] n= 3 x2 + 3x+ 1
ultimate strength is 8000 kg. Density of ice is
910 kg/m3. The maximum possible sag is 0 x = 3 n2 + 3n + 1 [Ans. :dl

6.3 m. Q.60 If a stranded conductor has 61 strands

Q.53 The working tension in the conductor is then the number of layers in which they
__ kg. are arranged is _

~ 4000 W 8000 ~4 W 5

[£] 16000 [ill 2000 [Ans.:al [£]6 [ill 3

Q.54 Wind force per unit length Q.61 If a stranded conductor has 37 strands
is
then the number of layers in which they
kg/m.
are arranged is _
~ 1.748 W 4.5591
~4 W 5
[£] 3.9 [ill 5.263 [Ans. :bl
[£]6 [ill 3
Q.55 The force due to ice coating is
Q.62 If the sag is small compared to the span
kg/m
then the shape of the conductor between
~ 4.5591 W 7.458
the supports is _
[£] 3.7832 [ill 1.762 [Ans.: cl ~ circular
Q.56 The total weight of the conductor [£] parabola hyperbola
including effect of ice and wind is [Ans.: cl
kg/m. If the sag is comparable compared to the
~ 2.5 W 4.5591 span then the shape of the conductor
between the supports is _
[£] 3.7822 [ill 6.5 [Ans. : dl
~ circular W catenary
Q.57 The span of the transmission line is __ [£] parabola [ill hyperbola
m. [Ans.:bl
~ 304.63 W 157.89 Refer following data for the Q.64 to Q.68.
[£] 176.12 [ill 200.56 [Ans.: c] A transmission line has a span of 150 m.
The conductor has an area of 2 em 2. It is
Q.58 The total diameter of stranded conductor
subjected to wind pressure of 1.5 kg/m. The
is given by __ if n is the number of
factor of safety is 5. The ultimate strength is
layers in which strands are arranged and
5000 kg/em 2. The specific gravity of the
d is diameter of each strand.
material is 8.9 g/cm3.
~ (n + l)d W (2n + l)d
[£] The weight
(2n + 1) / d [ill 2n+~
d kg/m.
[Ans.: b]
~ 1.78 W 2.895
Q.59 If n is the number of layers in which
strands are arranged in a stranded
[£] 2.327 @] 8.9

conductor around central strand and x in

Q.65 The total weight including wind pressure
the number of strands then these two are
is __ kg/m.
related to each other as _
 GJ 1.78
[}J 2.895
EJ 2.327
@J 8.9

The working tension in the conductor is

_kg.
GJ 1000
[}J 2000
EJ 4000
@J 10000

The sag is _ m. rAns.: bJ

GJ 2.5031 [}J 4.2178

EJ 6.785 @J 3.2734
rAns.: dJ
The vertical sag is __ m.
GJ 2.5031 [}J 4.2178
EJ 6.785 @J 3.2734
rAns.: aJ
000