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DESIGN & CONSTRUCTION OF
DISTRIBUION LINES & SUB-
Er. K.V. Surya Prakasa Rao, Former
Design of Electrical Sub
Transmission and Distribution
Design aspects Power Distribution System is governed by
Indian Electricity Rules.
Desired features of Power Distribution System are
Quality supply to consumers
To follow Standards in Design Construction, installation
Protection, Operation and Maintenance of electrical
equipment and lines, Substations
To lay down standard procedures and follow the same
with accountability to people
Voltages at which power is to be
L.T. single phase consumers – 240V
L.T. three phase consumers – 415V
H.T. consumers – 11,000V; 33,000V;
Permissible Voltage Variations:
Up to 650V supply + 6%
11KV and 33KV + 6% ; - 9%
132 KV + 10.0%; - 12.5%
220KV + 5%
Permissible Frequency Variation: + 3%
Pole sizes, spans and conduction sizes forthe
Type of Pole Length of Pole Voltage of Line Max Span (mts)
PSCC 9.1 meters 33KV 100
PSCC 8.0 meters 11KV 107
PSCC 8.0 meters 415/240V 67
Single Pole Support: 0 – 100
Double Pole Support: 100
Double Pole Support: 600
The table below gives sizes and other details of conductors standardized for use in
sub–transmission and distribution system for various types of lines. These are:
ACSR – Aluminium Conductors Steel Reinforced
AAC – All Aluminium Conductors
AAAC – Aluminium Alloy Conductors
Voltage Class No. and Diameterof Wire Type of
33 KV Lines 1. 7/3.35 mm (50 mm2
2. 7/4.09 mm (80 mm2
3. 6/4.72 mm + 7/1.57 mm (100 mm2
11 KV Lines 1. 7/2.11 mm (20 mm2
2. 7/2.59 mm (30 mm2
3. 7/3.35 mm (50 mm2
LT Lines 1. 7/2.11 mm (20 mm2
2. 7/2.59 mm (30 mm2
3. 7/3.35 mm (50 mm2
4. 7/2.21 mm (25 mm2
5. 7/3.10 mm (50 mm2
Stay Wires are used for anchoring of power line poles at
dead ends and at angular locations. The individual wire
used to form “stranded stay – wire” is to be of Tensile
grade 4 having minimum tensile strength of 700 N / mm2
as per IS–2141. Three sizes have been standardized
and are tabulated below:
No. of Wires & Const. Wire Dia (mm) MinimumBreaking
Load of Single -
Breaking Load of
7 (6/1) 2.5 3.44 22.86
7 (6/1) 3.15 5.45 36.26
7 (6/1) 4.0 8.79 58.45
Factor of Safety:
The factor of safety adopted is
2 with the following working
loads:Working Load on
Wind Pressure Zone
(where to be used)
50 kg / m2
75 kg / m2
100 kg / m2
Wind – Loads on conductors in Different Wind -
Wind – Load in Kg. permeterrun of
W.R. 75 kg /
W.R.100 kg /
W.R.150 kg /
ASCR 0.472 0.757 0.943 1.415
Racoon 7/4.09 ASCR 0.409 0.613 0.818 1.227
Rabbit 7/3.35 ASCR 0.335 0.502 0.670 1.005
Weasel 7/2.59 ASCR 0.259 0.388 0.518 0.777
Squirrel 7/2.11 ASCR 0.211 0.316 0.422 0.633
Amt. 7/3.1 AAC 0.310 0.465 0.620 0.930
Gnat 7/2.21 AAC 0.221 0.331 0.442 0.663
7/4.26 AAAC 0.426 0.639 0.852 1.278
Racoon 7/3.81 AAAC 0.381 0.572 0.762 1.143
Rabbit 7/3.15 AAAC 0.315 0.472 0.630 0.945
Weasel 7/2.5 AAAC 0.250 0.375 0.500 0.750
Squirrel 7/2.0 AAAC 0.200 0.300 0.400 0.600
Clearance above ground of the lowest conductor (including
guard – wires) is to meet the following conditions:
1. No conductor of an overhead line including service lines
erected across a street shall be at any part there of be at a
height less than:
a) For low & medium voltage line (i.e. up to 650 volts) – 5.791
b) For high voltage lines (up to 33 KV) – 6.096 mts
2. Along a street:
a) For low& medium voltage lines _5.486 mts
b) For high voltage lines(up to 33 KV) _ 5.794
3. Elsewhere other than 1,2 above
a) For high voltage up to 11KV - 4.572 mts
Clearance from Building, Structure etc
a. The vertical clearance above building (from the highest
point) on the basis of maximum sag shall not be less
than (2.439 m) 8 ft for low and medium voltage line (up
to 650 volts) and (3.64 m) 12 ft for 11 and 33 KV line.
b. The horizontal clearance is to be 4 ft (1.219 m) upto 11
KV line and 6 ft (1.820 m) for 33 KV line.
i) It may be mentioned that for Railway crossing, rules as
prescribed by Railways are to be followed
ii) Similarly the lines crossing or in proximity to the
telecommunication lines, the overhead line is to be
protected as per code of practice laid down by PTCC
The basic parameters for selection of materials and
standards, once decided will help in adopting sound
construction practices. Estimation of materials labour
and transport facilities, having been assessed the
materials should be made available at site, required
working gangs (equipped with tools & tackles) should be
formed as and when required.
The voltages of concern are:
a) 33 KV sub-transmission line,
b) ll KV Primary distribution line
c) 415-240Volts Secondary distribution line
Survey of the Proposed Route of
The first step to be taken prior to the design or construction
of any line is to conduct reconnisiance survey of the
country over which the line is to pass. Topo sheet map
of the area which would indicate towns roads,
streams/river, hills, railway lines, bridges, forest areas,
telephones telegraph and power lines may be taken and
the approximate route of the line marked on it. Before
finalising the route, the following parameters should be
kept in mind:
1. The shortest route practicable.
2. As close as possible to the road for easy maintenance
and approach during the construction.
3. Route in direction of possible future load.
4. Angle points should be less.
The areas to be avoided as far as
(a) Rough and difficult country side.
(b) Urban Development area.
(c) Restricted access for transport vehicles.
(d) Abrupt changes in line routes.
(e) Difficult crossing - river, railway.
(f) Proximity to aerodromes.
(g) Natural hazards like steep valleys, hills,
lakes, gardens, forests, playgrounds, etc.
The route selected for a distribution line shall be such
that it will give the lowest cost
considered over a period of years, consistent with
accessibility for easy maintenance, etc. This
includes many considerations such as original cost, tree
trimming and compensation, freedom
from vehicular damages future development and
availability for services.
The lines should be routed wherever possible to avoid
natural obstacle such as steep hills or valleys', swamps,
lakes, thick forests, rivers, etc. Lines should be so
located at a safe distance from buildings and from
possible fire, proximity to traffic and other hazards. Line
shall not cross school play grounds, cemetery, except
under special circumstances. Lines should be away from
the buildings containing explosives.
Transportation contributes a major portion of construction
cost. As such while finalising the route alignment, it may
be ensured that due to transportation cost should be as
low as possible.
Transport of RCC/PCC poles pose greater problem as
they are generally heavier than other types of supports
for same duty. The RCC/PCC poles are generally
stronger on the longer axis than on shorter axis. Care
should be taken on this aspect while handling, to prevent
excessive stressing of the pole at the time of transporting.
The unloading of poles from truck or trailer should also be
done carefully. Suitable skid boards must be used and on
no account, the poles be dropped. Several utilities have
special trucks made with side loading arrangement for
pole transportation or use trailers. It is preferably to
provide a chain pulley block with a beam arrangement in
the middle of the truck body to facilitate unloading/loading
of poles. The poles should not be dragged on rough
surface, but transported in small hand-cart
(a) Preliminary Walk Over survey
(b) Detailed survey
Having provisionally fixed the route, on the survey map,
a preliminary Walk Over survey is carried out, before
conducting the survey with ranging rods. As far as
possible, the line route is taken through areas with
minimum tree growth. If there are alternative routes, all
such routes are investigated for final evaluation of the
most economic route.
It may be mentioned here that the detailed survey can be
carried out by theodolite and angle points can be fixed
and marked with survey stones. A route map to a scale
of 1 cm— 0.5 km can be prepared showing the various
angles approach roads, near the line, routes detail of
railways, communication lines, EHT line crossing, river
crossing,, etc. but this is not necessary in case of small
lines as the local staff usually is conversant with the
topography and therefore marking of locations aligning
the line with ranging rods is found to be satisfactory.
Right of Way
(a) Once the route of the line is fixed approval has to be
From the railway authorities for Railway Crossings.
From the competent Forest Authorities for routing of
the line in Forest areas.
From the State level Power Tele-communication
Coordination Committee (PTCC).
(b) In addition if there are urban development Air-port and
similar other areas falling in the route of the line,
permission has to be obtained.
( c) Sometimes private gardens/orchards may fall on the
route and require free cutting. The details of trees are
to be marked. Compensation be got fixed from
Revenue Authorities and paid to the owner
Pole Locations :
In locating poles on lines, the following general principles are to be kept
1. Keep spans uniform in length as far as possible.
2. Locate to have horizontal grade
3. By locating the poles on high places short poles can be used and
will maintain proper ground clearance at the middle of the span. In
extremely hilly or mountainous country, poles are located on ridges
thereby increasing the spans without greatly increasing the pull on
the conductor. This is possible because the sag can be made very
large maintaining the required ground clearance.
4. Poles should not be placed along the edges of cuts or embankment
or along the banks of creeks of streams
5. Cut-point for a section could be at 1.6 km length (except in special
cases), where Double-pole structures are to be provided to take
tension of the conductors. It may have been already estimated that
10 supports (locations) are mostly required in one km. length of H.T.
lines and 15 supports for L.T. line
The construction activity of H.T. lines may b& divided as follows
(1) Pit marking, pit digging.
(2) Erection of supports and concreting.
(3) Providing of guys to supports.
(4) Mounting cross-arms, phi and insulators, and pin binding.
(5) Paying and stringing of the conductor. .
(6) Jointing of conductors.
(7) Sagging and tensioning of conductors.
(11) Testing and commissioning
For low tension lines the activities could be followed, with simplified
Pit Marking and Digging
After surveying, the pole location should be marked with
the peg. The pits should not be too large than
necessary, as otherwise, after erection of the 'pole and
filing there remains a possibility of tilting of pole. For
marking the pits, the dimensions of the pit and the
centre to centre distance of pits are required. Pits having
a dimension of about 1.2 m x 0.6 m should be excavated
with its longer axis in the direction of the line. The
planting depth should be about
1 /6 length of the support (1500 mm). Excavation is
generally done by using pickaxe crow bars,
and shovel, very hard or rocky soil may require blasting
of rock by small charges of gun
Erection of Poles and Concreting
After excavation of pits is completed, the supports/ poles to be
erected may be brought to the pit location by manual labour or by
cart. Then the pole may be erected inside the pit.
Erection of poles can be done by using Bipod/ wooden horse made
of 15 cm G.I pipe and 6 m long. The spread of the legs should be
10m. The tie wire for attachment of bipod to the pole is about 6 m
long and is made of 7/10 SWG. (3.15 mm) Stay wire and this wire
should be attached to the pole at 8 m. The pole is slid along the line
route. The pole is tied with 3 ropes.The rope at the bottom prevents
the pole from being dragged in the direction of the pull. To prevent
the support from moving side in raising, two guy ropes are fixed on
both sides and attached to temporary anchor. -
For smooth sliding and prefect placement of pole in the pit, an
inclined trench having15.2 cm (6 in.) width and 10.2 cm (4 in.) length
may be dug adjacent to the pit as shown below.
A piece of M.S. channel may be placed in the inclined position at the
other end of the pit for enabling the pole to slip smooth inside the pit.
The following figure shows the procedure for erection of pole.
The trench would facilitate the pole to skid smoothly into
the pit with jerks.
V3 from top of bipod' ' -- "' ;f»f y.|"V"J
The bipod is placed in position and attached to the pole
by means of tie wire. The pull for lifting the poles is
provided by rope pulley. When the pole has reached at an
angle of (35° to 40°) the bottom holding rope is slowly
released. When the pole assumes the vertical position,
the holding ropes should be tightened.
It should be ensured that the time of erection, four men
are at the ropes and the supervisor should be at a
distance for guiding correct position so that hi the event of
breaking of rope, if pole falls, it will not result into an
Before the pole is put into RCC padding or alternatively
suitable base plate may be . given below the pole to
increase the surface contact between the poles and the
soil. The padding will distribute the density of the
pressure due to weight of the pole on the soil.
Having lifted the pole the same should be kept in vertical
position with the help of manila rope of 20/25 mm dia.,
using the rope as a temporary anchor. As the poles are
being erected say from an anchor point to the next angle
point, the alignment of the poles should De checked and
set right by visual check. The vertical ties of the poles are
to be checked with a spirit level. After the pole erection
has been completed, and having satisfied that the
verticality and alignments are all right, earth filling and
ramming is to be done.
In swamp and special locations, before earth filling, the
poles are to be concreted up to the ground level of the pit.
After poles have been set, the temporary anchors are to
Erection of DPStructure forAngle
For angles of deviations more than 10°, DP structure
may be erected. The pit digging should be done along
the bisection of angle of deviation.
After the poles are erected, the horizontal/cross bracing
should be fitted and the supports held hi a vertical
position with the help of temporary guys of Manila rope
20/25 mm dial
Ensuring that the poles are held in vertical position (by
spirit level) the concreting of poles with 1:3:6 ratio may
be done from bottom of the support to the ground level.
Before lifting the pole in the pit, concrete padding of not
less than 75 mm thickness may be put up for the
distribution of the loads of the support ojn the soil or
anchor plate could be used. .
The concreting mixture 1:3:6 ratio, would
mean 12.8 bags of cement 100 Cft of 1 1/2
size gitti and 50 Cft of sand. It may be .noted
that while preparing the concrete mixture large
quantities of water should not be used as this
would wash away cement and sand
33 KV Line
(i) Provision at D.P. locations is for 6 guy-sets (20 mm Rod of turn-
buckle x 7/4 stay-wire, 8.5 kg in wt for each location). The quantity
of concrete at the rate of 0.5 cum for D.P. locations and 0.3 cum for
stay sets is 2.8 cum, irrespective whether D.P. locations are of
P.C.C. poles or Rail pole.
(ii) Provision for 8 tangent locations in a section of 1 km is for 3 Guy-
sets. These will require 0.9 cmt of concrete @ 0.3 cum per Guy-set.
A prefabricated base plate is to be provided at the bottom of the
P .C.C. support for uniform distribution of load. If this is not provided
then provision at the rate OX)5.cm_of concrete per location for 8
locations should be made for casting the base-pad before erection
of the P.C.C. support. Thus the total quantity of concrete required is
1.3 cum. Tangent locations are not concreted in several states but
boulder 1 filling is carried out to economies. If P.C.C. pole tangent
locations are to be concreted additional provision for concrete
quantity is to be made. However Rail pole or joist tangent locations
(if Rails or joists are used) should be concreted. Provision for
tangent location's concreting is to be at the rate of 0.5 cum per
The guys are made with 7/3.15 stay wire (5.5 kg)
turnbuckle rod is of 16 mm dia. 6 Guy-sets are required
at D.P. locations and 4 additional Guy-sets are required
in a km for 8 tangent-locations. The quantity of concrete
for Guy-sets is provided at the rate of 0.2 cum per Guy-
set. D.P. locations of P.C.C. poles require 0.3 cum
concrete per location. Boulder filling of tangent locations
could be adopted. If concreting is done for tangent
locations additional provision at the rate of 0.3 cum per
location should be made. Base-pad is to be used if not
additional provision for base^pad concreting should be
15 locations are there in 1 km. Provision for 9
guy-sets is made with 7/3.15 stay-wire (5.5
kg), the turn-buckle M.S. rod is of 16 mm dia.
concrete quantity at the rate of 0.2 cmt per
stay-set should be provided. Base pad should
be used if not additional provision for base
pad-concreting is to be made,
Providing of Guys to Supports
In spite of careful planning and alignment of line route, certain
situations arise where the conductor tries to tilt the pole from its
normal position due to abnormal wind pressure and deviation of
alignment, etc. When these cases of strain arise, the pole is
strengthened and kept in position by guys. One or more guys will
have to be provided for all support where there is unbalanced
strain acting on the support, which may result in tilting/uprooting
or breaking of the support.
Guy brackets or clamps are fastened to the pole. The most
commonly used form of guy is anchor guy. These guys are
provided at (i) angle locations (ii) dead end locations (iii) tee off
points (iv) steep gradient locations and (v) where the wind
pressure is more than 50 kg sq. m.
The fixing of guys stays will involve (i) pit digging and fixing stay
rod (ii) fastening guy wire to the support (iii) Tightening guy wire
and fastening to the anchor. The marking of guy pit, digging and
setting of anchor rod must be carefully carried out. The stay rod
should be placed in a position so that the angle of rod with the
vertical face of the pit is 30°/ 45° as the case may be.
G.I. stay wires of size 7/3.15 mm (10 SWG) or 7/2.5 mm
(SWG 12), and 16 mm 720 mm stay rods are to be
provided. For double pole structure (DP), four stays along
the bisectional the each direction and two stays along the
bisection of the angle of deviation or as required
depending on the angle of deviation are to be provided.
After concreting back filing and ramming must be done
well and allowed 7 days to set. The free end of the guy
wire /stay wire is passed though the eye of the anchor rod,
bent back parallel to the main portion of the stay/guy and
bound after inserting, the G.I. thimble, where it bears on
the anchor rod. If the guy wire proves to be hazardous, it
should be protected with suitable asbestos pipe filled with
concrete of about 2 m length above the ground level,
painted with white and black strips so that, it may be
visible at night. The turn buckle shall be mounted at the
pole end of the stay guy wire so fixed that the turn buckle
is half way in the working position, thus giving the
maximum movement for tightening or loosening.
Guy Strain Insulators
Guy insulators are placed to prevent the lower part of
the Guy from becoming electrically
energised by a contact of the upper part of the guy
when the conductor snaps and falls on them or
due to leakage. No guy insulator shall be located
less than 3.50 mm (vertical distance) from the
A type of insulators are to be used for L.T. Line-Guys
One C type of insulator is to be used for ll KV Guys
Two C type of insulators are to be used for 33 kV.
Fixing of Cross-Anns & Top-
After the erection of supports and providing guys, the
•cross-arms and top-brackets are to be mounted on the
support with necessary clamps bolts and nuts. The
practice of fixing the cross-arms brackets before the
pole erection is also there. In case, these cross are to be
mounted after the pole is erected, the lineman should
climb the pole with necessary tools. The cross-arm is
then tied to,a hand line and pulled up bytthe ground man
through a pulley, till the cross-arm reaches the line man.
The ground man should station himself on one side, so
that if any material drops from the top of the pole, it may
not strike him. All the materials should be lifted or
lowered through the hand line, and should not be
Insulators and Bindings
Line conductors are electrically insulated from
each other as well as from the pole or tower
by non-conductors, which we call 'Insulators'.
There are 3 types of porcelain insulator:
1. Pin type
2. Strain type
3. Shackle type
The pin type insulators are generally used for straight
stretch of line. The insulator and its r. . pin should be
mechanically strong enough to withstand the resultant
force due to combined effect , of wind pressure and
weight of the conductor in the span.
The strain insulators are used at terminal locations or
dead end locations and where the angle of deviation of
line is more than 10°.
The shackle type of insulators is used for LT. lines.
The pins for insulators are fixed in the holes provided in
the cross-arms and the pole top brackets. The insulators
are mounted in their places over the pins and tightened.
In the case of strain or angle supports, where strain
fittings are provided for this purpose, one strap of the
strain fittings is placed over the cross-arm before placing
the bolt in the hole of cross-arms. The nut of the straps is
so tightened that the strap can move freely in horizontal
Tying of Conductoron Pin
Conductors should occupy such a position on the
insulator as will produce minimum strain on the tie wire.
The function of the wire is only to hold the conductor, in
place on the insulator, leaving the insulator and pin to
take the strain of the conductor.
In straight line, the best practice is to use a top groove
insulator. These insulators will carry grooves on the side
as well. When the conductor is placed on the top
groove, the tie wire serves only to keep the conductor
from slipping out.
On corners and angles (below 5 deviation) the
conductor should be placed on the outside of the
insulators. On the far side of the pole, this pulls the
conductor against the insulators instead of away from
Kind and Size of Tie Wire to be
In general the tie wire should be the same kind
of wire as the line wire i.e., aluminium tie
wire should be used with aluminium line
conductor. The tie should always be made of
annealed wire so that it may not be brittle and
injure the line conductor. A tie wire should never
be used for second time. Good practice is to use
no. '6' tie wires for line conductor, (i) The length
of the wire varies from 1 m for simple tie of a
small insulators (Lt pin insulators) to 3 m (33 pin
Rule of Good Tying Practice
(i) Use only fully annealed tie wire.
(ii) Use that size of tie wire which can be readily handled yet one which
will provide adequate strength.
(iii) Use length of tie wire sufficient for making the complete tie,
including an end allowance for gripping with the hands. The extra
length should be cut from end if
the tie is completed.
(iv) A good tie should
(a) Provide a secure binding between line wire insulators and tie
(b) Have positive contacts between the line wire and the tie wire so
as to avoid shifting contacts.
(c) Reinforce line wire if the vicinity of insulator.
(v) Avoid use of pliers.
(vi) Do net use the wire which has been previously used.
(vii) Do not use hard drawn wires for tying.
Good helical accessories are available and can be used
Conductor Erection Paving and Jointing
Conductor erection is the most important phase in construction. The
main operations are :
(a) Transportation of conductor to works site.
(b) Paying and stringing of conductor.
(c) Joining of conductor.
(d) Tensioning and sagging of conductor.
The conductor drums are transported to the tension location. While
transporting precautions are to be taken so that the conductor does
not get damaged/injured. The drum could be mounted on cable
.drum support, which generally is made from crow-bar and wooden
slippers for small size conductor drums. The direction of rotation of
the drum has to be according to the mark in the drum so that the
conductor could be drawn. While drawing the conductor, it should
not rub causing damage. The conductor could be passed over poles
on wooden or aluminium snatch block mounted on the poles for this
The mid span jointing is done through compression crimping or if
helical fittings are used the jointing could be done manually. After
completing the jointing, tensioning operation could be
commenced. The conductor is pulled through come-along clamps
to stringing the conductor between the tension locations. Sagging of
conductor has to be in accordance to the Sag Tension
chart. In order to achieve it, it is preferred to pull the conductor to a
tension a little above the theoretical value so that while transferring
it from the snatch blocks to the, pit insulators and to
take care of temperature variation proper sag could be achieved.
Sagging for 33/11 kV line is mostly done by "Sighting". A horizontal
strip of wood is fixed below the cross-arm on the pole at the
required sag. The lineman sees from other end and the sag is
adjusted by increasing or decreasing the tension. The tension
clamps could then be finally fixed and conductor be fixed onpin-
insulators. All fittings, accessories like guys, cross-arms, etc., could
be checked as they
should not have deformalities.
Sagging and Tensioning
The conductor length in a section increases or decreases with variation in
atmospheric temperature. In summer when temperature is high the length
increases due to expansion and in winter, when the temperature is low the
length decreases due to contraction. With increase in length, the conductor
becomes loose, sag increases and tension reduces, while in winter the sag
decreases, tension increases. The conductor has to be properly keeping
the required sag at the strung atmospheric temperature.
It is known that sag d = WI2/2T where I is half the span length, T is-tension
conductor and W = V(w2+ww2), w is the weight of the unit length of the
vertically and ww is wind-pressure on the unit length of the conductor acting
horizontally. If we
design. The line for 75 kg/m2 wind-zonethen wind load on 1 m length of the
conductor and 2/3
projected Dia (Din mm) of the conductor ^
The line has to be designed to withstand the above load as
postulated by the I.E. Regulations. Hence it becomes necessary to
calculate the tension and sag under conditions occurring at the time
of erection. In practice the conductors are hung over Aluminium
rollers and pulled up through ropes over snatch blocks for the
required sag or tension and then transferred to the insulators. The
tension is not measured as it requires elaborate arrangements and
difficult to measure it accurately, the sag is only measured.
There are two important factors which vary the sag and tension : (i)
Elasticity of the conductor and (ii) Temperature. Sag is directly
proportional to Wand inversely proportional to T. If the length of the
conductor increases due to temperature increase then sag will
increase. This may be the case in summer, while it may be reverse
in winter. The tension accordingly decreases or increases.
In order that the sag and tension values under varied working
conditions may be kept according to the regulations, Sag-Tension
charts are prepared for different spans and temperatures for ACSR,
AAAC & AAC conductor.
(A) In case the lines cross-over the other lines or buildings, safe minimum
clearances are to be maintained as per IE Regulations. The clearances
have been tabulated for this purpose under design aspects. These
clearances should be maintained. The crossings could be for j
(i) Telephone/telegraph lines.
(ii) Buildings. .
(w) Lines of other voltages.
(iv) Roads, streets, other than Roads/Streets.
(B) River Crossing: Data for the highest flood-level should be obtained 'for
previous years. For medium voltage minimum clearance of 3 m be kept
over the highest floor level. Double pole or 4 pole structure would be
required to be specially designed, depending upon the span and conductor
size for the river crossing. The structures should be located at such places
that they could be approached under flood condition, also. The foundation
of structures should be sound so that it may not get eroded or damaged
due to rain water
Guarding is an arrangement provided for the lines, by
which a live conductor, when accidentally broken, is
prevented to come in contact with other electric lines,
telephone or telegraph lines, railway lines, roads, and
persons or animals and carriages moving along the
railway line or road, by providing a sort of cradle below
the main electric line. Immediately after a live conductor
breaks it first touches this cradle guard of G.I. wires
before going down further. This, in turn, trips the circuit
breakers or H.T7L.T. fuses provided for the H.T7L.T.
lines, and the electric power in the conductor or the line is
cut off, and danger to any living object is averted.
Guarding is not required for crossings of 66 kV and
higher voltage lines where the transmission line is
protected by fast acting, relay operated circuit breaker of
modern design with a tripping time of even less than the
order of 0.25 sec. from occurrence of fault to its
clearance. For all other crossings, nice Railway Tele-
communication lines and major road crossing guarding is
The minimum height between any guard wire and live
crossing conductor shall not be less than 1 .5 m in case
of a railway crossing.
The guarding consists of 2 G.I., bearer wires strung
between the two line supports, and G.I. Cross-lacings
connecting two-bearer wires at definite intervals. The
bearer fixed to the guarding cross-arms on the line
supports by means of threaded eyebolts for proper
tightening. In minor L.T. Lines, only two guard-stirrups
600 mm long on either side are normally used with
single G-.I. wire cross-lacing on either side, as a
measure of economy. Due to electrification of railway-
tracks nowadays, 1 1 kV & L.T. crossings have to be
through under-ground cables.
Earthing shall generally be carried out in accordance with the requirements of
Indian Electricity Rules, 1956 and the relevant regulations of the Electricity
Supply Authority concerned and as indicated below:
1 All metallic supports shall be earthed.
2. For RCC/PCC poles the metal cross-arms and insulator pins shall be bonded
and 5 earthed at every pole for HT lines and at every 5th pole for L T
3 All special structures on which switches, transformers, fuses, etc., are
mounted should be earthed.
4 The supports on either side of the road, railway or river crossing should be
5 All supports (metal, RCC/PCC) of "both HT and L T lines passing through
inhabited areas, road crossings and along such other places, where earthing
of all poles is considered desirable from safety considerations should be
In special locations, railway and telegraph line crossings, special structures,
etc., pipe/rod earthing should be done.
At other locations the coil earthing may be adopted. The coil earthing consists
of 10m length of 8 SWG. G.I. wire compressed into a coil 450 mm length and
50 mm dia and buried 1500 mm deep.
In order to prevent unauthorised persons from
climbing any of the supports of HT & L T lines
without the aid of a ladder or special
appliances, certain anti-climbing devices are
provided to the supports. Two methods
generally adopted are (i) barbed wire binding,
for a distance of 30 to 40 cm a height of 3.5 to
4 m from ground level, (ii) clamps with
protruding spikes at a height of 3 to 4m.
Testing and Commissioning
When the line is ready for energisation, jt should.be thoroughly
inspected in respect ofthe following.
1. Poles-Proper alignment, concerting and muffing.
2. Cross-arms-Proper alignment.
3. Binding, clamps and junipers - To check whether these are hi reach.
4 Conductor and ground wire - Proper sag and to check whether there
are any cuts, etc.
5 Guys : To check whether the Guy wire is tight and whether the Guy
insulators are hi
6. Earthing System: To check whether the earthing connections of
supports and fittings are intact. Measure earth resistance with earth
After the visual inspection is over and satisfied, the conductor is
tested for continuity/ ground, by means of megger. At the time of
testing through megger person should not climb on the pole or touch
the guarding, conductor, guy wire etc.
(1) Before charging any new line, it should be ensured that the required
inspection fee for the new line is paid to the Electrical Inspector and
approval obtained from him for charging the line.
(2) The line should be energised before the authorised officer.
(3) Before energising any new line, the officer-in-charge of the line shall
notify to the workmen that the line is being energised and that it will
no longer be safe to work on line. Acknowledgement of all the
workmen hi writing should be taken in token of having intimated
(4) Wide publicity by Tom-toming should be arranged hi all the localities
through which the line, that is to be energised passes, intimating the
time and date of energising and warning public against the risk hi
meddling with the line.
(5) The Officer-in-Charge of the line shall personally satisfy himself that
the same is in a fit state to be energised.
A model for functional responsibility is
attached for Divisions/Sub Divisions /
individuals. This could be drawn on the basis
of responsibility by the organisation
Particular of Works Sub Division Division
AE (CSD) EE ( O & M) EE ST/RE (Const)
1 Preparation and sanction of estimate - - X -
2 Issue of Work Order - - X -
3 Survey and routing - X - Supervision
4 Forest Clearance - - - X
5 PTCC Approval - - - X
6 Marking of pole location - X - Supervision
7 Identity of materials - X - Supervision
8 Material lifting - X - Supervision
9 Entry of materials received in register - X - Checking
10 Shifting of material to the site - X - Supervision
11 Pit digging and pole erection X Supervision - Sample Check
12 Fixing of V cross arm top clamp and DC cross arm X Supervision - Sample Check
13 Fixing of insulators X Supervision - Sample Check
14 Fixing of stays /Guays X Supervision - Sample Check
15 Stringing of conductor X Supervision - Sample Check
16 Fastening of conductor with insulators X Supervision - Sample Check
17 Guardining at crossing X Supervision - Sample Check
18 Fixing of danger Board X Supervision - Sample Check
19 Pole painting and numbering X Supervision - Sample Check
20 Fixing of anticlimbing device X Supervision - Sample Check
21 Connecton of earth X Supervision - Sample Check
22 Test charge of the line X Supervision - Sample Check
23 Preparation of completion report X Supervision - Sample Check
24 Handling over to O &M X Supervision - Sample Check
Sl. No Particular of Works Sub Division Division
Constn Gang Head AE (CSD) EE ( O & M) EE ST/RE (Const)
1 Preparation, Sanction of estimate - - X -
2 Issue of work order - - X
3 Survey and routing X Supervision - Sample check
4 Marking of pole location X Supervision Sample check
5 Identity of materials - Sample check
6 Lifting of material - Supervision
7 Entry of materials to the site - check
8 Shifting of materials to the site - supervision
9 Pit digging and pole erection X Supervision Sample check
10 Fixing of LT cross arm with shackle
insulators and earth
X Supervision Sample check
11 Fixing of stays /Guys X Supervision Sample check
12 Stringing of conductor X Supervision Sample check
13 Fastening of conductor with insulators X Supervision Sample check
14 Guardining at crossing X Supervision Sample check
15 Fixing of danger board X Supervision Sample check
16 Pole painting and numbering X Supervision Sample check
17 Earth connection X Supervision Sample check
18 Test charge of the line - Sample check
19 Preparation or completion report - Sample check
20 Handing over to ( O & M) - checking
REC has standardized the following sizes and types of supports for
11 KV and LT lines
Type Length Voltage Max Spn Type of With or
PCC 7.5 Mt 11 KV 107 Triangular Without
PCC 8.0 Mt 11 KV 107 Triangular With
PCC 7.5 Mt 415/240 V 107 Horizantal With
PCC 8.0 Mt 415/240 V 67 Vertical With
Stringing of the Line conductor
REC has standardized the following sizes of conductors for 33 KV, 11 KV and LT lines
Voltage Class No. and Diameter of wire Type of conductor
33 KV Lines i) 7/3 .35 mm (50 mm2) Al ACSR
ii) 7/4.09 mm (80 mm2) Al ACSR
iii) 6/4.72 mm + 7/1.57 mm (100
11 KV Lines i) 7/2.11 mm( 20 mm2) ACSR
ii) 7/2.59 mm (30 mm2) ACSR
iii) 7/3.35 mm (50 mm2) ACSR
LT Lines i) 7/2.11 mm ( 20 mm2) ACSR
ii) 7/2.59 mm (30 mm2) ACSR
iii) 7/3.35 mm ( 50 mm2) ACSR
iv) 7/2.21mm (25 mm2) AAC
v) 7/3.10 mm (50 mm2) AAC
Transformer Body /AB
Terminal of Pole
11KV /433-250V Distribution
Location Of Earth Pits And
1. The connections To The Three_earth Electrodes
Should Be As Follows;
(a).To one of The Earth Electrodes On Either Side Of
Double Pole Structure (X OR Y)
(1).One Direct Connection From Three 11Kv Lightning
(11).Another Direct Connection From The L.T Lightning
Arresters, If Provided
(b).To Each Of The Remaining Two Earth Electrodes.
(1). One Separate Connection From The Neutral (On The
Medium Voltage Side ) Of The Transformer
(11). One Separate Connection From The Transformer.
(111) One Separate Connection From The
Earthing Terminal Of The pole
2. 4mm (8Swg)G.I.Wire Should Be Used For Earth
All Dimensions in mm
33KV and 11KV Line Maintenance
Required to minimize interruptions and improve efficiency of power supply.
OH lines be inspected periodically for maintenance purpose to detect
any fault & repairs should be programmed.
OH lines be patrolled periodically at intervals (say 1month) at ground level
while line is live.
Pre mansoon inspection should be programmed before mansoon.
Nature of faults-
loose sags, snapping of conductors, tree branches touching line
conductors, tilting of cross arms, insulator failures (puncture) etc
1. Power Station Stepup Sub Station
2. Primary transmission line
3. Grid Sub Station
4. Secondary transmission line
5. HV Sub Station Primary Distribution line
6. Primary Distribution Line
7. Distribution transformer Station
8. Secondary Distribution Lines
Classification of Distribution
Type of Electric System -> AC or DC ; if AC
single Phase or Polyphase
Type of Delivery System-> Radial, loop or
network; Radial Systems include duplicate or
throw over systems
Type of construction: over load or
Principal features desired ->
Safety, smooth and Even flow of Power ;
There are three different ways through
which the primary distribution lines can
1. The radial Primary circuit
2. The loop primary circuit
3. The ring main system
When power in supplied to the consumers
through the secondary distribution system one
of the following arrangements used
3.Net work system
The main purpose of planning is
1. to make the system economical while
conforming to electricity rules of the
2. to minimize looses and maintain regulations
within the permissible limits
For proper planning of a distribution system load
survey and load fore casting of area are
In planning of an electrical distribution system it
is necessary to know three basic things.
The quanitity of the product or service desired ( per
unit of time)
The quality of the Product or service desired
The location of the market and the individual
Over Head Lines:
The rules have seen framed for
Providing quality service to the people
to lay down technical parameters and
specifications of materials to follow standards in
construction, installation protection, operation &
to follow laid – down principles & procedures with
accountability to people.
The main features fo O.H lines in
the rules are
Supports - Factor of safety 2 to 3.5
Conductors - Factor of safety 2.0
Stay wires, Guard & Bearer wires - 2.5
Wind load - 50 to 100 Kgs/m2 ( 150 Kg/m2)
Across Street Along Street Else where
i) up to 650V 5.791 mt 5.486 mt 4.572 mt
ii) 650 V to 33 KV 6.096 mt 5.791 mts 5.182 mt
Vertical clearances above
i) up to 650V 2.439 mt ( 8ft) 1.219 mt( 4 ft)
ii) 650 V to 33 KV 3.64 mt (12ft) 1.82 mt ( 6ft)
Voltage Regulation -
Frequency Variation -
Maximum clearance between supports
Insulator & Insulator fittings
Planning the 33/11 KV Sub Station
Involves the following Steps:
1. Tentative location based on available data of
the 11 KV Network
2. Capacity of the Sub Station
3. Selection of site
Orientation of the Sub Station
Planning of the Sub Station
Main equipments of Sub Station are
3. A) Circuit Breakers
B) HT fuse (HG Fuses)
4.Isolating Switches (Isolators)
5.Bus Bar arrangements
a) Current transformers
b) Potential transformers
9.Control and relay panels with relays, meters etc.,
10.Battery and Battery chargers
i) Power Cables
ii) Control Cables
16.Fencing, Retaining wall
17.Illumination, firefighting equipment, quarter
BASIC CONCEPT OF PLANNING
Awareness of the causes and their effects itself
would reduce the system irregularities to some
extent. All these difficulties ultimately lead to a
low voltage profile in the system.
The poor voltage profile causes loss of
equipments and energy. Thus, maintenance of
the voltage profile to keep the consumer voltage
at the declared level allowing the deviation
within the permissible limits would keep the
losses at control. The consumer voltage may be
kept at the desired level by controlling one or
more of the following variable on which it is
1. The voltage received at the grid sub-station
2. The range of tap changing gear available with the power transformers at
the grid sub-station
3. The percentage impedance of the power transformer at the grid sub-station
4. The voltage drop in the sub-transmission line (33 K.V. or 66 K.V. lines)
5. Tap available with the transformer at the primary distribution sub-station
6. The percentage impedance of the transformer at the PDS vii) vii) The
voltage drop in the primary transformers feeders
7. The percentage impedance of the secondary distribution transformers and
taps available with them
8. The voltage drop in the secondary distribution feeders
9. The voltage drop in the joints
10. Voltage regulators/booster and/or capacitors installed in the system
GUIDELINES FOR PLANNING
The following guidelines may be followed
while planning secondary distribution system
expansion or new:
1. Study the area carefully and estimate the load
densities, present and future.
2. Select the transformer size and conductor size from
the result of the optimization studies with due
consideration for the existing sizes.
3. Estimate the present status of the system with respect
of voltage regulation and losses
4. Estimate the optimal length of the feeders and the
optimal loading limit.
5. Mark the feed area for each secondary distribution
6. Determine the load centre
7 In the case of the existing distribution system, if the
transformer is not in the load centre, follow the
Check the length of the feeders from the load centre to the far-
end points and compare with the optimal lengths. If the length is
more than the optimal ones, do not try to shift the transformer to
the load centre
Taking the optimal feeder length into consideration, divide the
area into different feed area zones .
Determine the load centre of the different load Feed area zones,
following the procedure suggested in appendix.
If the existing location of the transformer does not fall into one of
these centres, then shift die transformer to the nearest centre
and provide new transformers at the other centres.
Modify the feeder layouts, keeping them as straight as possible
and the lengths within optimal limits
Operating instructions shall contain:
1. The designation of all the officers concerned to be
intimated in the event of abnormal situation of any
equipment /line and their telephone numbers.
2. General duties of the operator, the various checks he
has to perform on various equipment in shift.
3. Telephone number, and addresses of 'Fire'
'Ambulance1 'Hospitals' in addition to the telephone
number of all officers.
4. Single line layout of sub-station route map of each
feeder indicating cut points, roads, crossing etc.,
5. Detailed break down operations of lines.
Example of Operation Instruction of Lines/Equipments
CASE I 11 KV FEEDER
00.00 11 KV Feeder 'A' Trips note relay indications; Reset
00.01 Charge the Feeder If OK, supply is restored temporary fault is
cleared If trips, note Line relay indications, reset
00.03 Charge the Feeder,
If OK, Temporary fault is cleared supply is restored,
If trips, note the relay indications and reset
Examine the switch yard for any visible fault, If no fault is noticed, open
00.04. Charge lne OCB if OK, Trip the OCB,
Close the line isolator charge the feeder.
If OK Temporary fault is cleared, supply is 2: restored if trips, note the
relay indication & declare the feeder faulty. Inform all the
if the breaker trips at the time of test charging
the same, inform maintenance staff. In case, if at any time of test
charging the feeder, the power - transformer H.V. L.V. or group
control or incoming trips, declare the feeder faulty
00.00 33 KV feeder breaker (p) Trips at 132 KV sub station 'M'
00.01 Note the relay indications, reset, charge the feeder, If OK supply is restored.
If trips note relay indications, reset
00.03 Charge the feeder If OK supply is restored If Trips note relay indication, open
00.08 Charge the OCB, If trips, inform maintenance personnel for rectification If OK,
hand trip the OCB close line isolator charge the feeder. If OK supply is restored,
if trips proceed as
Contact Station 'A'
Ask the operator to open incoming isolator and 33 KV out going line isolator Ask
him to examine the switch yard and report.
If OK ask operator at 'A' to restore supply to the station 'A' and inform, If at the
time of charging any 11 KV feeder at station 'A' of the 33 KV
breaker 'P1 trips, isolate the 11 KV feeder, restore normalcy. If operator 'A'
confirms that station 'A1 is normal, contact station 'B' operator, Ask him if the
switch yard is normal. Ask him to open in-. coming 33 KV line isolator after 'B'-l
is opened hand trip 'P' ask operator at 'A' to close outgoing isolator A-2.
Charge the feeder.
If OK supply is restored
If trips, declare 33 KV line between ASB is faulty
Power Transformer Trips on
(A) Winding Temperature
(B) Bucholtz Relay
(C) Differential Relay
(D) O/L Relay
A WINDING TEMPARATURE :
Note the winding temperature is more than the set temperature?
If so, is the transformer overloaded? If sot
reduce the load on the transformer.
.Are the cooler fans, oil pumps functioning satisfactorily?
Is fuse blown out in Fan/Pump? If so, rectify.
Ts there any shortive between the contacts of
winding temperature relay due to vermin or ingress 'of moisture, if so take
B Bucholtz Relay:
Isolate the power transformer check bucholtz relay, is there any gas
collected, if so arrange for testing. If not check any shorting of the contacts
Megger the power transformer close HV/LV breakers. If OK hand trip,
inform maintenance personnel for check up
C Differential Relay
Isolate the power transformer inform maintenance personnel for detailed
D O/L Relay.
Check if any feeder relays indication is received without the feeder breaker
tripping isolate the feeder. Check the yard. If ok check is power transformer
OF EQUIPMENT IN THE SUB
Guide Lines of Erection of
33/11KV Sub Stations
The erection of Power Transformers comprises of following Works :
Unloading of Transformer form Tractor Trailer/Lorry at
the Sub Station.
Stacking aside wherever the Power Transformer plinth
etc are not ready.
Moving the transformer on to plinth
Assembly of all the mounting , accessories etc.,
Filling and topping up of transformer oil
Oil circulation through filter if required.
Unloading of Transformer form Tractor Trailer/Lorry at the Sub
Station :. Generally the higher capacity Power Transformers
are sent from the manufacturer duly dismantling, conservator
tank Radiators, Piping etc. in either tractor trailor or lorry. For
unloading the main tank from the vehicle we may use a
suitable crane or do manually. When manual unloading is
done, the following T & P and equipment are required.
Wooden Sleepers 8’ to 12’ length, 12” width, 6” or 8” thick– 40 Nos
10 Ton tirfur with rope -1
20 ton winch with rope -1
5 Ton Chain Pulley block -1
2 Ton Chain Pulley block -1
Hydraulic Jacks 10 Tons Capacity - 4 Nos
Wire rope – ¾” size - 20 Mtrs
Manila rope of different sizes & lengths
Rail Poles minimum 20 ft length - 4 Nos
General T & P
Wooden Packing pieces ¼”, ½” ,1”, 1 ½” , 2”, 4” thick – Set
Plat form up to the height if tractor trailor/ lorry is to be
built up with wooden sllepers. By using hydraulic jacks,
the main tank is to be lifted on all sides to a height so
that the rail poles can be in serted at the bottom of main
tank and main tank rests on rail poles The other end of
poles are to be on the wooden sleeper plat form Now
with the help of winch or tirfur the transformer main tank
will be dragged on rail poles up to wooden sleeper plat
form. When the main tank could be dragged correctly
over the platform, with the help of hydraulic jacks the
transformer main tank is raised slightly and rails are
removed, The main tank is (lowered to one sleeper
height by slowly removing the top sleepers on e after
another). During removal of sleepers following step by
step operations are done.
1. Keep two jacks under jack pads of transformers along the top sleeper ( Which is be removed
one jack each on either and of sleeper)
2. Operate the Jacks so that lifting pad of jacks are tightly positioned under transformer jack
3. Now slowly pressurize jacks equally on both sides simultaneously so that one side of the
transformer tank is raised slightly to enable to draw out the sleeper.
4. Now Place the wooden packing pieces one over the other by the sides of Jacks up to jack
5. Now remove the sleeper slowly with out hitting the jacks
6. Slowly lower the transformer tank, by releasing pressure in jacks slowly (both
simultaneously) and removing the packing pieces one after another
7. Now remove the Jacks, when the side of transformer is securely resting on the next bottom
8. Now place the jacks on the other side of the power transformer tank and carry pout above
operation and remove other side sleeper also.
9. After the transformer tank lowered to the height of one sleeper height, then sleepers are to
be placed along the rout to the plinth on which PTR is to be erected.
10. ON the sleepers rail poles are to be kept duly inserting under the tank and transformer tank
is to be dragged close to the plinth.
11. After dragging the transformer tank nearer to plinth the transformer tank is to be raised to the
level slightly above the plinth top level by using sleepers & Hydraulic jack
12. Then the Power Transformer tank is to be dragged on to the plinth slowly with the help of rail
poles and winch tirfor.
13. When the transformer tank is correctly positioned placed on the plinth further work is to be
Assembling of Transformerfittings , Mountings
The radiator dummy plates are to be removed
and ensured that no foreign material, moisture
is accumulated in the radiators, the radiators
can be fitted by using sleepers, Jacks/ Chain
pulley block. The radiator valves shall be
inclosed position only
Conservator tank is to be fixed by lifting the
Slicagel breather, vent pipe, Bucholtz relay
thermometes are to be fixed on to the power
Filling /toping Up of oil : Now New filtered tested
Transformer oil is to be filled in to transformer through
suitable clean pump & pipes slowly through one of the
top valves while filling oil slowly open bottom valve and
air releasing dummy of one radiator. When oil is filled
up to top of radiator then close the air releasing dummy
immediately open top valve of radiator. In the same way
all the radiators are to be filled and conservator tank is
filled up to 50% level approximately.
Then release the air once from all air releasing points.
Checkup oil level in the OLTC unit
Then earthing and jumpering is to be done as per
Erection of Breakers
Before Erection of Breaker, suitable plinths are to be
constructed duly embedding the foundation bolts as per
distances specified in the manual of the breaker
Once the curing period of plinth is completed and plinths
are perfectly cured, first the mounting structures of the
breaker is to be placed on plinth in position. Then the
breaker is to be brought near to plinth on rail poles or MS
Channels, lifted & erected with the help of chain pulley bock
& ropes. During erection of Breaker for tying the Breaker to
ropes lifting ring ears provided on the breaker are to be
used but not bushings or bushing collar frames
CT base channels are to be fixed and CTs are to be
positioned on the channels already fixed
Jumpering from Bus Isolators to Breaker; Breaker to CTs ;
CTs to line Isolators is done with Panther Conductor
through suitable clamps.
Double earthing of Breaker body & CT Body is to be done.
Erection of PTs, L Ass, CTs :
Before erection of these equipment base
dimension, distances between mounting holes
are to be noted.
Suitable holes are to be drilled in seating
structure on which the equipment is to be
The equipment is to be lifted by using lifting
holes provided to the equipment with the help
of chain pulley block manila rope
After positioning on the channels the base is to
be fixed to base channel with suitable coated
or GI bolts, Plain washers spring washers and
Earthing & Jumpering is to be done as per the
Electrical earthing is designed primarily to render electrical installation safe. The
purpose of earthing are :
1. Protection to the plant
2. Protection to the personnel and
3. Improvement in service reliability
Non- current carrying parts with conducting surface such as tanks of Power
Transformers, and frame work of circuits breakers, structural steel work in
switch yard instrument transformer cases, lightning arresters and armored
cables armoring should be effectively grounded for protection of equipments
and operating personnel. Earth connections of all equipments should be
made in duplicate.
Connecting lead should have sufficient current carrying capacity.
L A s should have independent earth electrode which should be inter connected
to the station grounding system.
All paints, enamel, seals should be removed from the point off contact of metal
surfaces before earth connections are made.
The resistances of earth system should not exceed 2 ohms for 33/11 KV Sub
But in the sub stations of Distribution companies Earth resistance Maximum of 1
Ohm is maintained.
Suitable grounding mat should be provided in the sub station yard.
In a Sub Statio n the fo llo wing shallbe e arthe d.
The neutral point of the systems of different voltages
which have to be earthed.
Apparatus, frame work and other non-current carrying
metal work associated with each system, for example
transformer tanks, switch gear frame work etc.,
Extraneous metal frame work not associated with the
power systems, for example, boundary, fence, steel
The earthing Means connecting of Electrical equipment,
machinery or an electrical system with the general
mass of earth is termed as earthing or grounding
FUNCTION OF AN EARTHING SYSTEM
The earthing system must provide an environment which
is free from the possibility of fatal electric shock.
The earthing system must provide a low impedance path
for fault and earth leakage currents to pass to earth.
The earthing conductors must possess sufficient thermal
capacity to pass the highest fault current for the required
The earthing conductors must have sufficient mechanical
strength and corrosion resistance.
A Sub Station earthing system has to satisfy
Earthing can be bro adly divide d as :
System Grounding ( System Earthing)
Equipment Grounding (Safety Grounding)
It is a connection to the ground of a part of the plant
forming part of the operating circuits for example the star
point of the transformer or the neutral conductor. The
grounding of the lighting, arrestors also comes under the
head of system grounding. The provision of system
ground reduces to considerable extent the magnitude of
the transient over voltages and there by increases the
life of electrical equipment besides minimizing the
Thus the fundamental purpose of system ground is the
protection of installation and improvement in quality of
service. The system ground also will ensure the safety
of the personnel to some extent, as it helps to clear the
Safety Grounds (Equipment Grounding)
It is a connection to the ground of non-current carrying parts of the
equipments like Motors, Transformer Tanks, Switchgear enclosures,
Metallic enclosures of all electrically operated equipments and also the
installations used to carry/ Support electrical equipments. The frames of
the equipments, if not earthed when come into contact accidentally with live
parts will have potential with reference to the ground. The potential
difference, when shunted between the hands and the feet of a person
touching the frame, produces current through the body which can result in a
fatality. By connecting body which can result in a fatality. By connecting
the frames to a low resistance ground system, a sufficiently high current
will flow into the ground when accidentally the live parts of the equipment /
Machinery touch the frames, and consequently saves the operating
personnel from fatal accidents.
Thus the equipment grounding is basically intended to safeguard to a great
extent from the hazards of touch voltages. The safety ground is so
designed that the potential difference appearing between the frames and
the neighboring ground is kept within safe limits.
Separation of systemand safety grounds:
During ground fault conditions, the fault current flows via
the system ground. When the system and safety
grounds are inter connected, the fault current flowing
(via) the system ground rises the potential of the safety
ground. Also the flow of current to safety ground results
in hazardous potential gradient in and around sub
station. In view of the above it is some times suggested
that separate system and safety grounds will avoid the
danger arising due to potential gradients. The idea is
that by connecting the system ground to a separate
earthing system situated in a in accessible spot, the
ground fault current does not flow through the safety
ground. However, this separate system of grounds has
many disadvantages and can be more hazardous as
With separate grounds we can avoid danger due to
potentials only for faults outside the stations.
Short circuit currents will be more if the fault occurs in
the sub stations.
The resistance may be more and in some cases
sufficient currents may not flow to operate the relays.
For effective separation of the earthing systems, the
system ground shall be installed at a distance of at
least twice the diagonal length of the sub station which
is covered by safety grounding. The neutral of the
transformer has to be connected to this remote
earthing by means of insulated leads. Even with this
arrangement one cannot always be sure about the
complete isolation of the two systems and there is
always a chance of inadequate electrical connection
through buried neutral pipes etc., Hence, this is
impracticable, complicated and costly. It is therefore a
common practice to install a common grounding
system and design the same for effective earthing and
safer potential gradients.
System earthing is governed by provisions of Rule - Of
I.E Rules, 1956. Unearthed systems have been tried
and due to the phenomenon of Arcing Grounds
associated with them, theses have been abandoned,
excepting in a few cases of power station auxiliaries
supply systems where other arrangements are made for
indicating earth faults. In an ungrounded system the
insulation of all the equipments, lines etc, will have to be
much higher values as compared to those of
equipments and lines of a grounded system. This
aspect greatly reduces the costs and ensures more
Types of SystemEarthing:
Earthing through a resistance.
Earthing through a reactance.
Earthing through a Peterson coil
Earthing directly or solid earthing.
Sub Station Earthing
Because of the difficulties and disadvantages involved in
marinating the system grounding and safety grounding separately
it is the common practice now to have a combined grounding
system at the sub stations. Provision of adequate earthing in a
sub station is extremely important for the safety of the operating
personnel as well as for proper system operation. The Primary
requirements of a good earthing system in a sub station are.
The impedance to ground should be as low as possible. The
impedance of the earth system shall not exceed the following
limits in the sub stations
Power Stations 0.5 Ohms
Major Sub stations above 110 KV 1.0 Ohms
Minor Sub Stations below 110 KV 2.0 Ohms
Distribution Transformer Station 5.0 Ohms
Transmission line supports 10.0 Ohms
The Step and touch potentials should be within safe limits
Touch Potential :
Touch potential is the potential difference between the ground surface
potential where a person is standing and the potential of his
outstretched hand (s) which are in contact with an earthed structure. It
is normally assumed that a person’s maximum reach is 1.0 meter.
Step Potential :
Step Potential is the potential difference between outstretched feet, at a
spacing of 1.0 meter without the person touching any earthed structure
The maximum potential difference between the centre of a mesh in an
earth grid, and an earthed structure connected to the buried grid
conductors. It is worst case scenario of a touch potential.
The transferred potential is a touch potential which is transferred some
distance by an earth referenced metallic conductor. For example,
consider a screened cable connecting two sub stations which are some
distance apart. If a person disconnects the earthed termination at one
end of a screened cable he may be subjected to the full ground potential
rise occurring due to an earth fault. This can be a very high touch
To keep the ground impedance as low as possible and
also to have satisfactory step and touch voltages, an
earthing mat will be buried at a suitable depth below the
ground and it is provided with grounding electrode at
suitable points. All the non-current carrying parts of the
equipments in the sub stations are connected to this grid
so as to ensure that under fault conditions, none of these
parts are at higher potential than the grounding grid.
Under normal conditions, the ground electrode make little
contribution to lower the earth resistance; they are,
however, desirable for marinating low value of resistance
under all weather conditions, which is particularly important
where the system fault currents are heavy.
Earthing in a sub station must conform to the requirements
of the Indian Electricity Rules and follow the directives laid
down in section I and III of IS : 3043-1966. the earthing
system has to be designed to have a low overall
impedance, and a current carrying capacity consistent with
Duration of fault.
Magnitude of the fault current.
Resistivity of the underlying strata.
Resistivity of the surface material
Material of the earth electrode.
1. 100 X 16 mm and 75 X 8mm size MS steel flats are being ordered for forming the
earthing system for EHT Sub station and 33/11 KV Sub Stations respectively
2. Earth mat shall be formed with the steel flats buried in the ground at a depth of
3. The earth mat shall extend over the entire switchgear yard and beyond the security
fencing of structural yard by at least one meter.
4. The outer most peripheral earthing conductor surrounding the earth mat shall be of
100 x 16 mm size MS flat.
5. The intermediate earthing conductors forming the earth mat shall be of 75 x 8 mm
6. All the risers used for connecting the equipment steel structures etc., to earth mat
shall be of 50 x6 mm size excepting for earthing of L A s and transformer neutrals
for which 100 x 16 mm or 75 x 8 mm size shall be used.
7. All Junctions (crossing of the steel flats while forming the earth mat and taking
risers from the earthmat for giving earth connections to equipments, steel structural
conducts, cable shearths shall be propersly welded.
8. Proper earthing lugs shall be used for connecting the earth terminals of
equipments to the earthing steel flat.
9. Provisions shall be made for thermal expansion of the steel flats by giving suitable
10. The earth mat shall be formed by placing 75 x8mm MS flat at a distance 5 meters
along the length & breadth of the sub station duly welding at crossing.
11. All the equipments, steel structural, conduits, cable sheaths shall be solidly
grounded by connecting to the earthing mat at least two places for each.
12. The ground mat of the switchyard shall be properly connected to the earth mat of
the control house at least at two points.
13. welding is done shall be given a coat of black asphalitic varnish and then covered
14. All paints, enamel and scale shall be removed from point of contact in metal
surfaces before applying ground connections.
15. The risers taken along the main switchyard structures and equipment structures
up to their top) shall be clamped to the structure at an interval of not more than
one meter with ground connectors.
16. 75 X 8 mm ground conductor shall run in cable trenches and shall be
connected to the ground amt at an interval of 5 meters.
17. Grounding electrodes 2.75 Mtrs length 100 mm dia 9 mm thickness CI Pipes
shall be provided at all their peripheral corners of the earthiong mat and also at
Distance of 10 Mtrs along length & width of switch gearand in the entire switch
18. The grounding electrodes shall be drived into the ground and their tops shall be
welded to a clamp and the clamp together with the grounding shall be welded to
the ground conductor.
19. The switchyard surface area shall be covered by a layer of crushed rock of size
25 x 40 mm to a depth of 100mm
20. Transformers and L A s and single phase potential transformer shall be
provided with earth pits near them for earthing and these earth pits in turn shall
be connected to the earth mat.
21. Power Transformers neutral shall be provided with double earthing. Neutral
earthing and body earthing of power transformers shall be connected to
separate earth electrode.
22. the entire earthing system shall be laid with constructional conveniences in the
filed, keeping in view the above points.
23. The joints and tap-offs where welding is done shall be given a coat of black
THE PERMISSIBLE LIMITS OF STEPPOTENTIAL ANDTOUCH
POTENTIAL SHALL BE
Maximum Acceptable step Voltage
Fault clearance times
Fault clearance times 0.2 Seconds 0.35 Seconds 0.7 Seconds
On soil 1050 V 600 V 195 V
On chippings 150mm) 1400 V 800 V 250 V
Maximum Acceptable Touch Voltage
Fault clearance times
Fault clearance times 0.2 Seconds 0.35 Seconds 0.7 Seconds
On soil 3200 V 1800 V 535 V
On chippings 150mm) 4600 V 2600 V 815 V
Item Material to be used
1 Grounding Electrodes CI pipe 100 mm (inner dia)
Meters long with a flange
at the top
2 Earth mat 75 X 8mm MS Flat
3 Connection to between
electrodes and earthmat
75 X 8mm MS Flat
4 Connection to between
earth mat and equipment
50 x 6mm MS Flat
The following are the minimum sizes of materials to used.
The size of trench for burying earth mat shall be 300mm
X 500mm. The earth mat shall be buried in the ground at
a depth of 500mm. The earth mat shall extend over the
entire switch yard.
All junctions and risers in the earth flat shall be properly
welded by providing additional flat pieces for contact
between two flats
Provision shall be made for thermal expansion of steel
flats by giving smooth circular bends Bending shall not
cause any fatigue in the material.
After welding, the joints and tap offs shall be given two
coats of Bitumen paint
Back filling of earth mat trench to be done with good
earth, free of stones and other harmful mixtures. Back fill
shall be placed in layer of 150mm, uniformly spread along
the ditch, and tampered by approved means
Earth electrodes shall be of CI pipe 100mm (inner dia) 2.75
meters long with a flange at the top and earth flat already
indicated and shall be connected to earth grid in the Sub
Station. All earth pits are to excavated and the preferred
backfill is a mixture of coke and salt in alternate layers. A
suitable size cement collar may be provided to each earth
electrode. All bolted earth mat connections and strip
connections to plant and equipment panel will be subject to
strict scrutiny. Transformer Neutrals shall be connected
directly to the earth electrode by two independent MS strips
of 75 X 8mm. The transformer body earthing shall be done
with 75 X 8mm flat. The independent connections of MS
strips with earth mat shall be given on either side of the
Transformer. All contact surface must be filled or ground flat
ensures good electrical connection, and the contact surface
shall be protected with a contact lubricant. Following this all
connections shall be painted with heavy coats of bituminous
black paint so as to exclude moisture.
Neutral connection earth pipe shall never be used for the
A separate earth electrode shall be provided adjacent to the
structures supporting Lightning Arrestors. Earth connection shall
be as short and as straight as practicable. For arrestors mounted
near for protecting transformers earth conductors shall be
connected directly to the tank.
An Earthing pad shall be provided under each operating handle of
the isolator and operating mechanism of the circuit breakers.
Operating handle of the isolator and supporting structures shall
be bonded together by a flexible connection and connected to the
All equipment and switchgear etc., erected shall be earthed as
per I.E Rules 1956.
SELECTION OF SITE FOR
While selecting site for Sub Station the following points should
be kept in view.
1.The Sub Station should be as near the load centre as feasible.
2.The Sub Station should be far away from the obstructions to
have permit easy and safe approach of HV over heads
3.The Sub Station should be easily accessible to the road to
facilitate transport of equipment
4.As far as possible near a town and away from built up areas
5.Sufficiently away from the areas where military rifle practices
6.The Site should have as far as possible good drinking water
7.The Sub Station should not be located within two miles of any
8.The site selected should have sufficient area to properly
accommodate the Sub Station equipment, Structures, Buildings
and also future extensions.
DESIGN & LAYOUT
The factors that determine the final layout of Sub
1.No, of incoming and out going feeders
2.Expected loads demand on Sub Station
4.Facilities of Operation and Maintenance.
5.The normal weather conditions
6.Expected fault levels at the busbars
Selection of Arrangement of
Switching of Functions
Relation of the Sub Station to the system as a
whole i.e the effect of a disturbance at a Sub
Station on continuity Power Supply in the
Relation of the individual switching functions to
the other system feeders & functions
Immediate as well as long range adaptability to
Evaluation of Bus Bar fault contingency.
Degree of switching flexibility to be provided for
equipment and line maintenance purposes.
P & T LINE CROSSINGS:
Posts & Telegraphs Department has to give clearance
for the crossing arrangement of power line with P &
T lines. A detailed sketch showing profile of
crossing span, angle of crossing and electrical
clearance shall accompany the proposal along with
the prescribed questionnaire duly answered.
Clearance for the power lines will be given if the
following conditions are fulfilled.
(i) The angle of crossing of the power line with the P&T
line is not less than 60o
(ii) The nearest power conductor shall be away from the
telecom line by not less than the distances tabulated
below under maximum sag conditions.
Clearance is to be obtained from the Railway Authorities
for the proposed power line crossing railway track. A
sketch showing full particulars such as Vertical
Clearance of the lowest power conductor over the
railway track, angle of crossing and the shortest distance
from the railway track from the nearest tower shall
accompany the proposal for railway crossing. The
prescribed questionnaire duly answered and Factor of
Safety Calculations shall also be sent along with the
proposals for railway crossing.
Clearance for the railway crossing will be accorded if the
following conditions are fulfilled. The power line shall
cross the railway track at an angle not less than 60o
The crossing span shall not exceed 80% of the normal
The minimum clearance of the lowest power conductor
over the railway track shall be as per the statement 2-1
on page II-4.
The clearance over the Railway Track and the
bottom most conductor for different Transmission
lines shall not be less than the distances below
under max. sag conditions.
For 132 KV Lines : 14.60 Metres.
For 220 KV Lines : 15.40 Metres
For 400 KV Lines : 17.90 Metres.
STRUCTURES DESIGN LOADS:
WIND PRESSURE ON STRUCTURES: In regions other than coastal regions 125
Kg/Sq. m. on 1.5 times the projected area of members of one face for latticed
structures and other non-cylindrical objects and on single projected area in the case
of other structures. In coastal regions the wind pressure may be assumed as 260
WIND PRESSURE ON CONDUCTORS : In regions other than coastal and hilly
regions, 75 Kg/Sq.m on two thirds projected areas. Coastal areas 125/150 Kg. /
Sq.m. In hilly regions 90 Kg. / Sq.m
Maximum tension per conductor of transmission line conductors strung from terminal
towers to station structures or strung buses :
1) 33 KV and 11 KV 450 Kg.
2) 66 KV – 450 Kg.
3) 132 KV and 220 KV – 900 Kg.
4) 400 KV – 1000 Kg.
5) Ground wire Tension – 450 Kg.
MAXIMUM SPANS OF LINES ADJACENT TO STATIONS
33 KV and below – 60.00 m
66 Kv and above – 150.00 m
UPLIFT OF ADJACENT SPANS:
Maximum slope (mean of the 3-phases) at the point of attachment 1 : 8 above
FACTOR OF SAFETY:
FOR STEEL: 2.0 based on maximum loading
conditions (on elastic limit for tension members and
crippling load for compression members).
FOR R.C.C. : 3.5 on ultimate breaking load
FOR SAFETY AGAINST OVER TURNING:
Steel–2, R.C.C – 2.0
The substation busbars can be broadly classified in
to the following three categories:
Out door rigid tubular busbars
Outdoor flexible ACSR or aluminum alloy busbars
These are of following forms:
1.Tubular aluminum conductors are supported on post
insulators made of porcelain. These are bolted to get
2. ACSR/AAC conductor is supported at each stringing
point on strain insulator .Such flexible busbars are
used for long spans with (beams and columns)
Commonly used Sizes of Aluminum Pipes:
Nominal Dia .
(External / Internal)
11 kV, 33 kV & 66 kV 42/35
Voltage Class 430 kV / 230 kV
Size 4" IPS (EH)
Outer diameter 114.30 mm
Inner diameter 97.20 mm
Thickness 8.51 mm
Cross sectional area 2825.61 sq.mm
Type of designation 63401 WP as per IS:5082
Tensile strength 20.5 kg/sq.mm
Weight 7.7 kg /m
Current rating at 75º C 3000 Amps
STRUNG BUS BARS
The materials in common use for strung type Bus
bars are ACSR conductors. Bundled conductors
(two or four) are used where high ratings for bus
bars are required. The size of conductors commonly
11 KV - 61/3.18 mm ZEBRA Twin
33 KV - 61/3.18 mm ZEBRA Twin
132 KV - 61/3.18 mm ZEBRA Single/Twin
220 KV - 61/3.53 mm MOOSE Single/Twin/Quadruple
400 KV - 61/3.53 mm MOOSE Quadruple
11kV, 33 kV
& 66 kV
61/3.18mm ZEBRA ACSR 420 737
37/3.00mm PANTHER ACSR 200 487
37/2.79mm LYNX ACSR 180 445
61/3.18mm ZEBRA ACSR 420 737
37/3.00mm PANTHER ACSR 200 487
61/3.18mm ZEBRA ACSR 420 737
61/3.53mm MOOSE ACSR 520 836
61/3.53mm MOOSE ACSR 520 836
Commonly Used Sizes of Conductor:
STANDARD BAY WIDTHS IN METERS:
11 KV - 4.7
33 KV - 4.7
66 KV - 7.6
132 KV - 12.2 & 11
220 KV - 17.00
400 KV - 27.00
STANDARD BUS AND EQUIPMENT ELEVATIONS
Equipment live terminal
elevation in meters
Main Bus /Buses
elevation in metres
11 & 33 2.8 to 4.0 5.5 to 6.5 9.0 6.5 to 8.5
66 4.0 6.0 to 7.0 9.0 to 10.5 9.5
132 3.7 to 5 8.0 to 9.5 13.5 to 14.5 12.0 to 12.5
220 4.9 to 5.5 9.0 to 13.0 18.5 15.0 to 18.5
The insulators, bus bars and connections should
not be stressed to more than one fourth of the
breaking load or one third of their elastic limit
whichever is lower.
CLEARANCES: The following are the minimum
clearances for out-door equipment and rigid
conductors in air.
Phase to phase spacing in
isolators and switches
11 75 400 310 610 920
33 170 400 320 760 120
66 325 750 630 1530 2140
Normally adopted phase spacings for strung bus
are indicated below:
11 KV - 1300 mm
33 KV - 1300 mm
66 KV - 2200 mm
132 KV - 3000/3600 mm
220 KV - 4500 mm
400 KV - 7000 mm.
a) The minimum clearance of the live parts to
ground in an attended outdoor sub-station and
the sectional clearance to be maintained
between live parts in adjacent sections for safety
of persons while working with adjacent sections
alive are given below:
Voltage rating (KV) Minimum Clearance to ground (mm.) Sectional Clearance (mm.)
11 3700 2600
33 3700 2800
66 4600 3000
132 4600 3500
220 5500 4300
400 8000 7000
The bottom most portion of any insulator or bushing in
service should be at a minimum height of 2500 mm
above ground level.