11KV/415V Overhead Line Specification(REC)

 

11KV LIGHTNING ARRESTER (IS: 3070 (Pt-II)).

Voltage Rating for LA:

  • The rated voltage of lightning arresters shall be 9 KV (rms).
  • This will be applicable to the effectively earthed 11 KV systems co-efficient of earth not exceeding 80 percent as per IS: 4004 with all the transformer neutrals directly earthed.

Normal Discharge Current Rating for LA:

  • The nominal discharge current rating of the lightning arresters shall be 5 KA.

Tests for LA:

  • The following routine and type tests as laid down in IS : 3070 (Part-I) shall be carried out.
  • Routine Test: Dry Power frequency spark over test.
  • Type Tests (Confirmation) :
  1. Voltage withstand tests of arrester insulation.
  2. Power frequency spark over test
  3. Hundred percent 1.2/550 microsecond impulse spark over test
  4. Front-of-wave impulse spark over test.
  5. Residual voltage test.
  6. Impulse current withstand test.
  7. Operating duty test.
  8. Temperature cycle test on porcelain housing.
  9. Porosity test on porcelain components.
  10. Galvanizing test on metal parts.

11 KV DROP-OUT FUSE CUTOUTS: (IS: 9385 (part-I to III).)

  • The distribution fuse cutouts shall be outdoor, open, drop-out expulsion type fuse cutouts suitable for installation in 50 Hz, 11 KV distribution system.
  • The rated voltage shall be 12 KV.
  • The rated current shall be 100 A.

Rated Lighting Impulse withstands Voltage for Fuse:

  • To earth and between poles 75 KV (Peak)
  • Across the isolating distance of fuse base 86 KV (Peak)

Rated One Minute Power Frequency Withstand Voltage (Wet & Dry) for Fuse:

  • To earth and between poles 28 KV (rms)
  • Across the isolating distance 32 KV (rms)

Temperature Rise Limit for Fuse:

  • Copper contacts silver faced 650C
  • Terminals 500C
  • Metal parts acting as spring The temperature shall not reach such a value that Elasticity of the metal is changed

Rated Breaking Capacity for Fuse:

  • The rated breaking capacity shall be 8 KA (Asymmetrical).

Construction Details for Fuse:

  • The cutouts shall be of single vent type (downward) having a front connected fuse carrier suitable for angle mounting.
  • All ferrous parts shall be hot dip galvanized in accordance with the latest version of IS : 2632. Nuts and bolts shall conform to IS : 1364. Spring washers shall be electro-galvanized.

Fuse Base Top Assembly:

  • The top current carrying parts shall be made of a highly conductive copper alloy and the contact portion shall be silver plated for corrosion resistance and efficient current flow.
  • The contact shall have a socket cavity for latching and holding firmly the fuse carrier until the fault interruption is completed within the fuse.
  • The top assembly shall have an aluminum alloy terminal connector. The top assembly shall be robust enough to absorb bulk of the forces during the fuse carrier closing and opening operations and shall not over-stress the spring contact. It shall also prohibit accidental opening of the fuse carrier due to vibrations or impact.

Fuse Base Bottom Assembly:

  • The conducting parts shall be made of high strength highly conductive copper alloy and the contact portion shall be silver plated for corrosion resistance and shall provide a low resistance current path from the bottom fuse carrier contacts to the bottom terminal connector.

Fuse Carrier Top Assembly:

  • The fuse carrier top contact shall have a solid replaceable cap made from highly conductive, anticorrosive copper alloy and the contact portion shall be silver plated to provide a low resistance current path from the Fuse Base Top Contact to the Fuse Link.
  • It shall make a firm contact with the button head of the fuse link and shall provide a protective enclosure to the fuse link to check spreading of arc during fault interruptions.
  • The fuse carrier shall be provided with a cast bronze opening eye (pull ring) suitable for operation with a hook stick from the ground level to pull-out or close-in the fuse carrier by manual operation.

Fuse Carrier Bottom Assembly:

  • The fuse carrier bottom assembly shall be made of bronze castings with silver plating at the contact points to efficiently transfer current to fuse base.
  • It shall make smooth contact with the fuse base bottom assembly during closing operation. The bottom assembly shall have a lifting eye for the hook stick for removing or replacing the fuse carrier.

Fuse Base (Porcelain):

  • The fuse base shall be a bird-proof, single unit porcelain insulator with a creepage distance (to earth) not less than 320 mm. The top and bottom assemblies as also the middle clamping hardware shall be either embedded in the porcelain insulator with sulphur cement or suitably clamped in position.
  • For embedded components, the pull out strength should be such as to result in breaking of the porcelain before pull out occurs in a test. For porcelain insulators, the beam strength shall not be less than 1000 kg.

Fuse Tube:

  • The fuse tube shall be made of fiber glass coated with ultraviolet inhibitor on the outer surface and having arc quenching bone fire liner inside.
  • The tube shall have high bursting strength to sustain high pressure of the gases during fault interruption.
  • The inside diameter of the fuse tube shall be 17.5 mm. The solid cap of the fuse carrier shall clamp the button head of the fuse link, closing the top end of the fuse and allowing only the downward venting during fault interruption.

Type Tests (IS: 9385 Part I & II) for Fuse:

  • Dielectric tests
  • Temperature rise test

Mounting Arrangement for Fuse:

  • The cutouts shall be provided with a suitable arrangement for mounting these on 74 X 40 mm or 100 X 50 mm channel cross arm in such a way that the center line of the base is at an angle of 15 to 20 deg from the vertical and shall provide the necessary clearances from the support.
  • Mounting arrangement shall be made of high strength galvanized steel flat and shall be robust enough to sustain the various stresses encountered during all operating conditions of the cutout.

11 KV PORCELAIN INSULATORS: (IS: 731 and IS: 3188):

  • The porcelain shall be sound, free form defects, through verified and smoothly glazed. Unless otherwise specified, the glaze shall be brown color.
  • The glaze shall cover all the porcelain parts of insulators except those areas which serve as support during firing are left unglazed for the purpose of assembly.
  • The design of insulators shall be such that stresses due to expansion and contraction in any part of the insulator shall not lead to deterioration.
  • The porcelain shall not engage directly with hard metal. Cement used in construction of insulators shall not cause fracture by expansion or loosening by contraction and proper care shall be taken to locate the individual parts correctly during cementing.
  • The cement shall not give rise to chemical reaction with metal fittings and its thickness shall be as uniform as possible.
  • The insulators should preferably be manufactured in automatic temperature controlled kilns to obtain uniform baking for better electrical and mechanical properties.
  • Both pin and strain insulators shall conform to Type B of IS : 731. The strain insulators shall be of Tongue and Clevis type.

 Test Voltage for Insulator:

  • Highest System Voltage : 12 KV (rms)
  • Visible Discharge Test: 9 KV (rms)
  • Wet Power Frequency Withstand Test 35 KV (rms)
  • Power Frequency Puncture Withstand Test (Pin Insulator) : 105 KV (rms)
  • Power Frequency Puncture Withstand Test (Strain Insulator):1.3 times the actual dry flashover voltage of the insulator.
  • Impulse Voltage Withstand Test : 75 KV (rms)

Failing Load for Insulator:

  • Mechanical Failing Load (For Pin Insulators only) : The insulators shall be suitable for a minimum failing load of 10 KN applied in transverse director.
  • Electro-Mechanical Failing Load (For Strain Insulators) : The insulators shall be suitable for a minimum failing load of 70 KN applied axially.

Creepage Distance for Insulator:

  • Highest System Voltage: 12KV
  • Heavily Polluted Atmosphere Pin Insulator: 320mm
  • Heavily Polluted Atmosphere Strain Insulator: 400mm

Tests: (As per IS: 731 ) for Insulator.

  1. Visual examination
  2. Verification of dimensions
  3. Visible discharge test
  4. Impulse Voltage withstand Test
  5. Wet Power Frequency Voltage withstand Test
  6. Temperature Cycle Test
  7. Mechanical Failing Load Test
  8. 24 hour Mechanical Strength Test for Strain Insulators
  9. Puncture Test
  10. Porosity Test
  11. Galvanizing Test
  12. Electro-Mechanical Failing Load Test

Routine Tests for Insulator

  1. Virtual examination
  2. Mechanical routine test
  3. Electrical routine test

Acceptance Test for Insulator

  1. Verification of Dimensions
  2. Temperature Cycle Test
  3. Electro-Mechanical Failing Load Test
  4. Puncture Test
  5. Porosity Test
  6. Galvanizing Test

Marking for Insulator:

  • Name or trademark of manufacturer
  • Month and year of manufacture
  • Minimum failing load in KN
  • ISI certificate mark, if any
  • Markings on porcelain shall be printed and shall be supplied before firing.

 Pin Insulators:

  • The pins shall of single piece obtained preferably by the process of forging.
  • They shall not be made by joining, welding, shrink fitting or any other process using more than one piece material. The pins shall be of good finish, free from flaws and other defects.
  • The finish of the collar shall be such that sharp angle between the collar and the shank is avoided. Aluminum ferrous pins, nuts and washers, except those made of stainless steel, shall be galvanized. The threads of nuts and taped hole when cut after galvanizing shall be well oiled or greased.

Dimensions  for Pin Insulators:

  • Pins shall be of small steel head type S 165 P as per IS : 2486 (Part-II) having stalk length of 165 mm and shank length of 150 mm with minimum failing load of 10 KN.

Tests: (IS: 2486 (Part-I)) for Pin Insulators

  • Checking of threads on heads
  • Galvanizing test
  • Visual examination test
  • Mechanical test
  • Galvanizing test
  • Mechanical test
  • Visual examination test

Helically Formed Pin Insulator Ties:

  • Helically formed ties used for holding the conductor on the pin insulator shall be made of aluminum alloy or aluminized steel or aluminum clad steel wires and shall conform to the requirements of IS : 12048. The ties shall be suitable for pin insulator dimensions of Pt.- I and conductor sizes specified.
  • Elastomer pad for insulator shall be used with the ties to avoid abrasion of the conductor coming into direct contact with the insulator.

Cross arm strap conforming to IS: 2486 (Pt. – II).

  • Aluminum alloy die cast thimble-clevis for attaching to the tongue of strain, insulator on one end and for accommodating the loop of the helically formed dead-end fitting at the other end in its smooth internal contour.
  •  The thimble shall be suitable for all sizes of ACSR conductors as specified. The thimble clevis shall be attached to the insulator by a steel cutting pin used with a non-ferrous split pin of brass or stainless steel.
  • The thimble shall have clevis dimensions as per IS: 2486 (Pt – II).
  • Helically formed dead end grip having a prefabricated loop to fit into the grooved contour of the thimble on one end and for application over the conductor at the other end.
  • The formed fitting shall conform to the requirement of IS : 12048.

Fittings for strain Insulators of Ball & Socket Type:

  • Cross arm strap conforming to IS: 2486 (Pt-II).
  • Forged steel ball eye for attaching the socket end of the strain insulator to the cross arm strap.
  • Forgings shall be made of steel as per IS: 2004. Aluminum alloy thimble-socket made out of permanent mould cast, high strength aluminum alloy for attaching to the strain insulator on one end and for accommodating the loop of the helically formed dead-end fittings at the other end in its smooth internal contour.
  • The thimble socket shall be attached to the strain insulator with the help of locking pin as per the dimensions given in IS : 2486 (Pt-II).

Tests

  • The helically formed fittings for strain insulators shall be subjected to tests as per IS : 12048.
  • The other hardware fittings shall be tested as per IS: 2486 (Part-I).

Fittings for strain Insulators with Helically Formed Conductors Dead-End Grips:

Fittings for Strain Insulators of Tongue & Clevis Type

  • The fittings shall consist of the following components:
  1. Cross arm strap conforming to IS:2486 (Pt.II)-1989.
  2. Aluminum alloy die cast thimble-clevis for attaching to the tongue of strain insulator on one end and for accommodating the loop of the helically formed dead-end fitting at the other end in its smooth internal contour. The thimble shall be suitable for all sizes of conductors ranging from 7/2.11mm to 7/3.35mm ACSR. The thimble clevis shall be attached tothe insulator by a steel cutter pin used with a non-ferrous split pin of brass or stainless steel. The thimble shall have clevis dimensions as per IS:2486 (Pt.II)-1989.
  3.    Helically formed dead-end grip having a pre-fabricated loop to fit into the grooved contour of the thimble on one end and for application over the conductor at the other end. The formed fitting shall conform to the requirement ofIS:12048-1987.
  • Note: As the helically formed fittings are made to suit specific sizes of conductors, the purchase should clearly specify the number of fittings required for each size of conductor

Fittings for strain Insulators with  Conventional Dead end Clamps Alternative to Fitting Covered:

  • Fittings for strain insulators with conventional dead-end clamps for use with tongue & clevis or ball & socket type insulators shall consist of the following components :
  1. Cross arms strap conforming to IS:2486 (Pt.II)-1989
  2.  Dead-end clamp made of aluminum alloy to suit ACSR conductors from 7/2.11mm to 7/3.35mm. The ultimate strength of the clamp shall not be less than 3000 Kg. The shape and major dimensions of clamps suitable for B&S and T&C insulators are shown in figures 7 & 8 respectively.

GUY STRAIN INSULATORS: (IS: 5300)

  • The porcelain insulator shall be sound, free from defects, thoroughly verified and smoothly glazed.
  •  The design of the insulator shall be such that the stresses to expansion and contraction in any part of the insulator shall not lead to its deterioration.
  • The glaze, unless otherwise specified, shall be brown in color.
  • The glaze shall cover the entire porcelain surface parts except those areas that serve as supports during firing.

Type for Guy Insulators:

  • The standard guy strain insulators shall be designations ‘A’ and ‘C’ as per IS: 5300.
  • The recommended type of guy strain insulators for use on guy wires of overhead lines of different voltage levels are as follows:
  • Power Line Voltage :11KV
  • Designation of Insulators: C
  • Dry one minute Power Frequency withstand Voltage: 27 KV (rms)
  • Wet one minute Power Frequency withstand Voltage: 13 KV (rms)
  • Minimum Failing Load: 88(KN)

 Tests: (IS: 5300) for Guy Insulators.

  1. Visual examination
  2. Verification of dimensions
  3. Temperature cycle test
  4. Dry one-minute power frequency voltage withstand test
  5. Wet one-minute power frequency voltage withstand test
  6. Mechanical strength test
  7. Porosity test
  8. Acceptance Tests : (to be conducted in the following order)
  9. Verification of dimensions
  10. Temperature cycle test
  11. Mechanical strength test
  12. Porosity test

Marking for Guy Insulators:

  • Name or trademark of the manufacturer.
  • Year of manufacture.
  • ISI certificate mark, if any
  • Marking on porcelain shall be applied before firing.

Type of Insulators for Guy Insulators:

  • The standard guy strain insulators shall be of designations ‘A‘ and ‘C‘ as per IS:5300.
  • The recommended type of guy strain insulators for use on guy wires of overhead lines of different voltage levels are as follows :
Line VoltageDesignation of Insulator
415/240VoltA Type
11KVC Type
33KVC Type (2 No’s of Strings in Series).

Basic Insulator Level for Guy Insulators:

Designation of InsulatorDry one min power frequency withstandWet one min power frequency withstand
A Type18 KV (rms)8 KV (rms)
C Type27 KV (rms)13 KV (rms)

 Mechanical Strength for Guy Insulators:

The insulators shall be suitable for the minimum failing loads specified as under:

Designation of InsulatorMinimum Failing Load
A Type44 KN
C Type88 KN

Routine Test as per Tests (IS: 5300) for Guy Insulators:

  1. Visual examination
  2. Verification of dimensions
  3. Temperature cycle test
  4. Dry one-minute power-frequency voltage withstand test
  5. Wet one-minute power frequency voltage withstand test
  6. Mechanical strength test
  7. Porosity test

Acceptance Tests (to be conducted in the following order):

  1. Verification of dimensions
  2. Temperature cycle test
  3. Mechanical strength test
  4. Porosity test

DANGER NOTICE PLATES:

  • As per provisions of IE Rules 1956, Danger Notice Plates in Hindi or English and, in addition, in the local language with the sign of skull and bones are required to be provided on power line supports and other installations.
  • It is further stipulated in the I.E. Rules that such Notice Plates are not required to be provided on supports like PCC, tubular, wood, steel rails, etc. which cannot be climbed easily without the aid of ladder or special appliances.
  • To adopt a uniform pattern and for helping easy procurement, a specification on Danger Notice Plates has been drawn up.

Standards of Danger Plate

  • The Danger Notice Plates shall comply with IS:2551-1982.

Dimensions of Danger Plate

  • Two sizes of Danger Notice Plates as follows are recommended:
  1. For display at 415 V installations – 200x150mm
  2. For display at 11 KV (or higher voltages) installations – 250x200mm
  • The corners of the plate shall be rounded off.
  • The location of fixing holes as shown in Figs. 1 to 4 is provisional and can be modified to suit the requirements of the purchaser.

Lettering of Danger Plate

  • All letterings shall be centrally spaced. The dimensions of the letters, figures and their respective position shall be as shown in figs.
  • The size of letters in the words in each language and spacing between them shall be so chosen that these are uniformly written in the space earmarked for them.

Languages of Danger Plate

  • Under Rule No. 35 of Indian Electricity Rules, 1956, the owner of every medium, high and extra high voltage installation is required to affix permanently in a conspicuous position a danger notice in Hindi or English and, in addition, in the local language, with the sign of skull and bones.
  • The type and size of lettering to be done in Hindi is indicated in the specimen danger notice plates shown in Fig. 2 and 4 and those in English are shown in Figs.
  • Adequate space has been provided in the specimen danger notice plates for having the letterings in local language for the equivalent of’ Danger’,’ 415′ ‘11000’ and ‘Volts’.

Material & Finishing of Danger Plate:

  • The plate shall be made from mild steel sheet of at least 1.6mm thick and vitreous enameled white, with letters, figures and the conventional skull and cross-bones in signal red color (refer IS:5-1978) on the front side. The rear side of the plate shall also be enameled.

Tests of Danger Plate:

  • The following tests shall be carried out :
  1. Visual examination as per IS:2551-1982
  2. Dimensional check as per IS:2551-1982
  3. Test for weather proofness as per IS:8709-1977 (or its latest version)

ACSR and AAC over head conductors:

  • A Conference on Standardization of Specifications and Construction Practices in Rural Electrification was held on 4th and 5th January, 1971 in New Delhi. Besides the Rural Electrification Corporation, representatives of various State Electricity Boards, the Indian Standards Institution (BIS), Central Water & Power Commission (CEA),Central Board of Irrigation and Power and many other organizations participated in the discussions.
  • Based on the consensus arrived at the Conference, REC Specification No. 1/1971 covering 7/2.21 mm (25 mm2 aluminum area) and 7/3.10mm (50mm2 aluminum area) AAC for use on LT lines and 7/2.59 mm (30mm2 aluminum area) and 7/3.35mm (50mm2 aluminum area) ACSR for use on 11 KV and LT lines was issued.
  • Subsequently, the Specification was revised to incorporate an additional size of ACSR viz 7/2.11mm (20mm2 aluminum area) for use on 11 KV and LT lines and then again to incorporate three more sizes of ACSR viz. 7/3.35mm (50mm2 aluminum area), 7/4.09mm (80mm2 aluminum area) and 6/4.72mm + 7/1.57 mm (100mm2 aluminum area) for use on 33 KV lines.
  • The sizes of conductors standardized for lines of different voltages are indicated below :

ACSR Conductor for 33KV Lines :

  1. ACSR 7/3.35mm (50mm2 aluminum area)
  2. ACSR 7/4.09 mm (80mm2 aluminum area)
  3. ACSR 6/4.72 mm + 7/1.57 mm (100mm2 aluminum area)

ACSR Conductor for11 KV Lines

  1. ACSR 7/2.11 mm (20mm2 aluminum area)
  2. ACSR 7/2.59 mm (30mm2 aluminum area)
  3.  ACSR 7/3.35 mm (50mm2 aluminum area)

ACSR Conductor for LT Lines

  1. ACSR 7/2.11 mm (20mm2 aluminum area)
  2.  ACSR 7/2.59 mm (30mm2 aluminum area)
  3. ACSR 7/3.35 mm (50mm2 aluminum area)
  4.  AAC 7/2.21 mm (25mm2 aluminum area)
  5. AAC 7/3.10 mm (50mm2 aluminums area)

Standards for ACSR Conductor :

  • IS: 398 (Pt.I)-1976 and IS: 398 (Pt.II)-1976.

Joint in Wires & Conductors:

  • All aluminums conductors: No joints shall be permitted in any wire.
  • Aluminum Conductor Steel Reinforced :

Aluminum Wires for ACSR Conductor:

  • No two joints shall occur in the aluminums wires closer together than 15 meters.

Steel Wires for ACSR Conductor :

  • No joints shall be permitted in steel wires used for ACSR of Sizes 20mm2 aluminums area (7/2.11mm), 30mm2 aluminums area (7/2.59mm), 50mm2 aluminums area (7/3.35mm) and 80mm2 aluminums area (7/4.09 mm).
  • In the case of ACSR of 100mm2 aluminums area (6/4.72mm + 7/1.57 mm) having seven galvanized steel wires, joints, in individual wires shall be permitted but no two such joints shall be less than 15 meters apart in the complete steel core.

Tests for ACSR Conductor:

  • The samples of individual wires for the tests shall normally be taken before stranding. The manufacturer shall carry out test on samples taken out at least from 10% of aluminums wire spools and 10% of steel wire coils. However, when desired by the purchaser, the test sample may be taken from the stranded wires.
  • The wires used for all aluminums conductors shall comply with the following tests as per IS:398(Pt.I)-1976.
  1.  Breaking load test
  2.  Wrapping test
  3.  Resistance test
  • The wires used for aluminums conductors, steel reinforced shall comply with the following tests as per IS:398
  1. Breaking load test
  2. Ductility test
  3. Wrapping test
  4. Resistance test
  5. Galvanizing test

Packing & Marking :(IS: 1778-1980)

  • The gross mass for various conductors shall not exceed by more than 10% of the values given in the following

Conductor Size Gross Mass for ACSR Conductor

(1) AAC

  • 25mm2 Al. area (7/2.21 mm) 500 Kg.
  • 50mm2 Al. area (7/3.10 mm) 500 Kg.

(2)ACSR

  • 20mm2 Al. area (7/2.11 mm) 1000 Kg.
  • 30mm2 Al. area (7/2.59 mm) 1000 Kg.
  • 50mm2 Al. area (7/3.35 mm) 1500 Kg.
  • 80mm2 Al. area (7/4.09 mm) 1500 Kg.
  • 100mm2 Al. area (6/4.72mm + 7/1.57mm) 2000 Kg.

Conductor Size Normal conductor length for ACSR Conductor

  1.  AAC

  • 25mm2 Al. area (7/2.21mm) 1.0 Km.
  • 50mm2 Al. area (7/3.10mm) 1.0 Km.

  1. ACSR

  • 20mm2 Al. area (7/2.11 mm) 2.0 Km.
  • 30mm2 Al. area (7/2.59 mm) 2.0 Km.
  • 50mm2 Al. area (7/3.35 mm) 2.0 Km.
  • 80mm2 Al. area (7/4.09 mm) 1.5 Km.
  • 100mm2 Al. area (6/4.72mm + 7/1.57mm) 2.0 Km.
  • Longer lengths shall be acceptable.
  • Short lengths, not less than 50% of the standard lengths, shall be acceptable to the maximum extent of 10% of the quantity ordered.

Marking for ACSR Conductor:

  • The following information shall be marked on each package:
  1. Manufacturers’ name
  2. Trade mark, if any
  3. Drum or identification number
  4. Size of conductor
  5. Number and lengths of conductor
  6. Gross mass of the package
  7. Net mass of conductor
  8. I.S.I certification mark, if any

Pre stressed Cement Concrete Poles(FOS 2.5) For 11KV & LT Lines:

  •  A research project for evolving economical designs of cement concrete poles for use on 11 KV and LT Lines was entrusted to the Cement Research Institute (CRI) of India.
  • The basic design parameters for these poles as given in Clause 6 of this Specification were approved by the Fifth Conference on standardization of Specifications and Construction Practices in Rural Electrification held in May, 1974.
  • Some of these design parameters which were based on certain foreign codes/practices and certain other provisions of this Specification, although at variance with the stipulations of IS:1678 – 1960, had been adopted to achieve economy in the designs. However, these modifications have since been incorporated in the revised IS:1678 – 1978.
  • This Specification covers PCC poles with an overall length of 7.5 M, 8.0 M and 9.0 M suitable for use in overhead 11 KV and L.T. power lines and double pole structures for 11/0.4 KV substations.

Application Standard for PCC Pole:

  • IS: 1678-1978, Specification for pre stressed concrete poles for overhead power, traction and telecommunication lines.
  • IS: 2905-1966. Methods of test for concrete poles for over-head power and telecommunication lines.
  • IS: 7321-1974. Code of practice for selection, handling and erection of concrete poles for over-head power and telecommunication lines.

Average Permanent Load for PCC Pole:

  • That fraction of the working load which may be considered of long duration over a period of one year.

Load Factor for PCC Pole:

  • The ratio of ultimate transverse load to the transverse load at first crack.

Transverse for PCC Pole:

  • The direction of the line bisecting the angle contained by the conductor at the pole. In the case of a straight run, this will be normal to the run of the line.

Transverse Load at First Crack for PCC Pole:

  • For design, the transverse load at first crack shall be taken as not less than the value of the working load.

Working load for PCC Pole:

  • The maximum load in the transverse direction, that is ever likely to occur, including the wind pressure on the pole.
  • This load is assumed to act at a point 600 mm below the top with the butt end of the pole planted to the required depth as intended in the design.

Ultimate Failure for PCC Pole:

  • The conditions existing when the pole ceases to sustain a load increment owing to either crushing of concrete, or snapping of the pre stressing tend on or permanent stretching of the steel in any part of the pole.

Ultimate Transverse Load for PCC Pole:

  • The load at which failure occurs, when it is applied at a point 600 mm below the top and perpendicular to the axis of the pole along the transverse direction with the butt end of the pole planted to the required depth as intended in the design.

Application for PCC Pole:

 7.5 M and 8.0 M Poles

  • These poles shall be used at tangent locations for 11KV and L.T. lines in wind pressure zones of 50 kg/M2, 75 Kg/M2 and 100 Kg/M2 in accordance with REC Construction Standards referred to in the following:

Pole length: 7.5M

  • 11KV lines without earth wire L.T. lines, horizontal formation.
  • Reference to REC Construction Standards: A-4, B-5

Pole length: 8M

  • 11KV lines with earth wire L.T. lines, vertical formation.
  • Reference to REC Construction Standards: A-5, B-6

9.0 M Poles

  • These poles shall be used for double pole structures of distribution transformer centers as per REC Construction Standards F-1 to F-4 and for special locations in 11 KV and L.T. Lines, such as road crossings etc.

Materials for PCC Pole::

(1) Cement

  • The cement used in the manufacture of pre stressed concrete poles shall be ordinary or rapid hardening port land cement conforming to IS: 269 – 1976 (Specification for ordinary and low heat port land cement) or IS: 8041 E-1978 (Specification for rapid hardening port land cement).

(2) Aggregates

  • Aggregates used for the manufacture of pre-stressed concrete poles shall conform to IS : 383 – 1970 (Specification for coarse and fine aggregates from natural sources for concrete). The nominal maximum size of aggregates shall in no case exceed 12mm.

(3) Water

  • Water should be free from chlorides, sulphates, other salts and organic matter. Potable water will be generally suitable.

(4) Admixtures

  • Admixtures should not contain Calcium Chloride or other chlorides and salts which are likely to promote corrosion of pre-stressing steel.

(5) Pre-stressing Steel

  • The pre-stressing steel wires, including those used as un tensioned wires  should conform to IS : 1785 (Part-I) – 1966 (Specification for plain hard drawn steel wire for pre stressed concrete. Part-I cold drawn stress relieved wire), IS: 1785 (Part-II) – 1967 (Specification for plain hard-drawn steel wire)., or IS : 6003 – 1970 (Specification for indented wire for pre-stressed concrete).
  • The type designs given in Annexure-I are for plain wires of 4 mm diameter with a guaranteed ultimate strength of 175 Kg/mm2.

(6) The concrete mix:

  • Itshall be designed to the requirements laid down for controlled concrete (also called design mix concrete) in IS : 1343 – 1980 (Code of practice for pre stressed concrete) and IS : 456 – 1978 (Code of practice for plain and reinforced concrete), subject to the following special conditions;
  1.  Minimum works cube strength at 28 days should be at least 420 Kg/cm2.
  2. The concrete strength at transfer should be at least 210Kg/cm2.
  3. The mix should contain at least 380 Kg. of cement per cubic meter of concrete.
  4. The mix should contain as low a water content as is consistent with adequate workability. If it becomes necessary to add water to increase the workability, the cement content also should be raised in such a way that the original value of water cement ratio is maintained.

Design Requirements for PCC Pole::

  • The poles shall be planted directly in the ground with a planting depth of 1.5 meters.
  • The working load on the poles should correspond to those that are likely to come on the pole during their service life. Designs given in Annexure-I are for 140 Kg. and 200 Kg. Applied at 0.6 M from top.
  • The factor of safety for all these poles shall not be less than 2.5.
  • The average permanent load should be 40% of the working load.
  • The F.O.S. against first crack load shall be 1.0.
  • At average permanent load, permissible tensile stress in concrete shall be 30 Kg/cm2.
  • At the design value of first crack load, the modulus of rupture shall not exceed 55.2 kg/cm2 for M-420 concrete.
  • The ultimate moment capacity in the longitudinal direction should be at least one fourth of that in the transverse direction.
  • The maximum compressive stress in concrete at the time of transfer of pre stress should not exceed 0.8 times the cube strength.
  • The concrete strength at transfer shall not be less than half the 28 days strength ensured in the design, i.e. 420 x 0.5= 210 Kg/cm2.
  • For model check calculations on the design of poles, referred to in Annexure-I, a reference may be made to the REC “Manual on Manufacturing of solid PCC

Dimensions and Reinforcements for PCC Pole:

  • The cross-sectional dimensions and the details of pre stressing wire should conform to the particulars given in Annexure-I.
  • The provisions of holes for fixing cross-arms and other fixtures should conform to the REC standards referred to in clause 4 of this specification and in accordance with the construction practices adopted by the State Electricity Boards.

Manufacture for PCC Pole::

  • All pre stressing wires and reinforcements shall be accurately fixed as shown in the drawings and maintained in position during manufacture. The un tensioned reinforcement, as indicated in the drawings, should be held in position by the use of stirrups which should go round all the wires.
  • All wires shall be accurately stretched with uniform pre stress in each wire.
  • Each wire or group of wires shall be anchored positively during casting. Care shall be taken to see that the anchorages do not yield before the concrete attains the necessary strength.

Cover for PCC Pole:

  • The cover of concrete measured from the outside of the pre stressing tendon shall be normally 20 mm.

Welding & Lapping of Steel for PCC Pole:

  • The high tensile steel wire shall be continuous over the entire length of the tendon.
  • Welding shall not be allowed in any case. However, jointing or coupling may be permitted provided the strength of the joint or coupling is not less than the strength of each individual wire.

Compacting for PCC Pole:

  • Concrete shall be compacted by spinning, vibrating, shocking or other suitable mechanical means. Hand compaction shall not be permitted.

Curing for PCC Pole:

  • The concrete shall be covered with a layer of sacking, canvas, hessian or similar absorbent material and kept constantly wet up to the time when the strength of concrete is at least equal to the minimum strength of concrete at transfer of pre stress. Thereafter, the pole may be removed from the mould and watered at intervals to prevent surface cracking of the unit, the interval should depend on the atmospheric humidity and temperature.

The pre stressing wires for PCC Pole:

  • Itshall be de tensioned only after the concrete has attained the specified strength at transfer (i.e. 210 Kg/cm2). The cubes cast for the purpose of determining the strength at transfer should be cured, as far as possible, under conditions similar to those under which the poles are cured.
  • The transfer stage shall be determined based on the daily tests carried out on concrete cubes till the specified strength indicated above is reached. Thereafter the test on concrete shall be carried out as detailed in IS: 1343 – 1960 (Code of practice for pre stressed concrete).
  • The manufacturer shall supply when required by the purchaser or his representative, result of compressive test conducted in accordance with IS : 456 -1964 (Code of practice for plain and reinforced concrete) on concrete cubes made from the concrete used for the poles.
  •  If the purchaser so desires, the manufacturer shall supply cubes for test purposes and such cubes shall be tested in accordance with IS: 456 – 1964 (Code of practice for plain and reinforced concrete).
  • The de tensioning shall be done by slowly releasing the wires, without imparting shock or sudden load to the poles. The rate of de tensioning may be controlled by any suitable means either mechanical (screw type) or hydraulic.
  • The poles shall not be de tensioned or released by cutting the pre stressing wires using flames or bar croppers while the wires are still under tension.

Separate eye-hooks or holes for PCC Pole:

  • It shall be provided for handling and transport, one each at a distance of 0.15 times the overall length, from either end of the pole.
  • Eye-hooks, if provided, should be properly anchored and should be on the face that has the shorter dimension of the cross-section. Holes, if provided for lifting purposes, should be perpendicular to the broad face of the pole.
  • Stacking should be done in such a manner that the broad side of the pole is vertical. Each tier in the stack should be supported on timber sleepers located at 0.15 times the overall length, measured from the end. The timber supports in the stack should be aligned in a vertical line.
  • Poles should be transported with their broad faces placed vertically and in such a manner that shocks are avoided. Supports should be so arranged that they are located approx. at a distance equal to 0.15 times the overall length from the ends.
  • The erection of the pole should be carried out in such a way that the erection loads are applied so as to cause moment with respect to the major axis. i.e. the rope used for hoisting the pole should be parallel to the broader face of the pole.

Testing of PCC Pole:

Transverse Strength Test

  • Poles made from ordinary Portland cement shall be tested only on the completion of 28 days and poles made from rapid-hardening cement only on the completion of 14 days, after the day of manufacture.
  • The pole may be tested in either horizontal or vertical position. If tested in horizontal position, provisions shall be made to compensate for the overhanging weight of the pole, for this purpose the over-hanging portion of the pole may be supported on a movable trolley or similar device.
  • The pole shall be rigidly supported at the butt end for a distance equal to the agreed depth of planting i.e. 1.5 M.
  • Load shall be applied at a point 600 mm from the top of the pole and shall be steadily and gradually increased to design value of the transverse load at first crack. The deflection at this load shall be measured.
  • A pre stressed concrete pole shall be deemed not to have passed the test if visible cracks appear at a stage prior to the application of the design transverse load for the first crack.
  • The load shall then be reduced to zero and increased gradually to a load equal to the first crack load plus 10% of the minimum ultimate transverse load, and held up for 2 minutes.
  • This procedure shall be repeated until the load reaches the value of 80 per cent of the minimum ultimate transverse load and thereafter increased by 5 per cent of the minimum ultimate transverse load until failure occurs. Each time the load is applied, it shall be held for 2 minutes.
  •  The load applied to pre stressed concrete pole at the point of failure shall be measured to the nearest five Kilograms.
  • The pole shall be deemed not to have passed the test if the observed ultimate transverse load is less than the design ultimate transverse load.

Measurement of Pole Cover for PCC Pole:

  • After completion of the transverse strength test, the sample pole shall be taken and checked for cover.
  • The cover of the pole shall be measured at 3 points, one within 1.8 meters from the butt end of the pole, the second within 0.6 meter from the top and the third at an intermediate point and the mean value compared with the specified value.
  • The mean value of the measured cover should not differ by more than(±)1 mm from the specified cover. The individual values should not differ by more than (±) 3 mm from specified value.
  • If these requirements are not met, the workmanship with reference to aligning of the end plates and pre stressing wires and assembly of moulds should be improved and inspection at pre-production stage tightened suitably.

Marking for PCC Pole:

  • The pole shall be clearly and indelibly marked with the following particulars either during or after manufacture but before testing at a position so as to be easily read after erection in position.
  1. Month and year of manufacture
  2. Transverse strength of pole in Kg.
  3. Maker’s serial No. and mark

Main Points should be look after for Overhead Line Installation:

Overhead lines:

  • The general precautions during storage and handling of shall be taken in accordance with relevant IS code.
  • While laying the conductor shall betaken from top of the drum and the repeated in the direction of arrow on it.. Care shall be taken to avoid contact with steel works, fence, etc by giving soft wood protection, using wooden rollers.
  •  Proper tools shall be used during stringing work. During stringing operation standard sag table or chart shall be followed and care shall be taken to ensure that there are no kinks in the conductor. Joints in conductors shall be staggered. Mid span joints in conductors shall be avoided.
  • After stringing the conductor, it shall be clamped permanently with shackle or strain clamps. An angle or section shall be selected while pulling up conductors.

Jumpers:

  • While stringing, sufficient length shall of conductors be kept at shackle terminations for making jumpers. Jumpers shall be neat and as far as possible symmetrical to run of conductors. These shall be made to prevent occurrence of faults due to wind or birds. PG clamps may be preferred to binding of conductors at jumper location or service taps.

Cross Arms :

  • The cross arms shall be made of MS Structural steel. The length of cross arms shall be suitable for accommodating the number of insulators on them with spacing of conductor. A gap of minimum 50 mm shall be left from the center of pin hole to end of cross arm on either side. The cross arm shall be complete with pole clamp made of MS flat of size not less than 50 x 6 mm with necessary nuts, bolts, washers, etc. The length of cross arm for carrying guard wires shall always run not less than 300 mm beyond outer most bare conductor of configuration.
  • Cross arms shall be properly clamped to the support taking into consideration the orientation of lines.

Porcelain insulators and fittings:

  • The porcelain insulators shall be confirming to IS 731 – 1971 for overhead lines. This shall be glazed, crack / burr free.
  • The insulator shall have adequate mechanical strength, high degree of resistance to electrical puncture and resistance to climatic and atmospheric attack.
  • All iron parts shall be hot dip galvanized & all joints shall be airtight. Pin insulators / shackle  insulators / disc insulators shall be erected on cross arms and ‘D’ iron clamp shall be used or as specified by Engineer-in-charge. Shackle insulators shall be used in conjunction with ‘D’ iron clamps when configuration of conductor is vertical.
  • These shall also be erected on cross arm at intermediate support in case of long lines, deviation from straight lines. Care shall be taken that insulators are not damaged during erection.

Binding material:

  • Binding of conductor with the insulator shall be done with soft aluminum wire / conductor. The binding of conductor to insulator shall be sufficiently firm and tight to ensure that no intermittent contact develops. The end of binding wire shall be tightly twisted in close spaced spiral around the conductor to ensure good electrical contact and strengthen the conductor.

Supports and spacing of poles:

  • Support of overhead line shall be of adequate strength confirming in all respects to rules 76 of Indian electricity rules.
  • Pole spacing and clearance between lowest conductor above the ground level across / along the street shall be in accordance with rule 85 of Indian electricity rules. Suitable foundation shall be provided for erection of poles.
  • The foundation shall include excavation in all types of soil and rocks and back filling, RCC, reinforcement, formwork.
  • Excavation for foundations for poles / stay / strut : After the location of supports / stay are pegged accurately, the excavation work shall be taken up and care should be taken while excavating that pits are not oversized.
  • The pit should be excavated in the direction of the line. The depth and size of pit shall be such that normally 1/6th of the length of pole is buried in the ground and suitable for foundation of support.
  • For stay the position of pit shall normally be such stay makes as large an angle as possible with the support and it shall be in the range of 40 to 60 degrees.
  • The length of stay rod shall project 450 mm above the ground level. The pit for strut shall be located at a distance not less than 1.8M from the pole.
  • The depth of pit shall be such that at least 1.2M of the strut is buried in the ground.

Stay / strut:

  • Stay set shall consist of stay rod, anchor plate, bow tightened / turn buckle, thimbles, stay wire and stain insulators.
  •  The stay rod shall be with stay grip in case of turn buckle is used instead of bow tightened. The entire stay set assembly shall be galvanized. The stay wire shall be either 7/4.0 mm diameter or 7/3.15 mm diameter GI having tensile strength of not less than 70 kgf/sq mm and confirming to IS 2141. T
  • The anchor plate shall be of MS galvanized and not less than 300 mm x 300 mm x 6.4 mm thick. The stay rod / buckle rods shall be minimum 16/19 mm diameter galvanized steel rod having tensile strength not less than 42 kgf/sq mm. Minimum length of stay rod and buckle shall be 1800 mm and 450 mm respectively.

Erection stay sets:

  • The anchor plate shall be galvanized MS plate. The stay rod with anchor plate shall be embedded in cement concrete 1:3:6. A stay shall be provided at all angle and terminal poles. Double stay shall be provided at all dead ends and in such case, these shall be as far as possible to be set parallel to each other.

Cage guard:

  • All metal supports of overhead lines and metallic fitting attached shall be permanently and effectively earthed. Cage guard / cradle guard shall be made of 6 SWG GI wire confirming to IS 2633 including netting, stretching and jointing of cage and lacing by 10/12 SWG GI wire, binding by 14/16 SWG GI wire.

Danger boards:

  • All supports carrying HV lines shall be fitted with danger plates confirming to IS 2551 at height of 3 M from ground indicating the voltage of line. The script shall be both in ‘English/Hindi’.

Anti climbing devices:

  • Necessary arrangement for preventing unauthorized persons from ascending any of the supports and structure carrying HV lines without the aid of ladder or special appliance shall be made.
  •  Unless otherwise specified barbed wire confirming to IS 278 having four points barbed spaced 75 +/- 12 mm apart shall be wrapped helically with a pitch of 75 mm around the limb of support and firmly commencing from the height of 3.5 M and up to 5 or 6 M as directed by the engineer.

Lightning arrestor:

  • Lightning arrestor suitable for HT lines shall be installed one unit per phase at terminations, transformer stations, etc.
  • The devices shall be connected ahead of fuse provided if any. Independent earth electrode shall be provided for LA.
  • The earth lead from earth electrode to LA shall be continuous. The LA shall confirm to IS 3070 and shall be non linear distribution class.
  • The LA shall be non-linear type, distribution class, outdoor type suitable for effectively earthed system. The LA shall consist of line terminal stud, earth terminal stud, number of spark gaps in series with non-linear resistor, the whole assembly housed inside a hermetically sealed porcelain bushing.
  • Neoprene rubber gasket shall be provided between metal caps and porcelain bushing. Non-linear resistor shall be silicon carbide blocks metalized at both ends to ensure good electrical contact between terminals, non-linear resistor & spark gaps.
  • Mounting bracket shall be hot dip galvanized suitable for mounting LA on structure.

Cable Laying Direct in Ground:

  •  The method shall be adopted where the cable route is through open country, along road / lanes, etc and where no frequent excavations are encountered and re excavation is possible without affecting other work.

Width of trenches:

  • The width of trench for laying single cable shall be 35 cm Where more than one cable are to be laid in the same trench in horizontal formation, width of trench shall be increased such that the inter-axial spacing between the cables for 415 volts shall be 20 cm and for 11 shall be 35 cm.

Depth of trenches:

  • Where cables are laid in single formation, the total depth of trench shall not be less than 75 cm for cable up to 1.1 KV grade and shall not be less than 120 cm for cable above 1.1 KV grade. Wherever more than one tier formation is unavoidable and vertical formation is adopted, the depth of trench shall be increased by 30 cm for each additional tier to be formed.

Protective covering:

  • Cable laid in trenches shall have covering of clean dry sand not less than 170 mm above the base cushion of sand before the protective cover is laid.
  • The cables shall be protected by B class/second class brick of not less than 20 cm x 10 cm x 10 cm or protective cover placed on top of the sand and both sides of cable for full length of the cable to the satisfaction of Engineer-in-charge.

Back filling:

  • The trenches shall be back filled wit excavated earth free from stones or other scrap edged debris and shall be rammed and watered, if necessary, in successive layers not exceeding 300 mm unless otherwise specified.

Route marker:

  • Route marker shall be provided along straight runs of cables and at points of change in direction as approved by Engineer-in-charge and in general at intervals not exceeding 100 meters in straight run. Route marker shall be made out of 100 mm x 100 mm x 5 mm GI/Al plate bolted or welded on 35 mm x 35 mm x 6 mm MS angle iron of 600 mm long.
  • Such route markers shall be mounted and grouted parallel to and 0.5 meter away from the side of trench. The work “cable” with voltage grading and size of cable shall be inscribed on the marker.