Transmission line accessories are the fittings, plates, hardware and earthing parts that complete an overhead line once the tower steel, conductor and insulators are in place. Indian tenders use the term for two families. The first is the set of accessories fixed to the tower itself: plates, anti-climbing devices, step bolts, bird guards and earthing. The second is the hardware that holds the conductor and earth wire: clamps, dampers and joints. This guide works from the foundation of the tower upward, so each item appears in the order a tower crew meets it. Every section names the Indian standard or utility specification that governs the item and links to the matching VLI product page.

What are transmission line accessories?

Transmission line accessories are every fitting on an overhead line that is not tower steel, conductor or insulator. POWERGRID, the national transmission utility, shows the split in its 400 kV specification. The scope lists “tower accessories like phase plate, circuit plate, number plate, danger plate, anti-climbing device, bird guard” as one supply item. A separate item covers “earth wire, hardware fittings and conductor and earth wire accessories” (POWERGRID Standard Technical Specification, Revision 6, March 2018, clause 1.1.2).

Buyers also say “transmission tower parts” for the steel members, bolts and stubs, and “transmission line hardware” for the conductor side. An enquiry should say which group it means, because the two groups follow different documents. Tower accessory drawings are standardized by the utility, while conductor hardware follows the IS 2121, IS 2486 and IS 9708 families described later in this guide.

What does a transmission tower carry, from foundation to earth wire peak?

A lattice transmission tower carries eight groups of fittings: stubs, earthing, plates, an anti-climbing device, step bolts, bird guards, attachments for insulators and earth wire, and, where required, aviation lights. Bolts, nuts and washers hold the steel together throughout. The table follows the tower from the ground upward.

The stub definition above follows the tower specification of Odisha Power Transmission Corporation (OPTCL), clause 4.1.5. The fixing height of 2.5 to 3.5 m comes from POWERGRID Technical Specification Section IV-C, Revision 7 (December 2023), clause 1.3. IS 5613 gives the same range (IS 5613 Part 2 Section 1, clause 6.7.7). The sections below take each group in turn. VLI’s transmission tower structure and transmission line accessories pages list the range.

Which accessories does each tower type carry?

Plates, anti-climbing devices, step bolts and earthing go on every tower. What changes between tower types is the attachment for the insulator string and the bird guard. POWERGRID places bird guards on all suspension towers and strain plates under each cross-arm tip of tension towers (Section IV-C, clauses 1.3.2 and 1.3.5.1). A suspension string hangs the conductor from the tower. A tension string takes the pull of the conductor where the line turns or ends.

Tower type follows the angle the line turns at the tower, called the deviation. OPTCL’s specification for 132, 220 and 400 kV lines sets four standard types:

A transposition tower is one where the three phase conductors swap positions along the line, which is why the phase plates need fixing points on both transverse faces. Other utilities follow the same A to D pattern but set their own deviation limits and labels, so read the tower schedule in the tender before ordering. The tower type table is from OPTCL’s transmission line specification, clause 4.1.2.

How is a transmission tower earthed?

Each tower is earthed with pipe earthing or counterpoise wires, as required at the location, and the tower footing resistance must not exceed 10 ohms. POWERGRID adds that the contractor measures the resistance of every tower in dry weather, after erection and before the earth wire is strung (Section IV-C, Revision 7, clause 1.7).

The same clause sets the counterpoise length by soil resistivity. A counterpoise is a set of galvanized wires buried radially from the tower legs. Below 1,500 ohm-metres the earthing uses four wires of 30 m, which is 120 m of wire per tower. Above 1,500 ohm-metres it uses four wires of 70 m, which is 280 m per tower. Counterpoise sites also need a footing impedance reading from a high-frequency tester, and that value must stay under 10 ohms as well.

Where normal pipe or counterpoise earthing does not reach 10 ohms, the specification calls for chemical earthing with a ground enhancement compound. The compound should conform to IEC 62561 Part 7, must not depend on moisture to stay conductive, and must have a set-form resistivity below 0.12 ohm-metre. River crossing towers on pile foundations use a 50 x 6 mm galvanized strip on two legs, joined with 16 mm bolts. Where hard rock stops the counterpoise from reaching its normal 1 m depth, the specification allows a shallower layout shown in its drawing.

For electrode sizes and materials, see VLI’s earthing rod guide. The product pages cover earthing materials, earthing pipe, counterpoise earthing and the cleat clamps that connect the earthing flat to the structure.

What do danger, number, phase and circuit plates do?

Danger plates warn the public and number plates identify the tower. Phase plates mark which conductor carries which phase, and circuit plates mark which circuit of a double-circuit line a conductor belongs to. POWERGRID requires every tower to carry a danger plate, number plate and circuit plate. A single-circuit tower also carries one set of phase plates, and a double-circuit tower carries two sets.

All plates must have rounded corners, and the danger, number and circuit plates must have no sharp edges. POWERGRID also requires phase plate fixing points on both transverse faces of transposition towers (Section IV-C, clause 1.3.5). Face A is the transverse face on the near side of the tower, as defined in IS 5613 Part 2 Section 1, Figure 9 and clauses 6.7.1 to 6.7.4. Because the quantity of circuit plates differs between the two documents, follow the tender when the two disagree.

VLI manufactures vitreous enamel steel number plates and retro-reflective plates for utility and railway electrification work.

How does an anti-climbing device work and where is it fixed?

An anti-climbing device is a barbed-wire barrier fixed to the tower at a height of 2.5 to 3.5 m. A gate on the leg that carries the step bolts lets authorized linemen pass. POWERGRID states the purpose directly: anti-climbing devices and danger boards are installed at all towers to warn and dissuade the general public from climbing EHV (extra high voltage) transmission towers (POWERGRID policies).

POWERGRID’s tower specification requires a barbed-wire type device on all towers. The wire must conform to IS 278, size designation A1, and carry a chromating dip as set out in IS 1340 (Section IV-C, clause 1.3.4). IS 5613 takes a lighter approach. The standard says the device shall conform to the specification laid down by the user of the installation. It gives a typical detail, including the gate for linemen (IS 5613 Part 2 Section 1, clause 6.7.5 and Figure 8). The device is therefore required by the utility specification and drawing, and the standard supplies the typical design.

For the wire grades and sizes behind the IS 278 designation, see VLI’s barbed wire guide. VLI supplies the anti-climbing device and the barbed wire from the same product range.

Step bolts, hangers and fasteners

Step bolts form the ladder that linemen climb. POWERGRID calls for step bolts to IS 10238. Each bolt is at least 16 mm in diameter and 175 mm long, and the bolts are spaced no more than 450 mm apart from 2.5 m above ground to the top of the tower. They sit on two diagonally opposite legs. Each bolt has two nuts on the tower end and a button head on the outer end, and must withstand a vertical load of at least 1.5 kN (Section IV-C, clause 1.3.1). IS 5613 Part 2 describes a similar bolt, 16 mm minimum diameter and 150 mm long with a hexagonal head, starting on leg 1 (clause 6.7.6).

Hangers, D-shackles and strain plates connect the insulator strings to the cross-arms, and U-bolts connect the earth wire clamps. The specification requires each of these to carry the same rated strength as the insulator string or clamp it supports (clauses 1.3.2 and 1.3.3).

The fasteners that join the tower follow their own rules in the same specification:

• Hexagonal bolts and nuts to IS 12427, 16 or 24 mm diameter in property class 5.6 with class 5.0 nuts. A 12 mm bolt of property class 6.8 may replace the 16 mm bolt, but 12 mm and 16 mm bolts cannot be mixed within one package.
• No fully threaded bolts. The threaded end must protrude between 3 and 8 mm beyond the tightened nut.
• A spring washer under every nut, and nuts tightened with a torque wrench at the torque the bolt manufacturer recommends.
• Threads punched after tightening, so the nut cannot loosen. A nut that slips over the thread is replaced together with its bolt.

What galvanizing do tower parts and accessories need?

POWERGRID requires a minimum zinc coating of 610 grams per square metre, with an average thickness of 87 microns, on fabricated tower parts and stubs 5 mm thick or more. Sections and plates thinner than 5 mm need 460 grams per square metre and 65 microns. Where the bid proposal sheet (BPS) names coastal, creek, marine or aggressive-soil locations, the minimums rise to 900 and 610 grams per square metre (Section IV-C, Revision 7, clause 1.6).

The specification applies IS 4759 to the coating and uses IS 2629 as guidance on the process. Bolts and nuts are galvanized to IS 1367 Part 13 by a centrifuge process, and small galvanizing damage is repaired with zinc rich paint before erection. BIS has since published IS 2629 Part 1:2025, which combines IS 2629, IS 4759 and IS 6159 into one document (BIS preview of IS 2629 Part 1:2025). Check which edition your tender cites. VLI runs its own galvanizing facility in Jaipur for tower components and transmission line hardware.

Why do suspension towers carry bird guards, and what do diverters add?

A bird guard stops birds perching above a suspension insulator string and fouling it with droppings. A bird flight diverter makes the earth wire visible so birds do not fly into it. The two devices solve different problems and sit in different places.

POWERGRID also sets a service life of at least 15 years for a flight diverter. Weight is limited to 500 g for a non-LED unit and 800 g for an LED unit, and a line that mixes types needs at least 20 percent LED units. Type tests must come from an accredited laboratory, and the radio interference and visual corona tests apply to lines of 220 kV and above (Section IV-C, clauses 1.3.7 and 1.3.7.3).

Diverters also carry a court-driven requirement in Great Indian Bustard habitat. In M K Ranjitsinh v. Union of India, the Supreme Court’s order of 19 April 2021 addressed overhead lines in priority and potential bustard areas, and its order of 21 March 2024 modified those directions. The 2024 order asked an expert committee to consider the efficacy and suitability of bird diverters on existing and future power lines. Check the current position for any line that crosses bustard habitat.

VLI’s range includes the bird guard, the bird flight diverter in LED and non-LED forms, and the anti-bird disk.

When does a tower need aviation obstruction lights?

A tower needs aviation lights only where the aviation authority or the project’s bid proposal sheet names it. POWERGRID puts span markers, night markers (obstruction lights) and tower painting in the contractor’s scope “wherever indicated in BPS”. The night and day visual aids follow IS 5613 and the requirement of the Directorate of Flight Safety (Section IV-C, clause 1.3.6).

One set of lights on a tower consists of one medium intensity light and two or four low intensity lights, with a battery, solar module and control panel:

• Medium intensity light: fitted on the top of the tower and flashing red at 20 flashes per minute. Effective intensity is 1,600 candela (a unit of light intensity) at night, with an IP 55 rating for dust and water protection.
• Low intensity lights: steady red, minimum 32 candela, IP 55, placement drawing submitted by the contractor.
• Lamp life: at least 15,000 burning hours, with failures inside that period replaced by the contractor at no cost.
• Power: sealed maintenance-free battery bank in a lockable tower-mounted box, charged by solar modules installed below the bottom cross-arm level.

VLI manufactures red LED aviation obstruction lights for transmission line towers and other tall structures.

Conductor and earth wire hardware completes the line

Conductor and earth wire hardware is the second family: the clamps that hold the wire, the dampers that limit vibration, and the joints that connect lengths. Each follows its own Indian standard, and OPTCL’s specification lists them. IS 2121 covers conductor and earth wire accessories: armour rods and binding wires (Part 1), mid-span joints and repair sleeves (Part 2), earth wire accessories (Part 3) and non-tension joints (Part 4). IS 2486 covers insulator fittings for lines above 1,000 volts, and IS 9708 covers Stockbridge vibration dampers.

POWERGRID’s 400 kV specification also puts composite long rod insulators and OPGW, with its fittings, in the contractor’s line hardware supply. This guide stays on the tower and ground side of the line, which is where VLI’s manufacturing range sits.

What foundation and erection items does a tower project need?

A tower project needs stubs and template jacks to set the foundation, lifting equipment to raise the steel, and torque tools to close the joints. POWERGRID promotes the use of cranes where site conditions allow. Where they do not, erection uses a gin pole, derrick or centre mast with power-operated winches, and tractors are not allowed (Standard Technical Specification, Revision 6, clause 1.1.2). The later specification makes a gin pole or centre mast mandatory for conventional erection of 400 kV towers with extensions (added body height) of 18 m or more. A gin pole or centre mast is also mandatory for 765 kV towers with extensions of 9 m or more. In difficult site conditions a derrick is permitted for those towers only with regional approval (Section IV-C, clause 3.3).

The same clause lists the checks that decide whether the accessories can be fitted without rework:

• Lower sections are fully braced and bolted before an upper section goes on.
• Each tower is fitted with its number, danger, circuit and phase plates, its anti-climbing device and, where the drawings call for one, its bird guard.
• Blank holes left for fixing the accessories are filled with bolts and nuts of the correct size after erection.
• The tower stands no more than 1 in 360 out of vertical before stringing.

Lattice towers in the OPTCL specification stand on stubs cast into the footing. Where a design drawing instead calls for base plates, foundation bolts are the anchor, and the drawing decides which applies. VLI manufactures template jacks for tower foundations from 33 kV to 1200 kV towers, derrick poles in common heights such as 6+3 and 6+6, and foundation bolts. When a project runs short of a component through theft, damage or a specification change, the tower shortage supply service covers the gap.

Voltage class: what changes between 66, 132, 220 and 400 kV

The accessory set stays largely the same across voltage classes; the governing code, the ground clearance and the bird diverter tests change. IS 5613 Part 2 covers lines above 11 kV and up to and including 220 kV, which takes in 66, 132 and 220 kV lines. IS 5613 Part 3 covers 400 kV and 765 kV lines, and its design section has a 2024 edition (BIS preview of IS 5613 Part 3 Section 1:2024).

The sources used for this guide give no 66 kV figures, so the tower specification of the owning utility governs a 66 kV line. Ground clearances come from the clearance table in OPTCL’s specification, section 4.1. Always confirm them against the tender, because each utility publishes its own values.

Tower accessories checklist for an enquiry or bill of quantities

An enquiry that lists each accessory with its quantity basis and its standard gets a comparable quotation from every supplier. The checklist below follows the specifications cited in this guide. Replace the quantity basis with the figures from your tower schedule.

The step bolt row shows the arithmetic. The first bolt sits at 2.5 m, and 450 mm spacing fits 83 full intervals between 2.5 m and 40 m, so a leg needs 84 bolts. A real count comes from the drawing and the actual tower height.

Where should you start an accessories enquiry?

Start from the tower schedule. List each tower type, count the suspension and tension towers, and note which are single or double circuit. The accessory quantities then follow from the rules above. Send VLI the schedule and the standard or tender clause that applies, and the team will quote the accessories against it through the contact page. The full range sits on the products page.

Frequently asked questions

How many plates does a transmission tower need?

POWERGRID calls for a danger plate, number plate and circuit plate on every tower. A single-circuit tower also carries one set of phase plates, and a double-circuit tower carries two sets. Transposition towers need phase plate fixing points on both transverse faces. IS 5613 shows the circuit plate as a set of two for double-circuit towers, so confirm the count in your tender.

What is the maximum tower footing resistance?

10 ohms. POWERGRID requires the contractor to measure the resistance of each tower in dry weather after erection and before the earth wire is strung. Counterpoise sites also need a high-frequency footing impedance reading, and that value must stay under 10 ohms too.

Are bird guards fitted to every tower?

No. POWERGRID places bird guards on all suspension towers, to stop birds perching above the insulator string. Tension towers carry strain plates for their tension strings instead, and any additional bird protection depends on the project. Bird flight diverters are a separate device, fitted on the wire in identified spans.

How much zinc coating must tower parts carry?

POWERGRID requires a minimum of 610 grams per square metre, averaging 87 microns, on tower parts and stubs 5 mm thick or more. Thinner sections need 460 grams per square metre. Coastal and aggressive-soil locations named in the bid proposal sheet need 900 and 610 grams per square metre. Ask the supplier for the galvanizing test report with each lot.

Is an anti-climbing device mandatory under IS 5613?

IS 5613 does not make the design mandatory. It says the device shall conform to the specification laid down by the user of the installation and shows a typical detail. In practice the utility specification makes it a requirement: POWERGRID requires a barbed-wire anti-climbing device, to IS 278 size A1, on all its towers.

Cover photo: electricity towers silhouetted against a sunset sky, by Jonathan Borba, via Pexels, Pexels licence.