Shield Wire vs OPGW: Choosing the Right Earth Wire for a Transmission Line

Whatever it is made of, the wire at the tower peak has three duties, and every candidate — steel, ACS, small ACSR or OPGW — is judged against them first:

OPGW's telecom capability is always layered on top of these three duties; it must never be bought at the cost of weakening them.

• Lightning interception: the shield wire is positioned to give an adequate shielding angle over the phase conductors, so that strokes terminate on the earth wire and are diverted to ground through the tower and its earthing system instead of striking a phase conductor and causing a flashover. On tall EHV towers, electrogeometric shielding models typically push designers toward small or even negative shielding angles.
• Fault-current return: during a single-phase-to-earth fault, the earth wire provides a metallic return path in parallel with the soil. This lowers the ground potential rise at the faulted tower, improves step and touch voltages, and reduces the zero-sequence impedance seen by the protection.
• Induced-voltage reduction: a continuous earth wire noticeably reduces electromagnetically induced voltages on telecom circuits, pipelines and fences running parallel to the right-of-way.

Before OPGW existed, three material families covered the job, and all three remain valid choices today:

• Galvanized steel strand: the cheapest and mechanically the most robust option, but its electrical conductivity is very low, so it returns only a small share of the fault current. Service life is governed by the zinc coating, which erodes fastest in coastal and industrial atmospheres.
• Aluminium-clad steel (ACS, often called alumoweld): steel wires with a relatively thick metallurgically bonded aluminium cladding. Depending on grade, conductivity is roughly 20-40 %IACS with far better corrosion resistance than galvanizing — a common pick for corrosive environments or lines with high fault levels.
• Small ACSR: in Iranian practice, Fox-class conductors to BS 215 have frequently been strung as guard wire. The aluminium layer gives good conductivity, manufacture is fully standardized under IEC 61089, EN 50182, ASTM B232 and DIN 48204, and line crews are already familiar with the fittings and stringing methods.

OPGW (optical ground wire) is, structurally, still a shield wire — but with an optical unit, usually a stainless-steel or aluminium tube containing the fibres, embedded in the construction, surrounded by aluminium-clad steel and/or aluminium-alloy wires. Design and type testing are covered by IEEE 1138 and the IEC 60794 series; the fibres are normally single-mode ITU-T G.652, with G.655 sometimes selected for long-haul DWDM routes.

With OPGW the utility gains a secure medium for teleprotection, SCADA and inter-substation communications, plus spare dark fibre that can be leased. But OPGW remains first and foremost an earth wire: it must satisfy every mechanical and electrical duty of a conventional shield wire, with the added constraints that the fibres impose a stricter temperature limit during short circuit and tighter rules on handling, bending radius and stringing.

The final choice is a trade-off across several factors that should be tabled at the concept-design stage:

• Fibre need: if the line will form part of the utility's teleprotection or telecom network, OPGW is close to unbeatable; if fibre is reliably available on another route, a conventional shield wire may be all that is required.
• Fault level: the thermal duty is checked as I²t in kA²·s against the wire's adiabatic short-circuit capacity; for OPGW the allowable temperature is set by the optical unit and is usually stricter than for an all-metal wire.
• Sag and span matching: the earth wire is commonly sagged to about 90 percent of the phase-conductor sag to preserve mid-span clearance; everyday tension, creep and ice/wind load cases must hold over the full life of the line.
• Aeolian vibration: earth wires are light and relatively highly tensioned, so they are vibration-prone; damper selection and placement matter for both options.
• Budget: beyond the higher cable price, OPGW adds splice closures and joint boxes, substation approach cables and optical terminal equipment to the project cost.
• New build vs retrofit: on a new line, OPGW is the cheapest way ever to get fibre onto the route; on an existing line, swapping the shield wire for OPGW means outages and re-stringing.
• Corrosion environment: in coastal and industrial areas, aluminium-surfaced options (ACS, ACSR, OPGW) outlast plain galvanized steel.

On 230 kV and 400 kV lines whose towers carry two earth-wire peaks, a common and economical practice is to string one OPGW on one peak and one conventional shield wire — galvanized steel, ACS or small ACSR — on the other. The fault current divides between the two wires, and the conventional wire can be sized to take the larger share, allowing a smaller and cheaper OPGW while keeping full lightning shielding on both peaks. The designer only needs to verify that the asymmetry in sag, weight and impedance between the two wires stays within acceptable limits.

Bandwidth demand on power networks almost never shrinks. Adding fibre later to a line built with a conventional shield wire means outages, re-stringing and sometimes tower-head reinforcement — so on new transmission builds, installing OPGW from day one is usually far cheaper than retrofitting fibre in the future.

The counter-argument is operational simplicity: a conventional shield wire needs no optical skills or splicing equipment to repair, its spares are trivial, and any line crew can restore it. For short industrial tie-lines, or corridors where reliable fibre already exists, a conventional earth wire remains an entirely defensible choice.