Earthing of Overhead Line Poles above 1 kV under PUE:2026: Resistance, Down-Conductors, Electrodes and Acceptance

Which poles of lines above 1 kV are earthed

Chapter 2.5 of the Rules for the Arrangement of Electrical Installations (PUE), 2026 edition, covers AC overhead lines above 1 kV up to 750 kV that are being designed, built new or reconstructed (cl. 2.5.1). Lines up to 1 kV fall under chapter 2.4, covered in the article Grounding Metal Lighting and 0.4–10 kV Line Poles. For lines above 1 kV, cl. 2.5.127 requires earthing of:

• poles with an earth wire or other lightning protection devices;
• reinforced-concrete and steel poles of 3–35 kV lines;
• poles carrying power or measuring transformers, disconnectors, fuses and other apparatus.

Wooden poles, including those with steel cross-arms, on a line without an earth wire or other lightning protection are not earthed; a wooden pole with an earth wire falls under the first item. The values below come from the PUE (2026 edition); the designer takes the final values for a specific line from the current text.

Table 2.5.29: the largest resistance of the earthing device

The permitted resistance depends on ρ, the equivalent soil resistivity at the pole: the worse the soil conducts, the more the PUE allows.

PUE (2026 edition), cl. 2.5.127, Table 2.5.29. Bounds given as ‘up to’ include the bound itself (cl. 1.1.18): at ρ = 500 ohm·m the value is 15 ohms.

Who the table applies to and what changes it

• poles with an earth wire or lightning protection: up to 50 m high, the table value; above 50 m, half of it;
• reinforced-concrete and steel poles of 3–20 kV lines in populated areas, poles that limit a crossing span with engineering structures (lines, pipelines) and reinforced-concrete and steel poles of all 35 kV lines: no more than the table;
• poles with apparatus: on lines of 110 kV and above the table value, on 3–35 kV lines no more than 10 ohms unless the technical conditions or the equipment manual require otherwise;
• poles of large crossings with protective devices: no more than 10 ohms at ρ up to 1000 ohm·m and no more than 15 ohms at higher ρ;
• mountain lines above 700 m above sea level: the table values may be multiplied by three;
• double-circuit and multi-circuit poles: a reduction to half is recommended whatever the height.

On a line with an earth wire, an earth electrode made for lightning protection must give the required resistance with the wire disconnected, and for other purposes with the wire connected (cl. 2.5.127).

Unpopulated areas on 3–20 kV lines: natural conductivity and artificial electrodes

On 3–20 kV lines in unpopulated areas the resistance of poles is not specified. It comes from the natural conductivity of reinforced-concrete foundations and the underground part of poles while soil resistivity stays within the limit in the table below; above it the poles also carry artificial electrodes of no more than the stated resistance (cl. 2.5.127).

PUE (2026 edition), cl. 2.5.127.

When natural conductivity counts

On 3–20 kV lines in unpopulated areas, the natural conductivity of the underground part of reinforced-concrete poles counts if the longitudinal reinforcement rods in the shafts, the earthing conductors and the artificial electrode (where there is one) are bonded metallically. Vertical artificial electrodes should stand at least 0.5 m from the shafts (cl. 2.5.130).

Foundations and reinforcement: what counts and what does not

Reinforced-concrete foundations of poles on lines of 110 kV and above may serve as natural earth electrodes if the anchor bolts are bonded metallically to the foundation reinforcement and the concrete has no polymer waterproofing; a bitumen coating makes no difference (cl. 2.5.128).

In clay, loam, sandy-loam and similar soils with ρ ≤ 1000 ohm·m, the reinforcement of foundations and poles on lines of 110 kV and above is used as natural electrodes, alone or together with artificial ones. At ρ > 1000 ohm·m the required resistance comes from artificial electrodes alone (cl. 2.5.129).

On 35 kV lines and on 3–20 kV lines whose resistance follows Table 2.5.29, the required value is reached with artificial electrodes; the natural conductivity of foundations and underground parts of poles need not be counted (cl. 2.5.130).

At a railway crossing, the reinforcement of reinforced-concrete poles and stubs that limit the crossing span may not be used as an earth electrode (cl. 2.5.211).

Down-conductors: cross-section, number and the reinforcement of concrete poles

The cross-section of each earthing conductor (down-conductor) on a pole must be at least 35 mm², and a single-wire one needs a diameter of at least 10 mm (78.5 mm²). Poles of lines of 110 kV and above carry at least two (cl. 2.5.132).

On reinforced-concrete poles, the metallically bonded rods of the longitudinal reinforcement in the shafts, prestressed and non-prestressed, must be used as down-conductors. During a short circuit they must not heat by more than 60 °C, and guy wires are to be used as earthing conductors in addition to the reinforcement. If these conditions cannot be met, a conductor must be run outside the shaft or inside it. On 3–20 kV lines, a conductor that cannot be bonded metallically to the reinforcement of the underground part must be connected to an artificial electrode with a resistance no higher than in Table 2.5.29, in populated and unpopulated areas alike. The earth wire (earthed under cl. 2.5.120) and the insulator fittings on the cross-arm of reinforced-concrete poles must be bonded metallically to the earthing conductor (cl. 2.5.131).

Depth, corrosion and electrode sizes

Pole electrodes usually lie at least 0.5 m deep, and 1 m deep in arable land. In rocky ground, radial electrodes may be laid directly under a loose layer at least 0.1 m thick above the rock; otherwise they are recommended to be laid on the rock and covered with cement mortar (cl. 2.5.133).

In regions with a mean annual relative humidity of 60 % or more, and in medium or strongly aggressive environments, down-conductors should be protected from corrosion where they enter the ground, as DSTU-N B V.2.6-186 requires (cl. 2.5.132). If the electrodes themselves are at risk of corrosion, their cross-section must be increased or electrodes with a galvanic copper coating used (cl. 2.5.132, 1.7.117). Aluminium earthing conductors are not allowed in the ground (cl. 1.7.120).

The smallest sizes of steel electrodes in the ground are set by Table 1.7.5:

PUE (2026 edition), Table 1.7.5. In a strongly aggressive environment black steel should not normally be used (cl. 1.7.117).

Acceptance: what is checked and how many poles

Earthing devices on overhead lines are checked on all poles in populated areas, on sections with the most aggressive, landslide-prone, deflation-prone and poorly conducting soils and, in addition, on at least 2 % of all poles that have earth electrodes (cl. 1.8.202).

Resistance is measured after the natural electrodes are connected (cl. 1.8.206). The permitted values for lines above 1 kV are in Table 1.8.37: for poles with an earth wire or lightning protection, reinforced-concrete and steel poles of 35 kV, the same poles of 3–20 kV in populated areas and equipment electrodes on poles of 110 kV and above they equal Table 2.5.29 (half of it for poles with an earth wire or lightning protection above 50 m). For equipment on 3–35 kV poles the table refers to item 2.1 of Table 1.8.36, for 3–20 kV poles in unpopulated areas to chapter 2.5.

The point where the earthing device joins a reinforced-concrete pole must be reachable for measurements without climbing the pole (cl. 2.5.127). Separately (cl. 1.8.203), the cross-section, integrity and strength of the joints of electrodes and earthing conductors are checked, as is the paint on conductors where they enter the ground. Welds are tested with a hammer blow, and a contact joint is sound if its transition resistance does not exceed 0.05 ohm.

The required resistance is set by the earthing design and confirmed by measurements after the work. Poles: the page Power Line Poles; foundation anchor cages: the article Foundation Anchor Cages FPO, FPM, FKM.