Grounding Metal Lighting and 0.4–10 kV Line Poles: PUE Requirements

Grounding poles in a 0.4 kV network (lighting lines and low-voltage overhead lines)

In a solidly earthed neutral network (a TN system), metal poles are bonded to the PEN conductor, which combines the functions of the neutral and protective conductors. Under the PUE (2026 edition), cl. 2.4.35, on poles with a bare PEN this is done on every pole.

Repeated PEN earthing should be made at the start and end of a line and on branches longer than 200 m: natural and lightning-protection electrodes first, separate ones only if these are absent or insufficient (cl. 1.7.93). In populated areas with one- and two-storey buildings, lines not screened by tall chimneys or trees must have lightning-protection electrodes: 30 ohms at most each, no more than 100 m apart (cl. 2.4.40).

Under the PUE (2026 edition), cl. 1.7.95, the combined resistance of all earth electrodes connected to the line’s PEN conductor, natural ones included, must not exceed 5, 10 and 20 ohms at any time of year (three-phase source, line voltage of 690, 400 and 230 V respectively). The most common network in Ukraine is 400/230 V, usually called 380/220 V, so the benchmark is 10 ohms for the whole line. The resistance of each repeated earth electrode (a pole’s own, if it has one) must not exceed 15, 30 and 60 ohms respectively.

For soil with a resistivity above 100 ohm·m, these norms may be multiplied by 0.01·ρ (ρ is the soil resistivity in ohm·m), but by no more than 10 times (cl. 1.7.96). The exception is a network whose neutral earth electrode serves installations both up to and above 1 kV: there the increase is allowed only while the voltage on the earthing device stays within Table 1.7.3.

Grounding poles of 3–20 kV lines in populated areas

For steel and reinforced-concrete poles of 3–20 kV lines in populated areas, the largest resistance of the earthing device is set by the PUE (2026 edition), cl. 2.5.127, Table 2.5.29: 10 to 30 ohms for equivalent soil resistivity ρ up to 5000 ohm·m and 6·10⁻³·ρ ohms above that. This value must be achieved with artificial electrodes (cl. 2.5.130). The full table and other cases (35 kV lines, crossings, poles with apparatus) are in the article Earthing of Overhead Line Poles above 1 kV.

Grounding on our poles

Our metal multi-sided poles have a provision for connecting a grounding bolt (see Technical Requirements for Lighting Poles); per the project, an additional earth connection is made to the pole’s contact point with an M16 bolt (see Installing Multi-sided Poles).

What to keep in mind

The allowable grounding resistance for your site depends on the network voltage, the area (populated or not) and the soil resistivity measured there. The power supply design sets it; there is no single universal figure. The norms above are a guide for a preliminary estimate.

Example: measuring a pole’s grounding resistance

Resistance is measured after the earth electrodes are installed, by the fall-of-potential method. To measure one electrode, it is first disconnected from the PEN conductor and the other electrodes:

• two auxiliary electrodes, a current one and a potential one, are driven in a straight line from the earth electrode at the distances given in the meter’s manual;
• current is passed between the earth electrode and the current electrode, and the voltage is measured at the potential one; the meter calculates R = U / I;
• the measurement is repeated with the potential electrode moved: close results confirm the value.

Example for a 400/230 V network: four repeated earth electrodes, measured separately, at 24, 28, 30 and 26 ohms (each within 30 ohms). In parallel they give ≈6.7 ohms, below the 10-ohm limit for the line (electrode interaction ignored).

Who is responsible for what

Lightning grounding of a pole under DSTU EN 62305-3

For a pole with an air-termination, cl. 5.4.1 of DSTU EN 62305-3:2021 names the shape and size of the earth-termination system as important criteria and in general recommends a low resistance: up to 10 ohms at low frequency, where possible. This is not the PUE’s 10 ohms, which limit the combined resistance of a line’s repeated earth electrodes. If the resistance of the whole earth-termination system, measured at neither the mains frequency nor its multiples, is below 10 ohms, the minimum electrode lengths (table below) may be disregarded (cl. 5.4.2.1).

The standard prefers one combined earth system for lightning protection, power and telecommunications. Each down-conductor needs a test joint that can be opened only with a tool, except natural ones connected to foundation electrodes (cl. 5.3.6).

Type A or B and electrode length

Type A (cl. 5.4.2.1): horizontal or vertical electrodes at each down-conductor, no fewer than two in total. Type B (cl. 5.4.2.2): a ring around the structure lying in soil for at least 80 % of its length, or a closed-loop foundation electrode. Type A suits rod air-termination and an isolated lightning protection system (LPS); type B is recommended for mesh air-termination, several down-conductors and bare rock (cl. 5.4.3; informative Annex E, E.5.4.2).

The minimum length l1 of a horizontal electrode at each down-conductor depends on the LPS class and soil resistivity ρ (Fig. 3). Approximate values of l1:

A vertical electrode needs 0.5·l1, and 0.5 m is added to the length for frost (cl. 5.4.3, Note 2); electrodes are advised to be laid at least 0.5 m deep (cl. 5.4.3). The l1 values are read off the graph in Fig. 3 (±1 m); exact ones come from the figure. For type B the mean radius re of the area inside the ring has to be at least l1; otherwise electrodes of length l1 − re (horizontal) or (l1 − re)/2 (vertical) are added. Lowering the resistance by lengthening electrodes is practically acceptable up to 60 m; above 3000 Ω·m type B or a mix of types is recommended.

Earth electrode corrosion: what the standard advises

This is about grounding electrodes and conductors, not the pole shaft. Table 5 of the standard (general recommendations): hot-dip galvanised steel suits air, concrete and good soil, in the ground only as a solid conductor; it may corrode in clay or wet soil and should not pass from concrete into soil.

Steel in concrete and steel in soil, if connected, form a galvanic couple of about 1 V that dissolves the steel in the soil (informative Annex E, E.5.4.3.2). For soil electrodes connected to steel in concrete, copper, copper-clad steel or stainless steel is recommended; a galvanised one is advised to be joined to steel in concrete through isolating spark gaps, not directly (E.5.6.2.2.1).

Touch and step voltage near the pole

Near down-conductors, touch and step voltage can be dangerous to life (Section 8). The risk is acceptable if any one of these holds: no people within 3 m of the down-conductors in normal use; a system of at least 10 down-conductors made per cl. 5.3.5; the contact resistance of the surface soil layer within 3 m of the down-conductors is at least 100 kΩ (per the note, 5 cm of asphalt or 15 cm of gravel usually gives this).

If none is met, the standard’s measures against touch voltage are insulation of an exposed down-conductor (100 kV, 1.2/50 µs impulse withstand) and/or inaccessibility with warning signs (cl. 8.1); for step voltage (cl. 8.2) the conditions are the same and the protective measures are chosen in the design.

Common mistakes

• treating a one-off measurement as constant: in a drought or in frozen ground the soil resistance is higher than after rain;
• tightening the grounding bolt on a dirty or painted surface;
• concreting the foundation without the grounding lead.