Ice, Rime and Wet Snow on Power Lines: Types of Deposit and Design Thickness

Why icing loads matter

Icing deposits increase the weight, wind area and tension of conductors and earth wires, so together with wind they set the design loads on poles and foundations. A general overview of wind, ice and temperature effects and design conditions is in Loads on Overhead Lines and Poles; the wind regions are in Wind Regions of Ukraine.

Types of deposit and their parameters

Ice deposits include hoar frost, crystalline and granular rime, glaze and mixed deposits. Hoar frost and crystalline rime add no significant load to conductors. Much harsher conditions come from granular rime, glaze and a mix of the two.

• Granular rime is a loose, snow-like ice that builds up on the windward side of conductors in drizzle, mostly windy, especially in mountains. Deposits reach many centimetres in thickness, with a bulk density of up to 0.4 g/cm³. In mountain regions the diameter of deposits sometimes reaches 150–200 mm, and they can stay for 10 days or more. Because of the large windage they can cause considerable, sometimes catastrophic, loads.
• Glaze is a layer of dense ice that builds up on the windward side as drops of supercooled rain or drizzle freeze. It is seen mostly at 0 to −3 °C, more rarely down to −16 °C. The crust reaches several centimetres in thickness (50–60 mm in some areas), with a density of 0.6–0.9 g/cm³. Glaze can form over anything from a few hours to a few days and stay for up to 15 days. When thick, the weight of the ice exceeds the weight of the conductor many times over, and the greater windage considerably raises the wind pressure.
• A mix of glaze and rime in mountain and foothill areas sometimes reaches 70 mm, and with alternating layers twice as much. Its density is from 0.2 to 0.6 g/cm³ (on average 0.35 in mountains, 0.2 on plains). The mix can stay on conductors for up to 40 days.
• Wet snow sticks at air temperatures up to about +1 °C. If the temperature then falls below zero, the snow freezes on and is not blown off. The density of frozen snow is from 0.1 to 0.5 g/cm³, less than ice, but deposits can be very large (20–30, sometimes 50 mm in diameter). In a cross wind these conditions are no easier than with glaze.

Deposits can also form on the poles themselves and add load on the members.

The amount of deposit depends on local conditions. In central Ukraine, for example, glaze deposits are relatively rare and weigh 0.5–1 kg per metre of conductor, while in some areas of Donbas glaze and rime are common and deposits weigh 2–5 kg/m. The intensity and frequency of icing grow with height above sea level, and near large bodies of water icing is heavier and more frequent: the influence of a sea extends tens and hundreds of kilometres. In most of Ukraine light crystalline rime is seen, while heavy icing occurs in western Ukraine, Crimea and Donbas. The worst conditions are on open mountain ridges and windward slopes.

Weather stations watch icing on short wires or rods 5 mm in diameter about 2 m above the ground, whereas line conductors hang much higher and are thicker. So the design ice load on a line conductor is found with correction factors for suspension height and diameter: it rises with height, because at height the wind is faster and carries more supercooled droplets. Under the Rules for the Arrangement of Electrical Installations (PUE), 2026 edition, cl. 2.5.27, 2.5.35, Fig. 2.5.1, the characteristic value (N/m), from which the ice wall thickness is calculated, is set for a 10 mm conductor at a height of 10 m above the ground on plain terrain, from a map of six regions, for a mean return period of 50 years.

PUE ice regions: from 8 to 40 N/m

The PUE (2026 edition) divide Ukraine into six regions by icing (Fig. 2.5.1). For each region the map gives the characteristic ice load gp, N/m; the ice wall thickness b for pole members more than 70 mm across is calculated from gp as b = √(35.4 · gp + 25) − 5 (cl. 2.5.35, 2.5.37).

For lines of reliability class 3 or 4 (110–330 and 500–750 kV) the values of gp are taken from the regional maps in Annex A (a separate album), for classes 1 and 2 (up to 35 kV) from the same maps or from the PUE map (Table 2.5.2, cl. 2.5.29).

On sections in hard-to-reach terrain, on hydroelectric dams and near cooling ponds, gp must be increased by 2 N/m (regions 1–3) or by 5 N/m (regions 4–6) if there are no observation data (cl. 2.5.32); the map values may be refined from many years of observations (cl. 2.5.30).

The table reproduces the map legend (Fig. 2.5.1); for region 6 the formula gives b of about 33 mm. The designer takes the final values for a specific site from the current PUE text.

Example: weight and windage of ice

An illustrative example, not a regional value. Conductor diameter d = 10 mm, ice wall thickness b = 10 mm, density 0.9 g/cm³. The ice ring area is π · b · (d + b) = 3.14 · 10 · 20 ≈ 628 mm²; cross-check using the iced diameter (d + 2b = 30 mm): π/4 · (30² − 10²) ≈ 628 mm². The ice weighs 6.28 cm² · 100 cm · 0.9 g/cm³ ≈ 565 g per metre (0.57 kg/m).

On level ground with adjacent spans of 300 m and 260 m, the weight span is (300 + 260) / 2 = 280 m, and the ice on it adds 0.565 · 280 ≈ 158 kg to the weight of the conductor. The iced diameter of 30 mm is three times the conductor diameter, so the windage per metre of conductor also triples.

From gp to the load on a conductor

The PUE (2026 edition) give the design ice load on a conductor, earth wire or other round member up to 70 mm in diameter as Gmp = k1 · μ1 · gp · γfG, N/m (cl. 2.5.33, 2.5.34). The factor k1 depends on the height of the reduced centre of gravity of the conductor: 5 m — 0.7; 10 m — 1.0; 20 m — 1.3; 30 m — 1.7; 50 m — 2.2; 70 m — 2.7; 100 m — 3.3 (Table 2.5.3). The factor μ1 depends on the diameter and on the load itself and equals 1.0 for d = 10 mm (Table 2.5.4). The factor γfG depends on the return period T: 5 years — 0.46; 10 — 0.63; 15 — 0.72; 25 — 0.84; 50 — 1.00 (Table 2.5.5).

For conductors, T is set by the reliability class of the line: 5, 10, 15 or 25 years for classes 1 to 4 (Table 2.5.1); class 2 covers 1–35 kV lines and class 3 110–330 kV lines (Table 2.5.2). For the strength of poles and foundations the periods are longer: 30, 50, 150 and 500 years.

An illustrative example for the conductor itself (cl. 2.5.25), not a value for a real line. Let gp = 15 N/m, conductor d = 10 mm (μ1 = 1.0), height of the centre of gravity 30 m (k1 = 1.7). A 35 kV line (class 2, T = 10 years, γfG = 0.63): Gmp = 1.7 · 1.0 · 15 · 0.63 ≈ 16.1 N/m. A 110 kV line (class 3, T = 15 years, γfG = 0.72): Gmp = 1.7 · 1.0 · 15 · 0.72 ≈ 18.4 N/m. The difference is about 14 %, because 0.72 ÷ 0.63 ≈ 1.14.

Ice on the pole itself

The PUE (2026 edition) do not always require counting ice on the pole itself. For steel poles of structural sections it is counted when the pole is taller than 50 m, stands in ice region 5 or 6, or is in mountains where the maximum characteristic ice load exceeds 30 N/m; for reinforced-concrete, multi-sided and wooden poles and steel poles with tubular members it is not (cl. 2.5.36).

Wind during icing in the PUE

The PUE (2026 edition) set wind during icing separately. For iced conductors and earth wires it is a line load in N/m from a separate map (Fig. 2.5.5, cl. 2.5.54, 2.5.56); for flat elements wider than 70 mm (pole members, insulators), a characteristic pressure of 150 to 400 Pa from a map of six regions at 10 m on plain terrain (Fig. 2.5.4, cl. 2.5.51, 2.5.53). In the normal mode this wind is combined with ice at 0.9 of the design value, at minus 5 °C (Table 2.5.14, cl. 2.5.61).

Checklist and common mistakes

Check:

• the type of deposit and its density: 0.9 g/cm³ is the density of glaze ice, while rime and wet snow are much lighter; for conductors the PUE (2026 edition) set ice as a load in N/m by region (Fig. 2.5.1), not as a density, and name 0.9 g/cm³ only for pole members more than 70 mm across (cl. 2.5.37);
• the ice region, with gp and b, from the PUE map or the regional maps; the return period from Table 2.5.1, separately for conductors and poles;
• the conductor diameter, suspension height and weight span (see Distance Between Power Line Poles);
• windage, taken from the iced diameter.