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IEC TR 62248: Atmospheric Icing of Structures — Measurement and Design Guide
Understanding ice accretion mechanisms, meteorological measurement methods, and structural design implications for atmospheric icing
IEC TR 62248, published as a Technical Report in 2002 by IEC Technical Committee 11 (Overhead Lines), provides essential guidance on the measurement, characterization, and structural design implications of atmospheric icing. Ice accretion on structures poses a significant hazard to infrastructure worldwide, from transmission towers and overhead power lines collapsing under accumulated ice weight to wind turbines suffering production losses and fatigue damage from uneven ice buildup. This report consolidates meteorological science and engineering practice into a unified framework for managing icing risk.
Atmospheric icing is estimated to cause billions of dollars in infrastructure damage annually across Northern Europe, North America, and East Asia. In China alone, the 2008 ice storm affected over 200 million people and caused direct economic losses exceeding 150 billion RMB, demonstrating the critical importance of proper icing design standards.
Icing Mechanisms and Classification
IEC TR 62248 classifies atmospheric icing into three primary types based on the physical mechanism of ice formation. In-cloud icing (also known as rime icing) occurs when supercooled water droplets in clouds or fog freeze on impact with a structure. This is the most common type affecting high-altitude infrastructure and wind turbines. The severity depends on the liquid water content (LWC) of the cloud, the droplet size distribution (median volume diameter, MVD), the wind speed, and the ambient temperature. Rime ice typically forms at temperatures between -15 deg C and 0 deg C, with rapid accretion rates when wind speeds exceed 10 m/s.
Precipitation icing (glaze ice or freezing rain) occurs when rain falls through a sub-freezing layer of air near the surface and freezes upon impact. Glaze ice is denser and more adhesive than rime, creating heavier loads per unit thickness. Freezing rain events are particularly hazardous because they can produce ice accumulations of 50-100 mm in a single storm, as seen in the 1998 North American ice storm and the 2008 Chinese ice storm. The duration of freezing rain and the wind speed during deposition critically determine the final ice load.
Hoar frost forms by direct sublimation of water vapor onto cold surfaces below 0 deg C under calm, humid conditions. While individual hoar frost events produce light, feathery deposits, repeated daily cycles can accumulate significant mass on bridge cables, tower lattice members, and overhead ground wires over weeks or months.
| Icing Type | Density (g/cm3) | Typical Temperature | Accretion Rate | Primary Hazard |
|---|---|---|---|---|
| Rime (in-cloud) | 0.1 – 0.6 | -15 to -3 deg C | Slow to moderate | Fatigue, aerodynamic imbalance |
| Glaze (precipitation) | 0.7 – 0.9 | -5 to 0 deg C | Rapid | Structural overload, conductor breakage |
| Hoar frost | 0.05 – 0.3 | Below -5 deg C | Very slow | Galloping, ice shedding |
| Wet snow | 0.3 – 0.6 | 0 to +2 deg C | Moderate | Sleeve formation, unbalanced loads |
| Mixed (rime + glaze) | 0.3 – 0.7 | -10 to -2 deg C | Moderate to rapid | Combined weight and aerodynamic effects |
The density of ice accretion is a critical parameter in load calculations. Assuming glaze ice density (0.9 g/cm3) when the actual accretion is rime (0.3 g/cm3) leads to grossly conservative designs, while the opposite assumption leads to unsafe structures. Site-specific icing measurements are essential for calibrating design assumptions.
Measurement Methods and Data Interpretation
IEC TR 62248 describes several methods for measuring ice accretion on structures. Passive methods include the use of ice accretion rods (cylindrical or flat-plate collectors) exposed at representative heights, which are periodically measured for ice mass and thickness. Active methods employ heated sensors that detect ice formation through changes in vibration frequency, capacitance, or thermal impedance. The report provides detailed guidance on sensor placement to avoid turbulence and wake effects from the supporting structure itself.
Meteorological measurements required for icing characterization include wind speed and direction at 10 m height, ambient temperature, relative humidity, and atmospheric pressure. For in-cloud icing assessment, the liquid water content and droplet size distribution must be measured using rotating cylinder collectors or optical particle counters. The standard recommends continuous monitoring at 10-minute intervals during icing events, with data logging for at least 5 consecutive winters to establish reliable statistical extremes.
Statistical analysis of icing data follows extreme value distribution methods (typically Gumbel or generalized extreme value distribution). The report recommends calculating ice loads with return periods of 50 years for standard structures and 150-500 years for critical infrastructure such as nuclear power plant transmission lines. Regional icing maps dividing territory into zones of equal icing severity are the most common tool for communicating design values to engineers.
| Infrastructure Category | Return Period (years) | Examples |
|---|---|---|
| Standard distribution lines | 15 – 30 | Rural MV/LV distribution |
| Transmission lines | 50 – 75 | 110 kV – 220 kV lines |
| Major transmission corridors | 50 – 150 | 400 kV – 800 kV highways |
| Critical infrastructure | 150 – 500 | Power plant connections, substations |
| Wind turbines | 50 (IEC 61400-1) | Onshore and offshore turbines |
Modern remote monitoring using ice detection sensors combined with SCADA systems allows real-time assessment of icing conditions on critical infrastructure. Early warning systems can trigger preventive actions such as increased current flow for de-icing transmission lines, shutting down wind turbines during heavy icing, or dispatching maintenance crews proactively.
Engineering Design Insights for Icing Environments
Structural design for icing involves three interconnected considerations: mechanical load capacity under ice weight, dynamic response to wind-on-ice conditions, and ice shedding behavior that can create non-uniform loads. For overhead transmission lines, the combined ice and wind load case is typically the governing design condition, with load factors combining nominal ice thickness (typically 10-50 mm radial) and concurrent wind pressure (typically 0.15-0.5 kN/m2 acting on the projected ice-coated surface). Differential icing between adjacent spans due to varying elevation or exposure can create dangerous longitudinal unbalanced forces on towers and fittings.
For wind turbines, icing affects both structural integrity and aerodynamic performance. Ice accretion on blades alters the airfoil profile, reducing power output by 10-50% during moderate icing events and causing complete shutdown under severe conditions. Uneven ice shedding from rotating blades creates severe imbalance loads that can trigger emergency stops and cause fatigue damage to the drivetrain. Design solutions include blade heating systems (electrical resistance or hot air circulation), hydrophobic coatings that reduce ice adhesion by 70-90%, and ice-tolerant control algorithms that limit power production during icing events while maintaining safe operation.
Bridge structures face unique icing risks from cable-stay and suspension cable icing, where ice accretion increases self-weight and wind load while ice falling from cables poses safety hazards to traffic below. De-icing systems using heated cables, passive shedding designs with low-adhesion sheath materials, and exclusion zones beneath iced cables are all established mitigation strategies.
Climate change adds a complex dimension to icing engineering. While warming temperatures generally reduce icing frequency, they may increase the severity of freezing rain events in some regions as warmer air holds more moisture. Engineers designing infrastructure with 50-100 year service lives must consider potential shifts in icing zones and incorporate adaptive capacity into structural designs.
Q1: What is the difference between rime ice and glaze ice in terms of structural danger?
A: Glaze ice is significantly denser (0.7-0.9 vs 0.1-0.6 g/cm3) and adheres more tenaciously to surfaces, creating heavier loads per unit thickness. Glaze forms during freezing rain events and can accumulate rapidly (10-30 mm/hour), while rime forms from cloud droplets and accretes more slowly. However, rime ice on structures like wind turbine blades can accumulate asymmetrically and cause significant aerodynamic and dynamic problems even at relatively low mass loads.
A: Glaze ice is significantly denser (0.7-0.9 vs 0.1-0.6 g/cm3) and adheres more tenaciously to surfaces, creating heavier loads per unit thickness. Glaze forms during freezing rain events and can accumulate rapidly (10-30 mm/hour), while rime forms from cloud droplets and accretes more slowly. However, rime ice on structures like wind turbine blades can accumulate asymmetrically and cause significant aerodynamic and dynamic problems even at relatively low mass loads.
Q2: How is design ice thickness determined for a specific location?
A: The process involves: (1) collecting meteorological data from nearby weather stations over at least 20-30 years, (2) identifying icing events from temperature, humidity, wind, and precipitation records, (3) converting meteorological parameters to ice loads using physical accretion models, (4) fitting extreme value distributions to the annual maximum ice loads, and (5) reading the design ice thickness corresponding to the selected return period from the distribution. National icing maps provide pre-computed values for most regions.
A: The process involves: (1) collecting meteorological data from nearby weather stations over at least 20-30 years, (2) identifying icing events from temperature, humidity, wind, and precipitation records, (3) converting meteorological parameters to ice loads using physical accretion models, (4) fitting extreme value distributions to the annual maximum ice loads, and (5) reading the design ice thickness corresponding to the selected return period from the distribution. National icing maps provide pre-computed values for most regions.
Q3: Can ice accumulation be prevented on structures?
A: Complete prevention is generally not feasible for large structures, but several mitigation methods are effective: passive approaches include aerodynamic shaping to reduce accretion surface area, hydrophobic coatings, and structural design for safe ice shedding. Active methods include electrical resistance heating, hot air circulation (used in wind turbine blades), chemical anti-icing compounds (limited applications), and mechanical de-icing (manual or robotic). The choice depends on the structure type, available power, and required reliability.
A: Complete prevention is generally not feasible for large structures, but several mitigation methods are effective: passive approaches include aerodynamic shaping to reduce accretion surface area, hydrophobic coatings, and structural design for safe ice shedding. Active methods include electrical resistance heating, hot air circulation (used in wind turbine blades), chemical anti-icing compounds (limited applications), and mechanical de-icing (manual or robotic). The choice depends on the structure type, available power, and required reliability.
Q4: How does IEC TR 62248 relate to other structural design standards?
A: IEC TR 62248 provides the icing meteorology and measurement framework, while structural design standards such as IEC 60826 (overhead line design), IEC 61400-1 (wind turbines), and national building codes specify the load combination factors, partial safety factors, and structural resistance verification methods that use the icing data as input. The report is designed to serve as the meteorological foundation for these application-specific standards.
A: IEC TR 62248 provides the icing meteorology and measurement framework, while structural design standards such as IEC 60826 (overhead line design), IEC 61400-1 (wind turbines), and national building codes specify the load combination factors, partial safety factors, and structural resistance verification methods that use the icing data as input. The report is designed to serve as the meteorological foundation for these application-specific standards.
