EN 50341-2-9-2017 pdf download.Overhead electrical lines exceeding AC 1 kV – Part 2-9: National Normative Aspects (NNA) for Great Britain and Northern Ireland.
NOTE. Unless stated otherwise in the project specification, wind from every direction from 0 to 345’ shall be considered for the design in 15’ increments.
GB.2 Seasonal factor, c.., Where a temporary loading condition will remain in place for less than 1 year, the appropriate
factor may be applied in the calculation of mean wind speed as indicated in Table NA.2.7 in National Annex to BSEN 1991-1-4:2005. Note that the appropriate factor will not be applied in conjunction with wind speeds of less return period than 50 years.
GB.3 Orography factor, C,, The orography factor, co shall be taken as 1.0 where the average ground slope is not greater
than 5% (1:20), measured over a distance of 10 times the height of the supports from the line. For greater slopes, reference shall be made to Figure NA.2 in National Annex to BSEN 1991-1- 4:2005+A1, “Definition of significant orography [definition of symbols given in A.3(3))’. For sites lying within the shaded area of that figure. the method given in BSEN 1991-1-4 A.3 for calculation of c0 may be used. As an alternative, or if the topography is complex, calculation by wind engineering specialists using digital terrain models may be less labour intensive and give more accurate results.
GB.4 Loading on conductors
For calculation of the load on supports due to wind loading on conductors (excluding those indirect effects due to conductor tension) the magnitude of the height above ground (h) adopted for the calculation of Vh(h) shall generally be taken as the average height to the attachment of the support considered except in the case of spans crossing deep valleys, river estuaries or hills where the attachment heights would not be rePresentative of the actual heights to the conductors away from the supports. In these cases, the value of h adopted shall be adjusted to approximately represent the mean height from the ground or water level to the attachment points on the supports, Alternatively, advice from a wind engineering specialist may be sought.
4.3.3 Mean wind pressure
GB.1 Air density
Air density in Great Britain shall be taken as 1,226 kg/rn3. Table 4.2 in BSEN 50341-1:2012 shall not be used.
Wind forces on overhead line components
G8.1 Design Approach 3
Table 4.4.1IGB,1 details the design wind pressures and drag factors to be adopted for design Approach 3.
The span factor G shall be assumed to be 1,0 for wind span lengths up to 200m and (0.75L + 30)IL metres for wind span lengths greater than 200m. Normal and High altitudes are defined as follows:
GB.6 Wood pole lines (design Approach 3)
For wood pole lines with conductors not exceeding 35mm2 copper (or 60mm2 for aluminium- based conductors), the wind only loading case may be used for all altitudes, with no applied ice loading. For all other lines, the following ice loadings are applicable when additionally subjected to wind:
Normal altitudes 9,5mm radial thickness
High altitude 12.5mm radial thickness
A greater radial ice thickness may be defined in the Project Specification, The following unit weight of ice/snow are considered depending on locality and altitude: glaze ice 9 kN/m3. wet snow! rime ice 5 kN/m’, The specific unit weight of ice to be used will be detailed in the Project Specification.
Temperature effects
Design Approach I
GB.1 Temperature effects
(a) Minimum temperature, with no other climatic load is not generally a critical loading condition in the GB but for supports subject to uplift loading may generate maximum uplift loads.
(b) The normal ambient temperature for extreme wind speed conditions in GB shall be assumed to be 0°C for design Approach 1.
(C) A reduced wind speed combined with a minimum temperature condition is not a critical loading condition in GB and need not be considered.
(d) The temperature to be considered for both icing in still air and combined wind and ice In GB shall be assumed to be -10°C.
Design Approach 3
The temperature to be considered for both icing in still air and combined wind and ice in GB shall be assumed to be -5,6°C
Security loads
GB.1 Security loadings (Failure containment or Broken Wire Conditions)
Towers shall be designed to resist the torsional or longitudinal loads, which would be generated by combinations of broken conductor(s), and)or earthwires.
Full details of failure containment conditions shall be given in the Project Specification, which will specify the following:
1) The combinations of conductors and earthwires, which shall be considered to be broken simultaneously.EN 50341-2-9-2017 pdf download.