Enhanced Fujita scale damage indicators and degrees of damage, chapter 26

C-1 Electrical Transmission Lines (C-ETL)

Typical Construction:

  • Single wood poles with wood cross arms, 7-35 metres (m) in height and 15-60 centimetres (cm) in diameter (at 2 m above ground)
  • Single steel or concrete poles with metal cross arms
  • Metal trussed towers


  • Whether poles go down is related to size, composition (wood / concrete / steel) and load (wire tension, number of transformers)
  • For small diameter (~15-20 cm) or very old wood poles, decrease toward lower-bound wind speed; for large diameter (~45-60 cm) wood poles, increase toward upper-bound wind speed
  • Keep in mind - one weak or overloaded pole going down can cause other poles along the line to break due to wire tension
  • Trees falling on lines can bring down poles - do not use this DI if this is the case
  • Metal truss towers where lines change direction are often more strongly reinforced, increase toward upper-bound wind speed
Table 26. Degree of Damage (DOD) for Expected (EXP), Lower Bound (LB) and Upper Bound (UB)
DOD Damage Description EXP* LB* UB*
1 Threshold of visible damage 130 110 155
2 Broken wood cross member 155 125 175
3 Wood poles leaning 175 135 200
4 Broken wood poles 195 145 220
5 Broken or bent steel or concrete poles 210 180 240
6 Collapsed metal truss towers 220 185 255

*All wind speeds in km/h

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