According to ICAO – Annex 14, the High Intensity Obstruction Lights – Type A – are used to indicate the presence of structures whose height above the level of the surrounding ground exceeds 150m. It is also used when aeronautical studies indicates such lights to be essential for recognition of the structure by day and night.
The High Intensity Obstruction Lights – Type B are used to indicate the presence of a tower supporting overhead wires, cables, etc., where an aeronautical studies indicates such lights to be essential for the recognition of the presence of wires, cables, etc.; or where it has not been found practicable to install markers on the wires, cables, etc.
