Free sailing height

Sail-free height under a bridge is calculated from the average maximum sea height to the lowest point of the bridge or to the height of the line between the two red marks ("specified width)) reduced by a 0.5 metre safety margin for variation of sea level.

By average maximum sea height is meant the water level which is the average value of all recorded annual maxima for the water level.

Sail-free height under a power line is calculated from the average maximum sea height to the lowest point of the line reduced by reduced by a safety margin (foravoiding the possibility of an electric arc between the top of the mast and the power line) which is obtained the Swedish electrotechnical norm:

• 1.5 metre for powerlines < 170 kV
• 2.0 metres for powerlines >170 kV and < 245 kV
• 2.75 for power ines > 245 kV and < 420 kV

High-voltage powwer lines should have two square signs, one indicating the sailing free height, and the second indicating the high voltage nature of the line (thunder sign).