IEC 61400-2 standard as interpreted within e.g. the MCS certification scheme (of UK origin) is interoperable with the USA (for example where it corresponds to an AWEA small wind turbine standard) and other countries. Wind Turbine Generator (WTG) classes Wind turbines are designed for specific conditions. During the construction and design phase assumptions are made about the wind climate that the wind turbines will be exposed to. Turbine wind class is just one of the factors needing consideration during the complex process of planning a wind power plant. Wind classes determine which turbine is suitable for the normal wind conditions of a particular site. Turbine classes are determined by three parameters - the average wind speed, extreme 50-year gust, and turbulence. Turbulence intensity quantifies how much the wind varies typically within 10 minutes. Because the fatigue loads of a number of major components in a wind turbine are mainly caused by turbulence, the knowledge of how turbulent a site is of crucial importance. Normally the wind speed increases with increasing height due to vertical wind shear. In flat terrain the wind speed increases logarithmically with height. In complex terrain the wind profile is not a simple increase and additionally a separation of the flow might occur, leading to heavily increased turbulence. The extreme wind speeds are based on the 3 second average wind speed. Turbulence is measured at 15 m/s wind speed. This is the definition in IEC 61400-1 edition 2. For U.S. waters however, several hurricanes have already exceeded wind class Ia with speeds above the 70 m/s (156 mph), and efforts are being made to provide suitable standards. In 2021, TÜV SÜD developed a standard to simulate a new wind class T1 for tropical cyclones. List of IEC 61400 parts IEC 61400-1:2005+AMD1:2010 Design requirements IEC 61400-2:2013 Small wind turbines IEC 61400-3:2009 Design requirements for offshore wind turbines IEC 61400-4:2012 Design requirements for wind turbine gearboxes IEC 61400-6:2020 Tower and foundation design requirements IEC 61400-11:2012 Acoustic noise measurement techniques IEC 61400-12-1:2005 Power performance measurements of electricity producing wind turbines IEC 61400-12-2:2013/COR1:2016 Power performance of electricity-producing wind turbines based on nacelle anemometry / Corrigendum 1 IEC 61400-12-1:2017 Power performance measurements of electricity producing wind turbines / Remote sensing devices like Sodar & lidar measurements IEC 61400-13:2015 Measurement of mechanical loads IEC TS 61400-14:2005 Declaration of apparent sound power level and tonality values IEC 61400-21:2008 Measurement and assessment of power quality