IEC 60309 (formerly IEC 309) is an international standard from the International Electrotechnical Commission for plugs, socket-outlets and couplers for industrial purposes. IEC 60309-2 specifies a range of mains power connectors with circular housings, and different numbers and arrangements of pins for different applications. The following colour coding is used: yellow = 100-130V; blue = 200-250V; red = 380-480V. Purple and white housings are used for less than 100V, while black and green are reserved for 500V and up and frequencies exceeding 60 Hz. IEC 60309-2 variants are designed so that a plug of one type can only be inserted into a socket of the same type. Different current ratings are distinguished by different diameters of the circular housing. The 16A single and three-phase variants below are commonly used in Europe at campings, marinas, workshops and farms. They are rarely used for indoor domestic purposes. In the UK the IEC 309 standard was adopted as BS 4343 in 1968, and in 1999 replaced by the European equivalent BS EN 60309. In Britain these heavy duty plugs are often referred to as CEE plugs or Commando plugs (refers to the MK Electric company Commando range of connectors).