IEC 60309 (formerly IEC 309 and CEE 17, also published by CENELEC as EN 60309) is a series of international standards from the International Electrotechnical Commission (IEC) for "plugs, socket-outlets and couplers for industrial purposes". They are also used in the entertainment industry where they are commonly referred to as "CeeForm" connectors. The maximum voltage allowed by the standard is DC or AC; the maximum current, ; and the maximum frequency, . The ambient temperature range is to . There is a range of plugs and sockets of different sizes with differing numbers of pins, depending on the current supplied and number of phases accommodated. Connectors generally are specified by the voltage and current ratings, general configuration (number of pins), and rotational alignment ("keying"). The fittings are popular in open-air conditions, as the connectors have a minimum IP44 weather-proofing rating. They are also sometimes used in situations where their special capabilities (such as high current rating or three-phase facilities) are not needed, to discourage potential users from connecting domestic appliances to the sockets, as 'normal' domestic plugs will not fit. The cable connectors and sockets are keyed and colour-coded, according to the voltage range and frequency used; common colours for 50–60 Hz AC power are yellow for 100–130 volts, blue for 200–250 volts, and red for 380–480 volts. The blue fittings are often used for providing weather-proofed exterior sockets for outdoor apparatus. In camping situations, the large 32 A blue fittings provide power to static caravans, whilst the smaller blue 16 A version powers touring caravans and tents. The yellow fittings are used to provide transformer isolated 110 V supplies for UK construction sites to reduce the risk of electric shock, and this use spills over into uses of power tools outside of the construction site environment. The red three-phase versions are used for three-phase portable equipment. Standardization IEC 60309-1 "Plugs, socket-outlets and couplers for industrial purposes" specifies general functional and safety requirements. IEC 60309-2 "Dimensional interchangeability requirements for pin and contact-tube accessories" applies to plugs and socket-outlets, cable couplers and appliance couplers with pins and contact tubes of standardized configurations. IEC 60309-3 "Particular requirements for plugs, socket-outlets, connectors and appliance inlets for use in explosive gas atmospheres". This standard was withdrawn in 1998. IEC 60309-4 "Switched socket-outlets and connectors with or without interlock" applies to self-contained products that combine within a single enclosure, a socket-outlet or connector according to