IEC 60309

IEC 60309 (formerly IEC 309) is an international standard from the International Electrotechnical Commission (IEC) for "plugs, socket-outlets and couplers for industrial purposes". The highest voltage allowed by the standard is 690 V DC or AC; the highest current, 125 A; and the highest frequency, 500 Hz. The temperature range is −25 °C to 40 °C.

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. The fittings also include limited (IP44) weather-proofing, that is one reason for choosing these fittings when the connections will be used in open-air conditions; a further reason for using these (other than when the heavy-current or three-phase facilities are actually needed), is to deter potential users from connecting domestic appliances to the sockets, as 'normal' domestic plug-tops will not fit thereby deterring unauthorized use.

The cable connectors and sockets are colour-coded, according to the voltage range and frequency used; for example, common colours are yellow for 100–130 volts and 50–60 Hz, blue for 200–250 volts and 50–60 Hz, and red for 400–480 volts and 50–60 Hz. Color codes depend on the highest, phase-to-phase, voltage. In addition to the colour coding the voltage and frequency are encoded by the keyway position. The blue fittings are often used for providing weather-proofed exterior sockets for outdoor apparatus. 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. In camping situations, the large 32-amp blue fittings provide power to static caravans, whilst the smaller blue 16-amp version powers touring caravans and tents. The red three-phase versions are used for three-phase portable equipment.