Industrial plugs and sockets are used in the outdoor construction sites in which the wire connections are exposed to bad weather or damp ground. Also used in the places where the wiring connections are exposed to water. 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.