Short answer: The colour of an IEC 60309 plug encodes its voltage and frequency, and the housing is physically keyed so a plug cannot be inserted into a socket of a different voltage. Blue is 200-250V single phase, red is 380-415V three phase, yellow is 100-130V, and green is 50-70V. The colour is a safety device, not decoration.
The standard colour table
- Yellow - 100-130V - 50/60 Hz - UK-style 110V site tools, transformers
- Blue - 200-250V - 50/60 Hz - Single-phase tools, lighting, small machines
- Red - 380-415V - 50/60 Hz - Three-phase distribution, motors, welding sets
- Green - 50-70V - 50/60 Hz - Low-voltage safety lighting, hand lamps
There are also 480-500V and 600-690V variants in black and other colours, used mainly in heavy industry and mining; they are rare in general distribution.
Why the keying matters more than the colour
Inside the housing there is a keyway and an earth pin position that differ by voltage. This makes it physically impossible to push a 400V plug into a 230V socket. That physical interlock is the actual safety mechanism - the colour is only the visual cue that lets an electrician identify a circuit at a glance.
This is why adaptors that defeat the keying are dangerous. They exist, they are cheap, and they are the cause of a large share of site accidents involving industrial connectors. If a plug does not fit, the circuit is wrong, not the plug.
Common misunderstandings
Blue means waterproof? No. Colour is voltage; protection degree is IP. A blue IP44 plug and a blue IP67 plug are both blue and behave completely differently in rain.
Red is always 400V? In practice red covers 380-415V, which includes 380V, 400V and 415V distribution systems. The plug is the same; the supply determines the exact voltage.
Yellow is a warning colour? Yellow here means 110V, not danger. On UK construction sites it is the standard for reduced-low-voltage tool circuits.
Choosing in one line
Match the colour to the supply you have, match the pin count to whether you need a neutral, and match the IP rating to the environment. Those three decisions cover almost every industrial plug selection.
Frequently asked questions
Can a blue plug be used on a red circuit with an adaptor? No. It defeats the keying and puts equipment rated for 250V onto a 400V supply.
What colour is used for 480V? Black, in the 480-500V range, mainly in North American and heavy industrial installations.
Is colour different for 60 Hz? The same colour family applies; the frequency is marked on the device. Some 60 Hz markets use specific combinations - check the local standard before ordering.
About Hefeng Electric - Hefeng Electric is a manufacturer of IEC 60309 industrial plugs, sockets and connectors in Fujian, China. Full colour and pin range, IP44 and IP67, CE certified. Contact: sales@electroconn.com