[quote name='flyfisher' timestamp='1415622978' post='2602019']
That makes sense, but do the regulations require that sockets/conduit containing three phases have such warning labels? After all, it's not exactly 'normal use' for someone to open up a socket so if the normal users of such sockets cannot be exposed to three phases by simply plugging in a standard plug why bother with warning labels at all. After all, by definition, it's only qualified electricians who can be expected to properly understand what such labels actually mean in practice.
[/quote]
To the letter then it should if it contains 400v or has more than one supply inside, the 400v sticker is just confusing for joe public and doesn't really explain fully to any electricians working on the system, on the other hand you would not have room for an essay on a socket outlet, damned if you do damned if you dont?
The normal label if it was because the sockets nearby where on a second phase would be "Pottential 400v between socket outlets in this area" probably with a symbol of two arrows pointing at each other > < or one on each pointing at the other socket. In large installations the electrical engineer will insist on which phase each ring main is on to avoid that yet keep the load for the entire building divided equally. Of course a month later the jukebox engineer turns up and drills a hole through the wall to get at the nearest socket and you are back where you started!