djk,
what you are describing, sounds a lot like
2core Neutral-screened, but with the screen used as an Earth conductor.
Over here, any socket-outlets used outdoors
must have the same IP rating regardless of whether they are in use or not.
A common rating is IP66, and the plug has a screw-type locking/sealing ring as well as a spring-loaded rubber sealed flap and locking catch.
Apparently, these plug/socket arrangements, can handle a 3Mega Pascal jet of water, with no leakage. [Linked Image]