All mk sockets have to have all tree pins inserted befor the shutters will open ,as for heights thats building regs for new builds you can install at the same height as and existing socket or switch in any other house or existing installation
If 4" high then I would prob not code, but I have code 2 before on sockets that are less than 2" off the floor, how do we know what type of adapter or transformer will be plugged into the socket, can't assume it's your standard plug and flex everytime, my son has a bright green charging adapter for his electric car that is around 3" just for the plug
I don't see a problem with 4 or 5" but much lower than that i could see problems, not just for flex strain but pattress's accidentally being kicked off because there so low
As there is no given height in the BS 7671-2008, and as been posted, if there is no visible signs of damage to either socket outlets or flexible cables on the appliances, then really there should be no code for this applied.
I would be more concerend if there were no RCD protection and as a tester I considered there maybe a chance that mobile equipment for the use of outside could be plugged into the socket.
As malcom has said building regs give a height of 450mm but this is for new builds ,and infact you can install sockets at the existing height in and old installation even if you are rewiring it
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