Metal boxes are recessed into the wall and provide a fixing for the socket itself. Knockouts are provided in the back, sides and ends to allow the cable to enter the box. Rubber grommets are fitted round the hole so the cable doesn’t chafe against the metal edges.
Elongated screw slots allow box to be levelled when fixed to wall. Adjustable lugs enable final adjustments to level of faceplate on wall. Boxes are usually 35mm deep, but with single-brick walls boxes 25mm deep should be used, along with accessories having deeper-than-usual faceplates.