An socket (20) includes a base (30), a cover (50) slidably mounted on the base, a lid (60) attached to the base by plates (70A). The cover has guiding protrusions (56A) mated with guiding grooves (72A) of the plates, respectively. The guiding grooves each include first and second parts (705A, 707A) communicated smoothly with each other. A center line of the first part departs radially offsetly from a center point of the plate so that either of opposite sides of the first part can urge the guiding protrusion therein to make the cover move relative to the base between open and close positions. A center line of the second part has an arc shape whose center point is the center point. Thus, neither of opposite sides of the second part can actuate the guiding protrusion and the cover is kept immovable in the close position when the lid is over-urged.