a frame having a bottom panel with at least two openings corresponding to at least two socket holes of the power receptacle;

a sliding block assembly disposed in the frame, including a sliding block body and a wear-resistant block, wherein the sliding block body includes two inclined surfaces, at least one of the two inclined surfaces has a slot located at a position where an object inserted through a socket hole of the power receptable pushes against, the wear-resistant block being disposed in the slot; and

a resilient member disposed in the frame, wherein when the resilient member is in its initial state, it urges the sliding block assembly to a closed position where the sliding block assembly covers the at least two openings;

wherein the frame further includes a position limiting member configured to abut the sliding block body, and a balancing support member, wherein when an inserted object pushes on only one of the two inclined surfaces, the position limiting member limits a sliding motion of the sliding block assembly, and wherein when two inserted objects simultaneously push on the two inclined surfaces, the sliding block assembly is balanced on the balancing support member and slides along the frame to expose the at least two openings.