A power socket (100) includes an insulative housing (1), a first contact (21) and a plurality of second contacts (22) attached to the insulative housing (1) and means (185); the insulative housing (1) defines an annular first groove (16) and a first cylinder (18) disposed within the first groove (16); the first groove (16) defines a first inner surface (161) and a first outer surface (162); the first cylinder (18) defines a first passageway (181) extending through a center portion thereof along the front to back direction; the first contact (21) has a first contact portion (211) extending into the first passageway (181); each second contact (22) has a second contact portion (221) extending into the first groove (16); the means (185) extending into the first groove (16) for preventing a familiar round power plug from being inserted into the first groove (16).