A low voltage electrical appliance having mechanical interlock means, comprising at least two identically constructed low voltage electrical appliances, wherein each of the appliances comprises a support, a base located above the support, a top cover and a contact holding sliding part reciprocatedly mounted in the base, wherein every two electrical appliances which are arranged in parallel are controlled by a mechanical interlock means so as to prevent the contact holding sliding parts (13, 23) of the parallel appliances from being closed simultaneously, in said mechanical interlock means, a swingable interlocking element is disposed on the base (11, 21), a pivot shaft is disposed on the interlocking element to pivotally couple it to said base (11, 21) so that the interlocking element can swing about said pivot shaft; an inner tang and an outer tang are formed on said interlocking element, each of the inner tang and the outer tang has an end face (321, 331) at the end thereof; an inner locking aperture is formed in a side wall face on the side of the contact holding sliding part (13, 23) adjacent to the interlocking element, an outer locking aperture is formed in a side wall face on the side of the contact holding sliding part (13, 23) away from the interlocking element, and each of the inner locking aperture and the outer locking aperture is formed with a sliding face (132, 232) therein; a through hole is formed in a first outside wall of the base; a through slot and an aperture in communication with the slot are formed in a second outside wall of the base; when at least two low-voltage electrical appliances with the same construction are arranged in parallel, the inner tang and the outer tang of the interlocking element can swing in the through slot so that the inner tang can extend into the inner locking aperture of the contact holding sliding part of the appliance and the outer tang of the interlocking element of the electrical appliance can extend out of a base housing of the electrical appliance and then into the outer locking aperture of the contact holding sliding part of the adjacent electrical appliance; the structural parameters of the mechanical interlock means and the contact holding sliding part of each of the electrical appliances satisfy the following formula: L > H and d 1 + d 2 > L − H where, L represents the distance between two end faces (321, 331) of the inner tang and the outer tang of the interlocking element; H represents the distance between two side wall surfaces (131, 231) of the two contact holding sliding parts (13, 23); d1 represents a gap in which the end face of the inner tang can freely swing in the inner locking aperture starting from a central position of the swingable interlocking element; and d2 represents a gap in which the end face of the outer tang can freely swing in the outer locking aperture starting from a central position of the swingable interlocking element, characterised in that on both lateral edges of bottom surface of the supports (12, 22) of each of the electrical appliances are provided at least one connecting trough, the shape of the cross section of the connecting trough parallel to the bottom surface of the supports (12, 22) is a semi-dovetail shaped or L-shaped semi-bore which has an opening in the bottom surface edge of the support (12, 22), said opening extends along depth of the semi-bore so as to form a slot, so that more than two appliances are combined into an appliance group by a matched connection of a U-shaped connecting member (5, 6) and the connecting trough. L represents the distance between two end faces (321, 331) of the inner tang and the outer tang of the interlocking element; H represents the distance between two side wall surfaces (131, 231) of the two contact holding sliding parts (13, 23); d1 represents a gap in which the end face of the inner tang can freely swing in the inner locking aperture starting from a central position of the swingable interlocking element; and d2 represents a gap in which the end face of the outer tang can freely swing in the outer locking aperture starting from a central position of the swingable interlocking element, characterised in that on both lateral edges of bottom surface of the supports (12, 22) of each of the electrical appliances are provided at least one connecting trough, the shape of the cross section of the connecting trough parallel to the bottom surface of the supports (12, 22) is a semi-dovetail shaped or L-shaped semi-bore which has an opening in the bottom surface edge of the support (12, 22), said opening extends along depth of the semi-bore so as to form a slot, so that more than two appliances are combined into an appliance group by a matched connection of a U-shaped connecting member (5, 6) and the connecting trough.