A mounting box assembly comprising a mounting box and at least one fitting element, the mounting box comprising: a box case defining a mounting space for accommodating an electrical component, and having a box opening for receiving the electrical component into the mounting space; a first support surface defining a first support plane located at a distance from the box opening in an axial direction parallel to a normal vector of the first support plane, the first support surface facing a first axial direction and being located laterally outside the box opening; and a second support surface defining a second support plane parallel to the first support plane, the second support surface facing a second axial direction opposite to the first axial direction, the first support surface being located between the box opening and the second support surface in the axial direction, each of the at least one fitting element comprising: a first fitting surface defining a first fitting plane; and a second fitting surface defining a second fitting plane parallel to the first fitting plane and located at a distance from the first fitting plane, the second fitting surface facing opposite direction with relation to the first fitting surface, the fitting element being adapted to be attached to the mounting box in a first position in which the second fitting surface is in contact with the first support surface, characterized in that while the fitting element is in the first position the first fitting surface is located between the first support surface and the box opening in the axial direction, and the first fitting surface is located laterally outside the box opening, the fitting element is adapted to be attached to the mounting box in a second position in which the second fitting surface is in contact with the second support surface while the second fitting surface is located between the first support surface and the first fitting surface in the axial direction, the mounting box assembly comprises locking means for locking the fitting element to the second position, the locking means being adapted to prevent axial movement between the fitting element and the mounting box, and the locking means comprises at least one locking member in one of the fitting element and the mounting box, and a counterpart member for each of the at least one locking member in the other of the fitting element and the mounting box, each counterpart member being adapted to co-operate with corresponding locking member for providing the locking action. a box case defining a mounting space for accommodating an electrical component, and having a box opening for receiving the electrical component into the mounting space; a first support surface defining a first support plane located at a distance from the box opening in an axial direction parallel to a normal vector of the first support plane, the first support surface facing a first axial direction and being located laterally outside the box opening; and a second support surface defining a second support plane parallel to the first support plane, the second support surface facing a second axial direction opposite to the first axial direction, the first support surface being located between the box opening and the second support surface in the axial direction, each of the at least one fitting element comprising: a first fitting surface defining a first fitting plane; and a second fitting surface defining a second fitting plane parallel to the first fitting plane and located at a distance from the first fitting plane, the second fitting surface facing opposite direction with relation to the first fitting surface, the fitting element being adapted to be attached to the mounting box in a first position in which the second fitting surface is in contact with the first support surface, a first fitting surface defining a first fitting plane; and a second fitting surface defining a second fitting plane parallel to the first fitting plane and located at a distance from the first fitting plane, the second fitting surface facing opposite direction with relation to the first fitting surface, the fitting element being adapted to be attached to the mounting box in a first position in which the second fitting surface is in contact with the first support surface, characterized in that while the fitting element is in the first position the first fitting surface is located between the first support surface and the box opening in the axial direction, and the first fitting surface is located laterally outside the box opening, the fitting element is adapted to be attached to the mounting box in a second position in which the second fitting surface is in contact with the second support surface while the second fitting surface is located between the first support surface and the first fitting surface in the axial direction, the mounting box assembly comprises locking means for locking the fitting element to the second position, the locking means being adapted to prevent axial movement between the fitting element and the mounting box, and the locking means comprises at least one locking member in one of the fitting element and the mounting box, and a counterpart member for each of the at least one locking member in the other of the fitting element and the mounting box, each counterpart member being adapted to co-operate with corresponding locking member for providing the locking action.