An electrical installation box assembly for installation into an opening in a wall of a structure, comprising: a fastening part comprising a tubular side wall portion extending between a first end and a second end of the fastening part, and at least one flange portion at the first end of the fastening part extending radially from the side wall portion of the fastening part, and at least one flexibly mounted projection (13, 14) located at a distance from the at least one flange portion and extending radially from the side wall portion of the fastening part in a resting position and being adapted to engage the wall in its resting position when the fastening part is inserted, with the second end thereof ahead, inside the opening in the wall from a first side of the wall such that the at least one flange portion is in contact with the first side of the wall; and a box case comprising a tubular side wall portion extending between a first end and a second end of the box case, and a bottom wall portion at the first end of the box case, wherein an outer diameter of the side wall portion of the box case is equal to or smaller than an inner diameter of the side wall portion of the fastening part such that upon inserting the box case, with the first end of the box case ahead, into the fastening part, from the first end of the fastening part, the box case locks the at least one projection of the fastening part into its resting position, and wherein a length of the box case between the first end and the second end thereof is greater than a length of the fastening part between the first end and the second end thereof, such that when the box case is fully inserted into the fastening part, the first end of the box case extends through the fastening part, wherein, when the box case is fully inserted into the fastening part, the second end of the box case is essentially flush with the first end of the fastening part, wherein at least one of said at least one flexibly mounted projection of the fastening part is located at a first distance from the at least one flange portion, characterized in that said at least one of said at least one flexibly mounted projection of the fastening part is adapted to move towards inside of the side wall portion of the fastening part during the insertion of the fastening part, with the second end thereof ahead, into the opening in the wall from the first side of the wall, and to return to its resting position by itself upon entering a second side of the wall, when the fastening part is inserted inside the opening in the wall such that the at least one flange portion is in contact with the first side of the wall, thus engaging the wall such that the wall is sandwiched between the at least one flange portion and the at least one projection. a fastening part comprising a tubular side wall portion extending between a first end and a second end of the fastening part, and at least one flange portion at the first end of the fastening part extending radially from the side wall portion of the fastening part, and at least one flexibly mounted projection (13, 14) located at a distance from the at least one flange portion and extending radially from the side wall portion of the fastening part in a resting position and being adapted to engage the wall in its resting position when the fastening part is inserted, with the second end thereof ahead, inside the opening in the wall from a first side of the wall such that the at least one flange portion is in contact with the first side of the wall; and a box case comprising a tubular side wall portion extending between a first end and a second end of the box case, and a bottom wall portion at the first end of the box case, wherein an outer diameter of the side wall portion of the box case is equal to or smaller than an inner diameter of the side wall portion of the fastening part such that upon inserting the box case, with the first end of the box case ahead, into the fastening part, from the first end of the fastening part, the box case locks the at least one projection of the fastening part into its resting position, and wherein a length of the box case between the first end and the second end thereof is greater than a length of the fastening part between the first end and the second end thereof, such that when the box case is fully inserted into the fastening part, the first end of the box case extends through the fastening part, wherein, when the box case is fully inserted into the fastening part, the second end of the box case is essentially flush with the first end of the fastening part, wherein at least one of said at least one flexibly mounted projection of the fastening part is located at a first distance from the at least one flange portion, characterized in that said at least one of said at least one flexibly mounted projection of the fastening part is adapted to move towards inside of the side wall portion of the fastening part during the insertion of the fastening part, with the second end thereof ahead, into the opening in the wall from the first side of the wall, and to return to its resting position by itself upon entering a second side of the wall, when the fastening part is inserted inside the opening in the wall such that the at least one flange portion is in contact with the first side of the wall, thus engaging the wall such that the wall is sandwiched between the at least one flange portion and the at least one projection.