DHI 21cm 16cm HFF hgr, For easy installation of electrical equipment, electrics or switch boxes on insulated ceilings and walls, minimum installation depth in the load-bearing substrate = 50 mm, drilling diameter = 10 mm, minimum drilling depth = total length DHI + 10 mm. The 50 mm dowel part must find a hold in a solid substrate, screw in straight, correct screw size and no forceful processing.