The internal system (pipes, valves, fittings, taps, joints, special pieces) and the materials used must meet the requirements set out in Article 3, paragraph 2. The sizing of the gas fuel supply pipes, accessories, devices, special pieces and any pressure reducers that are part of the internal system must ensure the correct operation of the user devices, in compliance with the pressures established for each device by the respective manufacturer. The tightness test must be carried out in compliance with the technical standards in force or equivalent to them. The measuring group (device not included in the internal system), where provided, must be installed in compliance with the technical standards in force or equivalent to them. The route between the delivery point and the user devices must be as short as possible and outside and/or inside the buildings must be made in compliance with the technical standards in force or equivalent to them. In particular, the following installation methods are permitted inside buildings where the cases indicated below apply: • in special fire-proof housings, when passing through or crossing buildings or rooms intended for civilian use or for activities subject to fire prevention inspections and checks pursuant to Annex 1 of the Presidential Decree of 1 August 2011, no. 151; • in steel sheathing when passing through rooms not included in the previous point, permanently ventilated hallways, or cavities, provided that the route can be inspected. In the rooms where the appliances are installed, the route of the pipes is permitted to be visible and in any case according to the methods set out in the technical standards in force. Furthermore, in the hallway or – where applicable – in the smoke-proof filter (as defined pursuant to Ministerial Decree 30.11.83), installation in housing or in sheathing is not necessary provided that the crossings of the fire-break structures are sealed. When crossing horizontal load-bearing elements, the pipe must be protected by a sheath protruding at least 20 mm from the floor and the cavity between the pipe and the sheath pipe must be sealed with suitable materials. The use of plaster is prohibited. In the case of above-ground entrance halls and not above basements, the installation of pipes under the floor is permitted, protected by a sheath equipped with vents at the ends towards the outside. 10. For installations serving rooms or buildings used for industrial activities, the provisions set out in the decree of the Minister for Economic Development of 16 April 2008 apply. 11. Pressure reducers that are not an integral part of the installed user appliances and whose conformity is not included in that of the user appliance itself, must be installed outside the buildings. 12. Any free sockets in the internal system must be closed with threaded caps and are permitted inside the rooms if intended exclusively for the installation of appliances. 13. Outside the premises where the appliances are installed, a manual shut-off valve with quick-closing operation for 90° rotation and end-of-travel stop in the fully open and fully closed positions must be installed in a visible and easily accessible position on each gas supply pipe. This valve can also be installed in any access space, filter or fire-prevention cavity provided that it is easily accessible from the outside in an emergency. 14. In the case of cavities that are ventilated from above and connected to an open space, installation in sheathing is not required, provided that the pipes are metal with welded or brazed pipes. 2.3.3.1 Sheaths 1. 2. 3. 1. 2. The sheaths must be: • visible; • made of steel with a minimum thickness of 2 mm and a diameter at least 2 cm greater than that of the gas pipe; • equipped with at least one vent to the outside. If one end of the sheath faces inwards, it must be made watertight towards the inside by sealing it with non-combustible material; The pipes must not have mechanical joints inside the sheaths. Metal or flame-retardant plastic sheaths are permitted when passing through external walls or floors. 2.3.3.2 Fire-resistant housings The fire-resistant housing must: • be impermeable to gases; • be made with Italian class 0 or European class A1 materials; • have fire resistance characteristics equal to those required for the walls of the room or compartment being passed through and in any case not less than REI/EI 30. The ducts must not have mechanical joints inside the non-inspectable housings. The housings must be permanently ventilated towards the outside with openings at both ends. The ventilation opening at the lowest level must be equipped with a flame arrester and, in the case of gas with a density greater than 0.8, must be located above ground level, at a distance measured horizontally of at least 10 metres from other openings at the same or lower level.