This part of ISO 16486 specifies a code of practice dealing with unplasticized polyamide (PA-U) piping systems, intended to be buried outside buildings and used for the supply of gaseous fuels. It addresses the common basic principles for gas supply systems. More detailed national standards or codes of practice might exist in the ISO member countries. This part of ISO 16486 is intended to be applied in association with those national standards or codes of practice related to the above-mentioned basic principles. It is applicable to mains and service lines, the components of which are prepared for jointing by scraping and/or machining, and gives instructions for the design, storage, handling, transportation, laying conditions and fusion quality control of PA-U pipes and fittings up to and including 250 mm outside diameter, as well as subsequent joint testing, backfilling, pipe system testing, commissioning and decommissioning. The jointing methods covered by this part of ISO 16486 are butt fusion jointing and electrofusion jointing. No special precautions are necessary for areas exposed to the influence of mining and earthquakes other than those precautions mentioned in this part of ISO 16486. It is the responsibility of users of this part of ISO 16486 to take existing and new national regulations into account.