This part of ISO4437 specifies the characteristics of fusion fittings made from polyethylene (PE) as well as of mechanical fittings for piping systems in the field of the supply of gaseous fuels. It also specifies the test parameters for the test methods referred to in this part of ISO4437. In conjunction with ISO4437-1, ISO4437-2, ISO4437-4, and ISO4437-5, it is applicable to PE pipes, fittings and valves, their joints, and joints with components of PE and other materials intended to be used under the following conditions: the maximum operating pressure (MOP) is based on the design stress, determined from the compound minimum required strength (MRS) divided by the C factor, and taking into account rapid crack propagation (RCP) requirements; a temperature of 20°C as reference temperature for the design basis. NOTE 1 For other operating temperatures, derating coefficients are given in ISO4437-5:2014. NOTE 2 It is the responsibility of the purchaser or specifier to make the appropriate selections from these aspects, taking into account their particular requirements and any relevant national regulations and installation practices or codes. This part of ISO4437 is applicable for fittings of the following types: electrofusion socket fittings; electrofusion saddle fittings; spigot end fittings (for butt fusion using heated tools and electrofusion socket fusion); socket fusion fittings; mechanical fittings. The fittings can for example be in the form of couplers, saddles, equal and reduced tees, reducers, elbows, bends, or end caps. NOTE 3 Fabricated fittings are normally not used for gas applications except for larger dimensions or in the absence of other solutions. Guidance can be found in ISO4427-3:2007, AnnexB.[2]