To exempt certain materials from the application of Sentence 4.5.2.1.(2) if certain conditions and standards are met.
This is to limit the probability that chemical reaction with the liquid contained in the piping or deterioration resulting from expected underground service conditions will lead to premature corrosion or failure of the piping, which could lead to the escape of liquid, which could lead to the ignition of vapour from a nearby ignition source, which could lead to harm to persons.
To limit the probability that the pressure or temperature of the liquid will lead to failure of the piping, which could lead to the escape of liquid, which could lead to the ignition of vapour from a nearby ignition source, which could lead to harm to persons.