arpadveress 
A combustible gas in a distribution line must contain a natural odorant or be odorized so that at a concentration in air of one-fifth of the lower explosive limit (LFL 20%), the gas is readily detectable by a person with a normal sense of smell.
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Clear statement, but from that point of basic design it shall be considered as one of the basic Ex requirements for the total lifetime of the whole. Part of lifetime tracking.
Keep up good work!
Arpad
