(1) The power-ventilated enclosure referred to in Article 6.2.13.3. and its exhaust duct system shall,
(a) except as provided in Sentences (2) and (3), be constructed of noncombustible materials compatible with and chemically resistant to the flammable vapours and particles being exhausted, and
(b) be provided with access doors to permit inspection and maintenance of the fan assembly and exhaust ducts.
(2) Combustible materials are permitted in the power-ventilated enclosure described in Sentence (1) and its exhaust duct system if,
(a) such materials are required by the corrosive or reactive properties of the chemicals or liquids being used, and
(b) their flame-spread rating is not more than 25.
(3) The flame-spread rating required in Sentence (2) is permitted to be greater than 25 if an automatic fire suppression system is provided inside the power-ventilated enclosure and its exhaust duct system.