We first see the Zone system in Art. 505, which provides this alternative system for Class I locations (the combustibles are gases). The Zones are 0, 1, and 2. But if the combustibles are dust or ignitable fibers (Class II or III), the Zones are 20, 21, and 22 (see Zone System for Class II and III Locations), covered by the requirements in Art. 506. Documentation Article 500 provides documentation requirements for Class I, II, and III under the Division system [500.4]. The ...

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