(a) Hazardous location classification of adjacent spaces must comply with Clause 12.5 of either API RP 500 or API RP 505 (incorporated by reference, see §110.10-1).

(b) A differential pressure-monitoring device or a flow-monitoring device, or both, must be provided for monitoring the pressurization of spaces having an opening into a more hazardous zone. A running fan motor or a fan-rotation monitoring device indicator is insufficient to satisfy this requirement.

(c) During initial startup, or after shutdown of the pressurization or ventilation system, and regardless of the classification of the hazardous location, the space must be ventilated or purged, followed by pressurization or ventilation of the space, before any electrical apparatus within the space may be energized. The atmosphere is considered non-hazardous when the concentration of explosive gases or vapors is below 30 percent of the lower explosive limit at all points in the space, equipment enclosures and vent ducts.

(d) Only electrical equipment and devices that are necessary for the operational purposes of the space may be installed in spaces made non-hazardous by the methods allowed in this section.