Enclosure Marking Requirements

Sections 4.11 & 6.3 of the 2003 NFPA 496 require markings on all protected enclosures with a "permanent label," located in a "prominent location," near all doors and access covers. The labels must include the following or equivalent statements:

Class I Locations:
" WARNING - PRESSURIZED ENCLOSURE"
"This enclosure shall not be opened unless the area is known to be free of fl ammable materials or unless all devices have been de-energized."

Class II Locations:
" WARNING - PRESSURIZED ENCLOSURE"
"Power shall not be restored after the enclosure has been opened until combustible dust have been removed and the enclosure repressurized."

Section 5.3 requires the following or equivalent statement in addition to the statement required by Section 4.11 above. "Power shall not be restored after enclosure has been opened until enclosure has been purged for _ minutes at a flow rate of _."

A Note to Section 5.3 permits the use of minimum pressure in place of fl ow rate if the pressure can positively indicate a known flow rate.

An Exception to Section 5.3 allows placement of the start-up instructions on the pressurizing system, if they are referenced by the permanent label on the protected enclosure.

In addition, all permanent labels must include three other markings:

• Section 4.11: Class, Group and Division of surrounding area
• Section 4.11: NFPA pressurization Type X, Y, or Z
• Section 4.11: T Code (temperature identification number): see NFPA 70, The National Electric Code, Article 500, Table 500-3(d)

Exception No. 1 allows omission of the T Code marking if the hottest temperature does not exceed 100º.

Exception No. 2 allows omission of the T Code marking for equipment which is marked for specific use in gas or dust atmospheres and does not exceed 80% of the flammable or ignitable atmosphere's ignition temperature.

Special Marking Requirements

An Exception to Section 4.8.2 permits the use of an indicator on the protected enclosure if all isolation valves are adjacent to the enclosure and marked to comply with Section 4.11.5:

"WARNING - PROTECTIVE GAS SUPPLY VALVE"
"This valve must be kept open unless the area is known to be nonflammable or unless all equipment within the protected enclosure is de-energized."

Typical Enclosure Wiring Methods
In a general sense, protected enclosures should be wired similar to explosion proof enclosures, in accordance with Article 500 of the National Electric Code - NFPA 70.

Single conductor wiring should be placed in rigid metal conduit, seal-flex conduit or other mediums approved for use in the hazardous location surrounding the protected enclosure. Additionally, NFPA 496 requires the use of approved seals on all pressurized enclosure conduit wiring entries, in accordance with NFPA 70. The use of an approved seal is simply the most practical way to prevent excessive leakage through conduit connections.

However, while explosion proof enclosures require conduit seals on all cable entries in accordance with NFPA 70, other methods of sealed cable entries that are suitable for hazardous locations can be used, such as compression glands.

Two primary goals

1. Ensure that all associated wiring and cable is protected by pressurization or other means, such as explosion proof conduit or intrinsic safety barriers.
2. Ensure that all associated conduit and wireways are sealed to conserve protective gas, unless they are used to supply protective gas to other enclosures or devices.

A Typical Enclosure Wiring Connection