Raised Floor Fire Spread: NFPA 75 Plenum and VESDA

Under-floor plenums in data centers and telecom rooms contain power cables, data cables, cooling pipes and PVC/foam dust. This concealed volume smolders slowly; surface smoke shows late, multiplying damage. NFPA 75 mandates CMP (Communications Plenum) rated cable, VESDA very-early smoke detection and plenum sprinklers for raised floors.

Plenum Fire Load

• Power cable: XLPE or LSZH — avoid PVC HCl off-gassing
• Data cable: CMP class (NEC 800.179) — UL 910 (Steiner Tunnel) 1500 BTU/sf
• Plenum openings: hot aisle/cold aisle airflow spreads fire

VESDA Air Sampling

VESDA (Very Early Smoke Detection Apparatus) is laser-based:

• Point detectors trigger at 0.04 percent/m — VESDA at 0.003 percent/m (more than 10x more sensitive)
• Perforated pipe network samples plenum air
• Four alarm levels: alert, action, fire 1, fire 2
• Critical for data centers where overcooling masks plumes

Plenum Sprinkler

• NFPA 75 9.3: Plenum >10 in height requires sprinklers
• Type: Preaction double-interlock (keeps water off the equipment if a sprinkler is accidentally damaged)
• Design: OH1 0.15 gpm/ft2 (6.1 mm/min) over 1500 ft2 (140 m2) (NFPA 13-2025 Table 19.2.3.1.1)
• Alternative: Clean agent: if only the space under the raised floor is protected, an inert gas (IG-541, IG-55 etc.) is mandatory (NFPA 2001-2025, 7.1.6.1); halocarbons (FM-200, Novec 1230) may be used where the room and the underfloor are protected together (7.1.6). Leaves equipment intact

Smoke Exhaust

• Smoke exhaust damper below plenum
• Activation triggers HVAC shutoff + purge fan
• Positive pressure above prevents upward spread
• NFPA 92 smoke control: typically 2-6 ACH for the plenum

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