3.2 Raised Floor, Ceilings, Lighting, EPS & EMF

The Raised Access Floor

The raised access floor (also called a raised floor or access floor) is a modular deck of removable tiles supported on pedestals and, in stiffer systems, connected by stringers, sitting above the structural slab. It serves two roles at once: it forms a pressurised plenum that distributes cooled supply air, and it provides a concealed space for power and data cabling. Typical plenum heights range from about 600 mm to 900 mm, and high-density or heavily cabled halls may go to 1200 mm to keep airflow unobstructed.

Loading and Airflow

Because fully populated cabinets can exceed 1500 kg, mission-critical tiles are rated for roughly 1465 kg/m2 (300 lb/ft2) uniform load, with additional concentrated and rolling-load ratings so cabinets can be wheeled into place without cracking a tile. Cooled air is pushed into the plenum by CRAC/CRAH units and rises through perforated tiles or floor grates positioned in the cold aisle. Correct tile placement and plenum pressure are critical: too many open tiles in the wrong place starve distant racks and create hot spots, so many operators validate airflow with CFD (computational fluid dynamics) modelling.

Air that leaks out of the plenum anywhere except the cold-aisle tiles is wasted bypass air. Brush grommets seal around cables passing through floor cut-outs, and blanking panels close unused rack U-spaces — both are low-cost, high-impact ways to protect plenum pressure and lower PUE. Note that brush grommets are airflow-management devices, not fire-rated barriers.

Common trap — zinc whiskers: Older electroplated floor tiles can grow conductive zinc whiskers on their underside. Disturbed during work, these can enter the airstream and cause short circuits. It is a raised-floor-specific hazard that pure slab designs avoid, which is one reason some modern halls use overhead cabling on a slab floor.

Raised Floor vs Slab

The raised floor is not the only option. On a slab (no access floor) design, power and data run in overhead cable trays and cooling is delivered by overhead ducting or in-row/rear-door units instead of an under-floor plenum. A slab supports higher structural loads and removes the zinc-whisker and plenum-leakage problems, but it gives up the flexible under-floor air distribution that made raised floors the classic choice. The exam contrasts them as under vs over: raised floor distributes air and cable beneath the room within a load-limited plenum, while slab keeps everything overhead with no floor plenum. Under-floor plenums also need aspirating smoke detection (VESDA-type sampling pipes) because airflow dilutes smoke before it reaches ceiling detectors — NFPA 75 recommends multi-zone detection including the under-floor void.

Suspended Ceilings

A suspended (dropped) ceiling hung on a T-bar grid below the structural slab creates an overhead return-air plenum, houses lighting and cable trays, and conceals fire-suppression piping and detection. In hot-aisle containment designs the ceiling plenum often becomes the hot return path back to the CRAH units, so its integrity directly affects cooling efficiency.

Data Centre Lighting

Lighting is a balance between operational visibility and heat: every watt of light becomes heat the cooling plant must reject. EN 12464-1 and TIA-942 guidance targets roughly 500 lux at floor level in IT equipment rooms, with at least 200 lux maintained throughout. Excessive levels such as 1500 lux add needless heat, while too little light creates safety and ergonomic problems. Motion-activated LED fixtures keep aisles dark and cool until staff enter, cutting both energy and heat load.

Emergency Lighting and EPS

Safety of egress does not depend on the utility. Emergency lighting is backed by an Emergency Power Supply (EPS) — a central battery system, a standby generator, or self-contained battery luminaires. NFPA 101 (Life Safety Code) and EN 1838 specify egress illumination, typically about 1 lux average along the escape route, so people can evacuate safely even during a full power failure before generators pick up.

EMF Sources and Shielding

Electromagnetic fields (EMF) in a data centre come from large transformers, motors, bus duct, UPS, and nearby high-voltage lines or rail systems. Strong fields can disrupt equipment or corrupt magnetic media, and shielding must be matched to the field type:

Magnetic-field mitigation is the classic exam point: because magnetic flux slips through non-ferromagnetic barriers, you use high-permeability materials to divert it, not a simple metal box. EMF reduction may be required to meet ICNIRP human-exposure limits and equipment-immunity specifications, and the cheapest fix is often to locate the white space away from the source in the first place.

Signal Reference Grid, Bonding and Earthing

Two related concepts underpin electrical safety and signal integrity. Bonding ties metallic parts together to a common potential; grounding/earthing provides a low-impedance path to earth for fault currents and static discharge. Together they protect personnel, equipment, and data. Inside the white space the best practice is a Signal Reference Grid (SRG): a mesh of copper conductors bonding all raised-floor pedestals, racks, and cabinets together and tying them to the building grounding electrode system. This equipotential plane minimises common-mode noise and equalises ground-voltage differences during transients and lightning. EN 50310 (bonding and earthing for buildings with ICT) and IEEE 1100 (the Emerald Book) detail these designs. Remember the SRG complements the life-safety electrical bonding network — it does not replace the code-mandated protective earth.