Cooling & Containment's Impact on ELV Layouts in Irish Data Centres

In Irish data centre design, the cooling architecture is not a mechanical engineering problem solved in isolation — it is the dominant physical constraint that determines where every ELV system runs, how fire detection zones are defined, where BMS integration points land, and how much coordination is required between the low-voltage and mechanical engineering disciplines. A data centre ELV designer who does not deeply understand the cooling strategy being deployed is designing blind.

Ireland's 1.3 PUE targets, driven by EirGrid grid capacity pressure and EU Taxonomy obligations, are pushing operators toward increasingly efficient and complex cooling strategies. Each step up in cooling sophistication — from basic CRAC units, through hot/cold aisle containment, to rear-door heat exchangers and direct liquid cooling — creates new ELV design challenges. This article systematically works through those challenges, covering raised floor plenums, aisle containment, fire detection in controlled airflow zones, BMS integration, and the emerging demands of liquid and immersion cooling.

The ELV Design Challenge: Cooling Architecture Constrains Everything

The fundamental tension in data centre ELV design is that the spaces that cooling systems need — plenums for air distribution, contained aisles for thermal management, service clearances around CRAC/CRAH units — are exactly the same spaces that ELV designers want to use for cable routing, detection coverage, and system access.

This is not a new problem, but it has intensified as Irish data centre operators chase lower PUE figures and higher rack densities. A traditional 3–4kW/rack air-cooled deployment with open aisles gave ELV designers relatively free routing options. A modern 20–40kW/rack deployment with hot aisle containment and supplemental liquid cooling on GPU clusters presents a far more constrained design environment.

The consequence of poor ELV–cooling coordination is severe: fire detection blind spots that breach IS 3218 compliance, cable routes that obstruct CRAC service access, BMS integration gaps that prevent remote monitoring of cooling plant, and costly remedial work during commissioning when clashes are discovered. BIM coordination using Navisworks clash detection between ELV Revit models (LOD 300) and mechanical cooling models is no longer optional on any Irish Tier III or above project — it is a contractual requirement on most frameworks.

Raised Floor Data Centres: ELV Cables vs. Cold Air Plenum

The raised floor plenum is one of the most contested spaces in traditional data centre design. Raised to 600mm or 900mm above the structural slab, the plenum acts simultaneously as the primary cold air delivery path (pressurised from CRAC/CRAH units), the primary ELV cable highway, and the primary power cable route. Managing all three functions without compromising any of them requires disciplined design.

Separation Requirements in the Plenum

TIA-942-B and EN 50600-2-4 both require that power cables and ELV/ICT cables are physically separated within the plenum, typically by routing on opposite sides of the plenum zone or by using separate cable trays with a minimum 300mm separation. This separation is not just an EMI requirement — it is a fire load management measure. A power cable fault that ignites nearby ELV cables in a plenum can propagate rapidly in an enclosed underfloor environment where suppression systems may not reach effectively.

ELV cable containment within the plenum must use metal cable tray (not plastic duct) to maintain fire compartment integrity. Cable jackets must be LSZH or ETFE rated — both achieve low flame propagation and low smoke/toxin emission in enclosed spaces. LSZH is generally preferred for Irish projects due to its broader availability and clear compliance with ETCI National Rules.

Perforated Tile Management and Airflow Impact

Perforated floor tiles deliver cold air from the plenum to IT equipment intake faces. The airflow volume delivered through each tile is determined by the plenum static pressure and the tile's open area percentage. ELV cable trays running under perforated tile zones obstruct plenum airflow and can create turbulence that reduces effective cold air delivery by 15–30%.

The ELV designer must work from the mechanical engineer's perforated tile layout drawing and ensure that cable trays do not route directly beneath high-airflow tile zones. Where unavoidable, elevated cable tray supports that allow air circulation beneath the tray are required. All plenum penetrations for vertical ELV cable risers must be sealed with intumescent blankets or pillows to prevent cross-zone air leakage.

Fireproof Barriers in Plenum Zones

IS 3218 requires that the underfloor plenum is divided into fire compartments aligned with the above-floor compartment boundaries. ELV cable routes crossing compartment boundaries must pass through fire-rated sleeve penetrations. The ELV designer must coordinate with the passive fire protection contractor to ensure all penetration locations are identified on drawings and sealed in the correct sequence during construction — before raised floor panels are reinstated.

Cable Containment Above Hot/Cold Aisle Containment

When hot or cold aisle containment is installed, the overhead space above the containment canopy becomes effectively a dead zone for ELV cable routing. Containment canopies typically extend to 2.4–2.7m above finished floor level, leaving only 300–600mm between the canopy top and the cable tray level in many Irish data hall designs with 3m floor-to-ceiling heights.

Overhead ELV cable ladder should be routed in the 400–600mm zone above containment canopy level, running parallel to the rack rows. Cross-aisle routing above canopies is particularly problematic — it requires that cables either pass over the containment (acceptable if structural clearance allows) or that the containment design incorporates cable routing ports at canopy level (specified by the ELV designer and supplied by the containment manufacturer).

Fire Detection in Contained Aisles: IS 3218 Requirements

This is the most safety-critical ELV–cooling interaction in the data centre. IS 3218 (the Irish standard for fire detection and alarm systems, aligned with EN 54) requires that all fire compartments within a building are adequately covered by fire detection. In a data centre, every contained aisle — whether hot or cold — constitutes a distinct detection zone that must be covered.

Why Conventional Spot Detectors Fail in Contained Aisles

Conventional point smoke detectors rely on the buoyancy-driven rise of smoke plumes toward the ceiling where detectors are mounted. In a cold aisle containment enclosure, cold air is positively pressurised downward from the top of the enclosure — the exact opposite of the natural buoyancy plume direction. In a hot aisle containment enclosure, hot exhaust air rises rapidly and exits through overhead chimneys before accumulating at detector level. In both cases, the standard EN 54-7 spot detector geometry is unreliable for early-stage fire detection.

VESDA as the Preferred Solution

Very Early Smoke Detection Apparatus (VESDA) — or equivalent aspirating smoke detection (ASD) systems — actively draw air samples from multiple points within the contained aisle volume and analyse them at a central detector head. This active sampling approach bypasses the airflow directional problem entirely. Sample pipe inlets can be positioned at the top of containment panels, within the overhead canopy, and at IT equipment exhaust level, providing multi-point coverage regardless of airflow pattern.

For IS 3218 compliance in contained aisles, the ELV fire detection designer should specify VESDA or ASD with sampling points at a maximum 6m spacing within the contained volume, with sample pipe routing coordinated with the containment manufacturer to ensure pipe penetrations are made through containment panels without compromising containment integrity.

CRAC/CRAH Units and ELV Service Clearance Zones

Computer Room Air Conditioning (CRAC) and Computer Room Air Handling (CRAH) units are large mechanical plant items — typically 500–700mm deep and 1000–1500mm wide — that require substantial service clearance for filter changes, coil cleaning, fan motor replacement and refrigerant servicing. ELV cable routes that encroach on these service clearances create ongoing operational problems and can constitute a Health and Safety violation under the Irish Safety, Health and Welfare at Work Act.

Standard service clearance requirements that ELV designers must design around:

• Front access (airflow side): minimum 1000mm clear space, no ELV cable tray at working height (below 2.1m)
• Side access (electrical/controls side): minimum 500mm, ELV cable tray may run at high level (above 2.1m) if not obstructing panel access
• Rear access (coil side): minimum 600mm where rear-access design, no overhead obstruction
• Top connections: flexible ductwork or pipe connections must remain accessible — no permanent ELV cable routes directly above CRAC connection points

BMS cabling to CRAC/CRAH units — typically RS-485 Modbus or BACnet MS/TP — must be run in dedicated conduit from the BMS controller location to each unit, with the final connection made via a flexible conduit or 300mm service loop to allow unit movement during maintenance. ELV designers must specify BMS termination locations on CRAC control panels and confirm these with the mechanical engineer before the cable route is finalised.

BMS Integration for Cooling Plant

The Building Management System is the ELV subsystem most heavily affected by cooling architecture. In an Irish data centre, the BMS cooling integration scope typically includes:

• Chiller plant: chilled water supply/return temperature, chiller on/off status, compressor load, condenser water flow, chiller efficiency (kW/kW) — communicated via BACnet/IP over the BMS network
• CRAC/CRAH units: supply air temperature, return air temperature, fan speed (%), cooling valve position, alarm status — typically Modbus RTU or BACnet MS/TP at unit level, with BACnet/IP gateway to floor-level BMS network
• Cooling tower / dry cooler: fan speed, entering/leaving water temperature, efficiency mode — BACnet/IP or Modbus TCP
• Chilled water valves (CW valves): valve position feedback and control — hardwired analogue or BACnet from valve actuator
• Glycol dosing system: glycol concentration, dosing pump status — Modbus or hardwired digital I/O
• Free cooling economiser: economiser valve position, plate heat exchanger approach temperature, changeover logic — BACnet/IP

The BMS network topology for cooling integration on large Irish data centres typically uses a hierarchical BACnet/IP architecture with floor-level field controllers (BCUs) communicating via the BMS LAN to a central SCADA/dashboard. The ELV designer is responsible for designing the BMS field cabling (Cat6A or RS-485 twisted pair, depending on the protocol), field controller panel locations, containment routes, and the integration interface schedule between the BMS and each piece of cooling plant.

Liquid Cooling Impact: Rear-Door Heat Exchangers and DLC

As rack densities in Irish data centres rise beyond 15–20kW/rack — driven by AI/ML workloads and dense compute configurations — air cooling alone becomes insufficient or thermally inefficient. Liquid cooling supplements or replaces air cooling, and each liquid cooling technology creates specific ELV requirements.

Rear-Door Heat Exchangers (RDHx)

RDHx units replace standard rack rear doors with a water-cooled heat exchanger panel. Hot air exhausted from the rear of IT equipment passes through the water coil, transferring heat to the chilled water circuit before leaving the rack. From the ELV perspective, RDHx deployment requires:

• Leak detection cabling: rope-type leak detection sensor routed along the base of each rack row where RDHx water supply manifolds run, connected to a zone-based leak detection controller panel
• BMS integration: RDHx supply/return temperature monitoring, chilled water flow rate, leak alarm — typically Modbus or BACnet from the leak detection controller to BMS
• Power interlock: dry contact output from leak detection controller wired to the rack PDU remote-off input, enabling automatic IT equipment power isolation on confirmed leak

Direct Liquid Cooling (DLC)

DLC delivers chilled water or dielectric fluid directly to processor cold plates within servers, achieving heat removal at 95%+ efficiency. Coolant Distribution Units (CDUs) in each rack row require BMS integration for supply temperature, return temperature, flow rate, and coolant leak alarm. The ELV designer must design BMS sub-panels at CDU locations with sufficient I/O points for each CDU, and route BMS cabling from CDU locations to BMS field controllers.

Immersion Cooling: Fire Detection Redesign

Immersion cooling — where IT equipment is submerged in dielectric fluid tanks — represents the most radical departure from conventional ELV design. When immersion tanks replace server racks, the IT equipment is no longer in the air-breathing environment where smoke-based fire detection is relevant. The fire risk profile changes from combustion of electronic components to:

• Dielectric fluid overheating (thermal runaway in the tank)
• Coolant fluid leak onto surrounding floor area
• Vapour accumulation from single-phase immersion coolants

Fire detection for immersion cooling zones therefore transitions from VESDA smoke detection to temperature monitoring (thermal detectors on tank external surfaces, integrated with BMS) and coolant leak detection (rope sensors on floor drain channels around tank perimeters). The ELV fire alarm designer must agree the detection strategy with the facility's qualified fire engineer and ensure the resulting design is compliant with IS 3218 for the modified risk environment.

Cooling Type vs. ELV Impact: Design Reference Table

BIM Coordination: ELV LOD 300 with Mechanical Cooling

On Irish data centre projects of any significant scale, BIM coordination between ELV and mechanical cooling disciplines is the mechanism that prevents the clashes described throughout this article from becoming physical rework on site. The process typically works as follows:

The ELV designer produces a Revit model at LOD 300 containing all cable trays, conduit routes, detection panel locations, BMS controller positions, and major equipment items. The mechanical engineer produces an equivalent model containing CRAC/CRAH units, chiller plant, pipework, and containment systems. Both models are federated in Navisworks Manage and clash detection is run weekly during the detailed design phase.

Critical clash categories for ELV–cooling coordination include: ELV cable tray within CRAC service clearance zones; cable tray below perforated floor tile zones in raised floor designs; VESDA sample pipe routing through containment panels without sealing; BMS cable conduit route conflicts with chilled water pipe runs; and overhead cable ladder elevation conflicts with containment canopy height.

The BIM coordination output feeds directly into the construction sequence — containment and BMS cabling must be installed before containment panels and raised floor tiles are reinstated, and this sequencing must be reflected in the project programme.