Underground car parks with EV charging present evolving fire risks. This technical guide covers ventilation design, structural protection, and the emerging challenge of electric vehicle fires.. Underground Car Park Fire Risks Underground car parks are increasingly complex fire environments. The growth of electric vehicles, combined with higher vehicle fire loads from modern cars with more plastics and larger fuel tanks, creates challenges that existing guidance may not fully address. Key Risk Factors Enclosed space with limited natural ventilation Modern vehicle fires producing 5 10 MW heat release rates EV battery fires with potential for thermal runaway and re ignition Toxic gas production (HF from lithium batteries, CO from all vehicle fires) Structural vulnerability of concrete elements to spalling Fire service access limitations in deep basements Ventilation Design BS 7346 7: Smoke Ventilation for Car Parks Impulse fan (jet fan) systems for single level car parks Ducted extract systems for multi level car parks Design fire: typically 4 8 MW (single vehicle to two vehicles) Extract rate: typically 10 air changes per hour (fire mode) Day to day ventilation for CO and NO₂ dilution CFD Modelling Essential for jet fan system design Models smoke movement and temperature distribution Demonstrates tenable conditions along escape routes Validates fan positioning and throw distances Assesses structural fire exposure temperatures Natural Ventilation Open sided car parks (BS 9999 definition: 25% of perimeter open) Reduced requirements for smoke ventilation Must still comply with structural fire protection requirements Wind analysis may be required for exposed sites EV Fire Challenges Battery Thermal Runaway Lithium ion battery fires can reach temperatures exceeding 1000°C Cell to cell propagation can last hours Toxic gas release: hydrogen fluoride, phosphorus pentafluoride Re ignition risk days or weeks after initial suppression Water is the most effective suppression agent (large quantities required) Design Implications EV charging zones should be located near ventilation intake/extract points Enhanced ventilation rates for EV charging areas Sprinkler protection of EV charging bays (higher design density) Structural fire protection review for prolonged EV fire exposure Gas detection for toxic gas monitoring Emergency power isolation for charging infrastructure Fire Service Operations Extended firefighting operations for EV battery fires Water supply requirements significantly greater than conventional car fires Containment of contaminated run off water Specialist PPE requirements for toxic gas exposure Post fire vehicle recovery and quarantine procedures Structural Fire Protection Concrete Spalling High strength concrete particularly susceptible to explosive spalling Spalling exposes reinforcement to fire temperatures Polypropylene fibres in concrete mix reduce spalling risk Applied fire protection to structural elements (intumescent coatings, boards) Design Fire Duration Vehicle fires can burn for 60 90 minutes (conventional vehicles) EV fires can continue for hours with re ignition potential Structural design fire duration should consider extended EV scenarios Eurocode parametric fire approach may not be appropriate Post Tensioned Structures PT tendons highly sensitive to temperature Loss of pre stress at temperatures above 300°C Enhanced fire protection required for PT elements in car parks Inspection and assessment methodology after fire events Sprinkler Protection Design Standards BS EN 12845: Ordinary Hazard Group 2 or 3 classification NFPA 13: Ordinary Hazard Group 2 FM Global DS 8 26: Specific car park sprinkler guidance Quick response sprinklers recommended for car park applications EV Specific Considerations Sprinklers may not extinguish a battery fire but control spread Under vehicle sprinkler protection being researched Deluge systems for EV charging zones (some insurers requiring) Integration with fire alarm and EV charging management systems Magnus Opifex SEVEN LTD — UK's Leading Fire Safety & Fire Engineering Consultancy 🌐 magnus opifex.co.uk 📞 +44 (0) 20 3488 1926 ✉️ info@magnusopifex.co.uk Founded by Daniel Sheridan, Magnus Opifex SEVEN LTD delivers award winning fire engineering, fire risk assessments, and building safety consultancy across the United Kingdom and internationally.