Risk Considerations for Data Center Fire Protection

Issue 76: Risk Considerations for Data Center Fire Protection

By Richard W. Bukowski, P.E., FSFPE

The NFPA 75 Technical Committee (TC) in its 2013 edition of the Standard¹ permits a fire risk analysis to be used to determine the construction, fire protection and fire detection requirements for a facility.  As defined in the standard, fire risk analysis is,

"A process to characterize the risk associated with fire that addresses the fire scenario or fire scenarios of concern, their probability, and their potential consequences.”¹

Risk factors to be considered include life safety and (direct or indirect) economic losses from loss of function (capacity) or data, loss of professional reputation, and the costs of redundant systems. Some guidance on the thermal sensitivity of typical equipment is provided, but data on design scenarios and their probabilities necessary for a fire risk analysis are not.  The NFPA 75 TC, in conjunction with the NFPA 76² TC, has formed a task group to develop additional guidance for the 2016 editions.  This activity is being supported by a fire protection research foundation project to identify and validate a computational fluid dynamics (CFD) model that can be used to assess the performance of detection systems in the challenging data center environment for a range of design fire scenarios.

To conduct the risk analysis for IT equipment or facilities as envisioned by the technical committees, engineers need to consider the range of design fire scenarios that may be expected to occur over the facility's operating life that could result in failure to meet the performance objectives for the center.  These design fire scenarios, weighted by their likelihood, quantify the risk of loss due to fire. 

Fires Originating in Digital Equipment

The risk of fires originating in digital equipment (servers, storage units) is very low because there is little energy available to any fault and little combustible material within the equipment,³ especially when listed.  Some internal components run hot due to high component densities and fast clocking rates, with most of these mounted on heat sinks or other devices and some including individual fans to improve cooling. 

In many cases, these components incorporate on-board temperature measuring devices such as thermistors that can shut down the equipment before excessive temperatures cause the component to malfunction.  Since these approaches would result in equipment shutdown before any fire could be ignited (if there was combustible material present), they virtually eliminate fire risk.  Many equipment manufacturers now employ smokeless design procedures that minimize smoke potential under any fault condition.

The exception is power supplies (including UPS) that contain much higher fault energy potential.  Most power supplies are operated from 240 VAC and are designed to be operated near maximum rated output power for optimum efficiency.  Power supplies (including UPS) can utilize smokeless design procedures and can be equipped with internal temperature sensors capable of shutting down equipment that is overheating, but the energy available can lead to a fire under some conditions.  Power supply sections of servers or similar equipment, especially those listed to UL 60950,⁴ are separated by internal enclosures or other barriers to prevent a fire from spreading within the unit.

Internal temperature sensors arranged to shut down overheating equipment are sometimes configured to provide a warning message to operators so that appropriate steps can be taken prior to an orderly shutdown.  These arrangements are intended to protect the equipment and to prevent fires, but, because they are not connected to the fire alarm system, they do not fall under NFPA 72⁵ jurisdiction and are not subject to approval by the AHJ.

Wire and Cable Fires

Data centers and telecommunications facilities contain large quantities of wire and cable.  Data cables do not carry sufficient energy to result in a fire under any fault condition, so they only represent potential fuel if exposed to an external fire source.  Power supply cables do carry sufficient energy to represent both a fire source under fault conditions and potential fuel when exposed to an external fire source.  Linear heat detectors run within bundles of power cables are used by the nuclear power industry to provide overheat warnings and more rapid fire detection without the need for additional detectors in the cable space. 

Wire and cable run in spaces used for environmental air (plenum spaces as defined in NFPA 90A⁶) are required to be plenum rated, meaning that they are low flame spread and low smoke producing.  Wire and cable run in spaces not used for environmental air do not need to be plenum rated, but products listed to suitable reaction-to-fire tests⁷ can minimize fire risk from wire and cable products.

Fires Originating in HVAC Equipment

HVAC equipment in data centers (often referred to as computer room air conditioning or "CRAC" units) extract heat and move large volumes of air by means of large fans pushing air past chillers and through filters. ⁸  The fan motors and filters are potential fire sources, but the cooling units, whether operating on gaseous refrigerants or chilled water, are unlikely to burn.  Smoke detectors located downstream of the filters are traditionally used to detect fires in the filters to shut down the fans, limiting distribution of smoke. 

A source of nuisance alarms involves economizers which introduce outside air into the air stream.⁹  These can pull in smoke from a fire outside the facility, so smoke detectors in the intakes may be needed to switch the economizer to recirculation mode.  Airside economizers typically use high efficiency (HEPA) filters to keep the cooling air clean.  These filters will remove smoke particles from fires, preventing activation of smoke sensors located downstream of the filter. 

Fires Originating under Raised Floors or Above Suspended Ceilings

Where these spaces are used for environmental air, they are treated as plenum spaces.  Those above suspended ceilings are subject to specific regulations in NFPA 90A⁶ to limit combustible materials.  Those below raised floors are subject to the requirements of NFPA 75¹ and the National Electrical Code.¹⁰ Materials used in plenums to construct or line the spaces and all materials contained within the spaces (including wire and cables, which must be specifically rated for use in plenums) must exhibit low flame spread and smoke production properties, and have limited potential heat.  Accessible abandoned wire and cable must be removed in accordance with the National Electrical Code. 

Thus, in the absence of significant ignition sources, such as power cables or heat producing fixtures, the fire risk in these spaces is low.  Piping carrying liquids (chilled water or refrigerant) even if constructed of plastic materials, cannot be ignited even by large sources due to the heat sink provided by the liquid, so they do not contribute to fire risk.  Where such spaces are not used for environmental air, they are not subject to these regulations, but if the same material restrictions are followed, fire risk is similarly low.

Fires Originating in Other Combustibles

By far, the greatest risk of fires in data centers comes from the presence of miscellaneous combustible materials in the space.  These may be cardboard boxes and packaging materials from equipment coming into or going out of the facility, papers related to facility operations, "temporary” storage of construction materials related to facility modifications, or even coffee cups.  Fires in such materials are usually detected by "open area” smoke detectors mounted on the ceiling because smoke rises by buoyancy.  But in these facilities, the high airflows and powerful cooling systems will carry the smoke in the airstream while diluting its concentration. 

In many cases, "special application” detectors that exhibit much higher than normal sensitivities or the ability to operate in higher temperatures or air velocities may be used where conditions dictate.  Strict enforcement of housekeeping rules can have a significant impact on this risk.

Risk-based Protection

Based on these observed fire risks, appropriate strategies for detector selection and placement, extinguishment system types and objectives across a range of design fires can be developed for specific data center configurations in accordance with the intent of NFPA 75 and 76.  The full paper to be presented at the SFPE's Annual Meeting will discuss such appropriate strategies and the use of CFD models validated under the current FPRF project to support the engineering analysis.

Richard Bukowski is with Rolf Jensen and Associates

1. NFPA 75: Standard for the Fire Protection of Information Technology Equipment, National Fire Protection Association, Quincy, MA, 2013.
2. NFPA 76: Standard for the Fire Protection of Telecommunications Facilities, National Fire Protection Association, Quincy, MA, 2012.
3. Mangs, J. and Keski-Rahkonen, O., "Full Scale Fire Experiments On Electronic Cabinets," VTT Building Technology, Publication 269, Espoo, Finland, 1996.
4. UL 60905, Information Technology Equipment - Safety, Underwriters Laboratories, Northbrook, IL, 2013.
5. NFPA 72, National Fire Alarm Code, National Fire Protection Association, Quincy, MA, 2013.
6. NFPA 90A, Standard for the Installation of Air-Conditioning and Ventilating Systems, National Fire Protection Association, Quincy, MA, 2012.
7. Babrauskas, V., Peacock, R., Braun, E., Bukowski, R., and Jones, W., "Fire Performance of Wire and Cable: Reaction-to-fire Tests - A Critical Review of the Existing Methods and of New Concepts," NIST TN 1291, National Institute of Standards and Technology, Gaithersburg, MD, 1991.
8. Patterson, M. and Fenwick, D., "The State of Data Center Cooling: A Review Of Current Air and Liquid Cooling Solutions," INTEL White Paper, Intel Corp., Santa Clara, CA, 2008.
9. Scofield, C. and Weaver, T., "Using Wet-Bulb Economizers in Data Centers," ASHRAE Journal, August 2008.
10. NFPA 70, National Electrical Code, National Fire Protection Association, Quincy, MA, 2014.

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