|Risk Considerations for Data Center Fire Protection|
Issue 76: Risk Considerations for Data Center Fire Protection
By Richard W. Bukowski, P.E., FSFPE
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
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
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.
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
Richard Bukowski is with Rolf Jensen and Associates
3rd Quarter 2013 –An Overview of Approaches and Resources for Building Fire Risk Assessment –Brian J. Meacham, Ph.D., P.E., FSFPE
1st Quarter 2011 –A Historical Perspective on the Evolution of Storage Sprinkler Design –HC Kung, Ph.D., FSFPE
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