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Feature Article*
April 21, 2004
Limiting the Plenum Cable Fire Risk
contributed by DuPont
Overview
Concerns are rising about the growing amount of cables in commercial
buildings required to service the ever-increasing demands of IT
networks. More workstations, each with
expanding capabilities and
increasing bandwidth requirements, are taxing the communications
infrastructure. Communications cabling, which carries important
data packets to and from computer networks, is as common in building
plenum spaces as duct work. While most cable selections are based
on electrical performance requirements, there are fire-rating
factors that are often overlooked beyond what is minimally required.
This fact places each building at a greater fire risk with each new
installation of communications cable. As a result, much discussion
in the industry has been focused on the new 2002 National Electric
Code (NEC), which calls for the removal of the accessible portion of
abandoned cabling. These concerns are also the thrust behind the
genesis of a new cable technology that significantly reduces these
fire safety risks.
National
Codes and Standards
The communications cable most often used in commercial buildings is
called “plenum” cabling. It is designed for use specifically in
hidden spaces within dropped ceilings that handle return airflow –
the plenum space. The National Fire Protection Association (NFPA)
sets the plenum cable requirements based upon three mandatory
attributes: smoke generation, flame spread, and fuel load. In the
early to mid 1970s, several significant fires occurred where cable
running in plenum spaces greatly increased the severity of the fire
damage (such as One World Trade Center, 1975). As a result, a
consortium of industry partners convened to address cabling types
used in this application. In the early 1980s, the NFPA Committee
90A, responsible primarily for developing standards for the HVAC
plenum space, issued primary requirements for all materials “exposed
to the air flow” to be non-combustible or “limited combustible.”
Technically speaking, a limited combustible material is categorized
as having a smoke generation index of less than or equal to 50 (as
compared to red oak, which has a smoke generation index of 100), a
flame spread index of less than 25, and a potential heat (fuel load)
of less than 8.1 mega Joules/kilogram. The fuel load is the heat
energy contained in a cable that could be released in the event of a
fire. In the 1980s, while many communications cables were tested,
none passed such stringent flame, smoke, and fuel load requirements
while also meeting the electrical performance outlined by the
National Electrical Code (NEC) and the Telecommunications Industry
Association (TIA). The result of this testing and development was a
compromised selection of plenum-rated cable (allowed only as an
exception to the original NFPA ruling) based on the best cable
construction available at the time – which was combustible in nature
and hazardous with respect to smoke generation levels. Over 25
years later, this selection of combustible cable still represents
the plenum cable we know today. But, times have changed, and the
cabling industry is now able to meet NFPA’s original challenge with
a new, safer cable for use in plenum spaces — Limited Combustible
Cable.
Differences in Materials equal Differences
in Fire Safety
One
of the core challenges the cabling industry has faced has been finding
materials that excel simultaneously at three, often divergent, goals: 1)
Excellent electrical insulation properties on copper and fiber, 2)
Reasonable processing parameters for cable manufacture, and 3) High
ratings on flammability, smoke generation, and fuel load. The three most
commonly used materials in cabling today are (acronyms for each polymer
material can be found on cable data sheets):
•
Polyethylene (PE)1
•
Polyvinyl chloride (PVC)1
•
Fluorinated ethylene propylene (FEP)
When
comparing the performance of each, polyethylene (PE)1 offers
excellent electrical properties for insulating copper. However, in a
fire, it is highly combustible, extremely high in fuel load, and readily
generates dense smoke, which creates significant life and equipment fire
safety hazards and risks. Polyvinyl chloride (PVC) has poor electrical
properties but offers better fire performance than PE; yet, it alone is
difficult to melt-process and has poor flexibility in cable
applications. To resolve these issues, other materials (such as lead)
are added to achieve greater processability, flexibility, and aging
stability. The resultant PVC compound represents an inexpensive
material that creates a relatively safe jacket for most plenum cables,
but remains combustible in nature. Lastly, Fluorinated ethylene
propylene (FEP), marketed by DuPont as Teflon® has electrical
insulation characteristics on copper equal to polyethylene, and only FEP
meets the highest performance criteria for flame spread, fuel load, and
smoke generation. The construction of a typical 4 pair UTP (unshielded
twisted pair) plenum cable is shown in Figure 1. It contains FEP as the
copper insulator and a FRPVC1 jacket.
Figure 1.
Plenum (CMP) Cable
In
contrast, the limited combustible cable encompasses several improvements
that include optimum fire-rated materials of construction, as well as
cable size reduction (10% smaller jacket). A typical limited
combustible cable construction is illustrated in Figure 2.
Figure 2.
Limited Combustible (LCC) Cable
Limited Combustible Cable
Through a collaboration among industry
customers, competitors, suppliers, and research firms such as UL and
NFPA’s Fire Research Foundation (NFPRF2), the limited
combustible cable construction has finally arrived. The 2002 NFPA 90A
recognizes this cable requirement and listing as approved for use in
plenum spaces. The NEC is currently in the process of reviewing the
limited combustible technology, and specific applications may be
established as early as the 2005 edition, setting it up for broad
acceptance in the future. Limited combustible cables significantly
exceed the flame, fuel load and smoke ratings of today’s minimal
code-approved exception cables, enough to earn the limited combustible
rating. In addition, they must pass a full array of other tests,
including temperature aging, humidity, and jacket slitting, all of which
may compromise the plenum cable safety and the integrity of data signals
over time. Finally, these cables are made with less total plastic, and
because only one type of stable plastic (FEP) is used they are more
easily recycled. Many leading cable manufacturers including Belden,
Mohawk, Krone and CommScope currently offer 4 pair UTP, coaxial, and
fire alarm limited combustible cables made with DuPont™ Teflon®.
The enhanced 4 pair UTP constructions come in everything from Category 3
to 6e (TIA Standards). The average increase in total installation costs
of a system that includes limited combustible cables is less than 10%
compared to conventional plenum cable. Limited combustible cable made
with Teflon® may also save additional money if local codes
require plenum sprinklers in sprinklered buildings (see your inspector
and NFPA 13). For more information on this new technology, check with
your local distributors and contractors, or contact DuPont at:
800-207-0756 or www.teflon.com/cablingmaterials
1
There are several different formulations of PE insulations and PVC
jacketing materials used in this industry.
2
The NFPRF is the National Fire Protection Research Foundation, a
research firm associated with the NFPA.
------------------------------------------------------
The DuPont Company has
provided solutions to meet the needs of an ever-changing world for over
200 years, and DuPont™
Teflon® fluoropolymers are used in a wide variety of applications where
maximum protection is paramount. One example is the latest innovation from
DuPont Communications Cabling Solutions, Limited Combustible data
communications technology. For more information on DuPont’s solutions,
please contact Stacy Geurin at 302-999-3739.
*CRE Partners is not responsible for the content, validity,
technical accuracy or other claims or information contained in this
article. Feature Articles are often authored by outside sources
and do not necessarily reflect the views or opinions of CRE Partners.
Further, publication of articles in the CRE Partners Newsletter and/or
web site is not meant to represent, promote, or endorse any company,
brand, product or solution.
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