Combustible Components of a Typical Plenum Cable

Plenum cables-whether copper or fiber optic-contain large quantities of plastics, and many plastics will burn and/or generate great amounts of smoke in a fire. A typical cable for data or voice transmission has two main components, a cable core made up of insulated copper wires twisted in pairs and a jacket. The industry standard cable is 4-pair UTP, with four twisted pairs of insulated wire, with "U" meaning "unshielded" and "TP" meaning "twisted pair." More than half of this cable is potentially flammable plastic. In an office with 100 workstations each with two cables run 100-feet (average) to each workstation can contain as much as 300 pounds of combustible cabling materials in the ceiling or floor plenum. Considering multiple floors and high-density areas like computer equipment rooms, it is easy to understand cabling's significant contribution to a building's fire load. Cable insulation and jacketing material are an undeniably significant fuel source, and, when they are heated, can generate a tremendous amount of smoke-a scenario with dangerous repercussions for building occupants and equipment.

Fuel Load of Commonly Used Cable Insulation and Jacketing Materials

The cabling industry has been challenged by developing materials that excel at three, often divergent, goals: electrical transmission properties; reasonable processing parameters for productive cable manufacture; and high performance ratings on flammability, smoke generation and fuel load.

The three most commonly used materials in cabling today are Polyethylene (PE), polyvinyl chloride (PVC), fluorinated ethylene propylene (FEP), all of which have a significant impact on the level of fire hazard. When comparing the performance of each, the first, PE, 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. The second, 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 are added. The resultant PVC compound offers better electrical performance, greater flexibility, and are easier to melt when the cables are manufactured, compared with PVC alone. However, while PVC compounds represent an inexpensive material that creates a relatively safe jacket for most plenum cables, it remains combustible in nature.

Lastly, FEP has electrical insulation characteristics on copper equal to PE, and needs no additives to meet fire safety standards. Meeting the highest performance criteria for flame spread, fuel load, and smoke generation, FEP has inherently good fire and smoke characteristics that help cables meet code and insurance requirements by a comfortable margin. In fact, PE and PVC, which are combustible materials by the NFPA standards, can increase the combustibility of a cable, and contribute up to eight times as much fuel as FEP.

Smoke Generation and Flame Spread Properties of Commonly Used Cable Types

Not all communications cables are alike-and not all pose the same fire risk. There are several basic cable types: Limited Combustible Cable (LCC), which has its FEP insulation and jacketing, communications multipurpose plenum cable (CMP), which has an FEP insulation but a PVC jacket; and CMR riser cable and CM (general purpose communications cable), both of which have PE insulation and PVC jackets.

In a fire test replicating a real world office fire, in a very short period of time the CMR cable generates large quantities of smoke, with the fire spreading down the length of the cable. The burning CMR cable produces temperatures of 1000°C-at 800°C temperature the integrity of a building's steel structure starts to be compromised. Better performance is seen in the CMP cable, which generates less smoke and less fire. The LCC fares even better. After a prolonged exposure to the heat source, the LCC evidences almost no smoke and no sustained ignition or flame spread.

One of the most telling battery of tests comparing these cable types was performed by the Building Research Establishment (BRE) facility in Bedford, England. This BRE test is more severe than the Steiner Tunnel test, specified under UL 910. Here, a real-scale test set-up was constructed which matches the environment found in the air plenum of a typical building. During the test the CMP cable generated approximately 15 times the smoke of LCC. But the fact that by the time only 30 seconds have elapsed, the CMP cable has generated approximately 10 times the smoke of LCC means people in a burning building could be engulfed in smoke literally in a matter of seconds, making a safe evacuation and a chance to protect expensive equipment virtually impossible.

Courtesy of: DuPont Cabling Solutions

Further data from the test fire show that flame-retardant PVC-as used in CMP cabling-has approximately the same fire loading as wood. This is comparable to sticks of wood in a plenum, on fire at one end, and being bathed in a stream of air-a graphic image of the extent of fire hazard of CMP cabling in a plenum. The quick spread of such a fire is compounded by the fact that flame retardant PVC catches fire at a much lower temperature than FEP.

Due to its FEP insulation, the LCC generates much less smoke, the lowest possible fire load, and is much less likely to burn than existing CMP plenum. In fact, when compared to minimal CMP, LCC has up to 20 times lower smoke generation as well as eight times lower fuel load and lower flame spread. It also resists the effects of aging and corrosive environments, and has enhanced performance over a wide temperature range.

To be listed as LCC, the cable must pass strict tests. The NFPA has developed a smoke index rating and a flame spread index rating. Under this system, burning Red Oak is defined as having a smoke index and a flame spread index of 100. To carry the LCC designation, the cabling must have a smoke index of 50 or lower on this test, and a flame spread index of 25 or lower. LCC cabling must also possess a maximum fire load (potential heat) of 8.1 mega Joules per kilogram (3500 BTU/lb), and it must meet the standards described under the new UL listing UL2424 for "Limited Combustible FHC 25/50 CMP" cabling.

While listed 25/50 limited combustible CMP meets or exceeds all current and proposed cable fire safety standards worldwide, it is important to note that conventional CMP cable still meets many minimum standards and fire codes, and is clearly superior to CM and CMR cable, and it represents a major safety improvement over those cable types. However, the polyethylene used in CMX, CM, and CMR cable has a fuel load comparable to gasoline.