Development
and Fire Testing of Plenum Cables
Background, History, and
Current Issues
UL 910 Test Objectives and Criteria
High Failure Rates on FUS in UL910 Test
Tunnel Harmonization Project by FPRF
Bibliography
1. Development of a Fire Test for Plenum Cables
The "Need" ¾ A Safe, Economic way to Install Communication Cable in Office Buildings
To an architect designing an office building, the plenum is an attractive area to place communications cable, for it saves the expense of under-the-floor conduit (which was the installation method originally recommended by the telephone companies). Recognizing the widespread use of plastic insulated communications cable in plenums, the National Electrical Code (NEC) in 1975 contained a provision that "Conductors having inherent fire-resistant and low smoke producing characteristics, approved for the purpose, shall be permitted for ducts, hollow spaces used as ducts, and plenums..." This provided incentive to develop fire resistant, low smoke producing cable without conduit for use in plenums.
To put this opportunity into practice, fire resistant- low smoke cables had to be developed and a test had to be developed for communication wire and cable which would 1) provide a numerical rank ordering of the flame spread and smoke producing characteristics, and 2) provide a test that would be meaningful with respect to actual fire conditions. The test that was developed in the late 1970’s, was the UL 910 plenum test and the first, fire resistant, low smoke producing cables were made of FEP fluoropolymer, a fluoropolymer copolymer of tetrafluoroethylene (TFE) and hexafluoropropylene (HFP). FEP generates very low smoke, and does not require compounding with fire and smoke retardants. In addition, FEP has a very low inherent fuel load compared to PVC and polyolefin. Subsequently, other low flame spread, low smoke producing fluoropolymer cables ¾ PVDF (polyvinylidene fluoride), and ECTFE (ethylene/chlorotrifluoroethylene) ¾ were also commercialized.
UL 910 Test Objectives and Criteria
In 1978, Underwriters Laboratories Northbrook facility conducted a Fact-Finding Investigation for Bell Laboratories, which was titled "Comparative Flame Propagation and Smoke Development Tests on Communication Cables in Various Geometry’s." UL's general charter is to establish, maintain, and operate laboratories for the investigation of materials, devices, products, equipment and systems with respect to hazards affecting life and property, while it’s general objectives are to report and circulate the results of investigations to organizations, public safety authorities, government bodies or agencies, and other interested parties. There were two specific objectives for the joint communications wire and cable fire test development program with Bell Labs. These were:
This led to the development of the "Test for Flame Propagation and Smoke-Density Values for Electrical and Optical-Fiber Cables Used in Spaces Transporting Environmental Air," called UL 910. The requirements for a test facility were met by the Steiner Tunnel, the test facility used in ASTM E-84. There were 15 Steiner Tunnels in North America in 1975. Since it was a widely used and well characterized large scale fire test, one had confidence in having a reproducible environment and test facility. The validity of the UL 910 test has been recognized for the past 24 years. As recently as 1995, studies sponsored by the Society of the Plastics Industry (SPI) have confirmed the suitability of the test. SPI’s work at British Research Engineering in Cardington, UK compared the fire characteristics of a full room fire test using a variety of cables to UL 910 results. The test results showed good correlation between a full room plenum cable fire and the UL 910 protocol
UL's Steiner Tunnel testing equipment used during the 1978 Fact-Finding Investigation is identical to the equipment being used today to test and list wire and cable products intended for installation in air handling plenums. The device used to measure smoke, the air inlet chamber, and the rectangular section and rectangular to round exhaust transition at the exhaust end of the tunnel, exist on the tunnel equipment today as they did in 1978. In short, no physical changes have been made to the tunnel equipment. As part of UL's routine maintenance program, UL has replaced parts of the test apparatus that have worn out. In addition, UL has periodically updated the data acquisition system, as new technology became available. In each case, each data acquisition update was verified to have no effect on the results obtained from the test.
2. Current Issue ¾ High Failure Rates on FUS in UL 910 Test
When Underwriters Laboratory approves a cable it must pass an initial series of tests, that include the UL 910 Steiner Tunnel Test. Once the cable meets all requirements UL will list it, but they will also randomly test cables from every manufacturer on a yearly basis (this is called the Follow up Service or FUS). They do not test every cable from each manufacturer yearly as many people make a large number of cables. However, over time, most cables will get picked up for follow up testing
The original rate of FUS failures for the UL 910 test in the 1980’s was low, roughly 10%. By the late 1980’s a greater number of cable designs included formulated PVC’s and polyolefins. Both groups of materials have higher smoke generation than fluoropolymers, leading to cables that passed the UL 910 with less margin. And, because these compounds are complex, due to the difficulty of getting PVC and polyolefin to pass a stringent fire and smoke test, they also have the potential to show more variability when processed commercially. UL noticed in the late 1980’s that the percentage of Follow-up Service (FUS) failures was increasing in the UL 910 Steiner Tunnel Test. By the mid-1990’s a full 50% of the FUS UL 910 tests were failing, and this was still true in mid-1999.
In every case of follow up failure, UL properly worked out a solution with each cable manufacturer. However, it became clear with time that the overall trend of high (50%) follow up failures was not being reduced. The current round of UL’s TAPCOM (Technical Advisory Panel for Communications and Power Limited W&C) activity was started in April, 1997 to address this issue.
As part of the analysis resulting from these TAPCOM sessions, UL properly investigated all options to help understand variability in the Tunnel. This included a rigorous mechanical inspection of the apparatus. It was found that the exit exhaust duct had corroded through over time. It is not known how long the leaking duct has existed. After being repaired the UL 910 test became more consistent with historical results but is more difficult to pass regarding smoke generation due to the changed air flow. This creates much concern among the wire and cable producers. Unfortunately it may be that the leakage began occurring during the same period that higher smoke producing polymers, like PVC and polyolefins, were being evaluated in plenum applications.
A standard insulated conductor was developed by Lucent under the auspices of TAPCOM for the calibration of the UL 910 test chamber. UL has adopted the use of this cable for calibrating the tunnel, replacing red oak, which is a natural material and has inherent variability. Lucent’s calibration cable has been proven useful and consistent in tests by UL’s quality control consultant.
In the1/99 TAPCOM report, tests by Doug Relyea, the statistical consultant hired by UL, showed a standard deviation in test results on a calibration cable of .0066 units in average smoke, .041 in peak smoke, and 0.5 units in flame spread. This demonstrated that a well maintained tunnel gave precise and reliable results.
During review of the UL 910 test protocol it was noticed that there are some subtle differences between UL 910 and NFPA 262 (the NFPA 90A standard actually calls out NFPA 262, not UL 910.) However, UL 910 existed first and NFPA 262 was written directly from it. This led to activities to update both NFPA 262 and UL 910 so that they would be identical. This was completed in November 1998. Some of the additional benefits of revising UL 910 was that the standard could be used to assist in the revision of NFPA 262 and also in the development of an ASTM Standard. It is also extremely important that UL 910 reflects what is and has been current practice because of the increased international interest of the test method.
In a June 15, 1998 bulletin, UL announced a Recognition Program and adopted identification tests for plenum-cable jacket and insulation materials. The program becomes effective December 15, 1999. The major reason for the compound recognition program is the use in plenum cable of complex plastic compounds. Fluoropolymers are consistent materials, so there has historically been little concern about variation in their flame performance. Because PVC’s contain more than 50% by weight additives, of which a large percentage are reactive flame retardants, variation in their compounding and the process in which they are used can seriously effect their consistency. Although compounding is an effective way to enhance a material’s performance, it requires such a tremendous loading of additives to allow PVC to pass the UL 910 that most of it’s properties are severely compromised (physicals, moisture absorption, aging characteristics, etc.)
3. Tunnel Harmonization Project by Fire Protection Research Foundation (FPRF)
The idea to harmonize all of the tunnels running NFPA 262 (UL 910) grew out of the TAPCOM sessions. The Fire Protection Research Foundation (FPRF) initiated a project to harmonize the construction and performance of UL 910 Steiner Tunnels around the world for wire and cable testing), using the UL Northbrook facility as the original reference tunnel and the latest UL 910 / NFPA 262 Standards. The research project provides for harmonization of the tunnels and precision and bias data for national and international use.
The objectives are 1) to make sure that the various chambers comply with the dimensional requirements of UL 910/NFPA 262 and then 2) do a round robin test of the chambers using the 910ST standard insulated (calibration) cable to determine any bias between chambers.
The following testing laboratories are participating in the round robin test program:
(We will add the city and country location for the laboratories. ERH)
Tunnel Harmonization Progress
The harmonization of the test tunnels running UL 910 is proceeding well. A separate report will be issued on this subject on this W&C web site.
1. "Low Smoke and Flame Spread Cables", L. J. Przybyla, E. J. Coffey, S. Kaufman, M. Yocum, J. C. Reed, and D. B. Allen, Journal of Fire and Flammability, Vol 12 (April 1981)
2. NFPA 262 "Standard for the Method of Test for Flame travel and Smoke of Wires and Cables for Use in Air-Handling Spaces" - 1999 Edition", National Fire Protection Association Quincy, Mass.
3. "Results of Full- Scale UK Fire Tests of LAN Data Communications Cables Used in Concealed-Space Applications" James Hoover, Loren Caudill, James Walnock, Theo Schroots and GuenterMann, DuPont de Nemours Technical Paper, portions presented at BICSI, Orlando Florida (January 1997)
4. "Underwriters Laboratories UL 910 "Test for Flame-Propagation and Smoke-Density Values for Electrical and Optical-Fiber Cables Used in Spaces Transporting Environmental Air", Underwriters Laboratories, Northbrook, ILL
5. ASTM E 84 Standard Test for Burning characteristics of Materials, ASTM, West Conshohocken, Pa.