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Article 01

 

A History of Plenum Cable Fire Safety Issues


THE EVOLUTION OF PLENUM CABLE FIRE STANDARDS
AND THE IMPACT OF THOSE
STANDARDS ON MATERIAL SPECIFICATION

By Gary Stanitis and Fred Dohmann, Ausimont USA, Inc.


The evolution of building practices during the second half of the 20th century included many general trends. One was the proliferation of very tall "High-Rise" buildings in metropolitan areas. In addition, changes in construction materials, furnishings, and interior decorative finishes have made fire loadings and fire load distributions within buildings change with time. There has been a dramatic evolution of voice and data communications systems and an increase in the routing of voice/data cable air handling plenums. Air handling plenums (the space above the ceiling used for air management) have become more common with the advent of integrated heating ventilation and air-conditioning.

To understand the impact that these changes have had on fire safety, the City of New York can be viewed as a representative population of modern high-rise office buildings. For the first half of the 20th century, even into the late 1960s, lives lost in New York City high-rise fires were few. This changed in 1968 and 1969 with three well-publicized fires, the Time and Life building, the Chemical Bank building, and an architect's office. The architect office fire, which was in a building only five-stories tall, killed ten occupants. In 1970 there was a fire at One New York Plaza and 919 Third Avenue that killed five people. As a response New York City established "Local Law 5", which established a number of requirements for high-rise office buildings for greater fire protection.

In 1975 there was a large fire at One World Trade Center that luckily occurred in the middle of the night when the building was unoccupied. The One World Trade Center fire was analyzed in great detail and a report was issued by The New York Board of Fire Underwriters Bureau of Fire Prevention and Public Relations. The information gathered in this investigation was used to further develop New York City Local Law 5. The following are some of the observations/ conclusions of the report.

"The [exposed] polyethylene (PE) and polyvinyl chloride (PVC) cable insulation and plastic back panel blocks burned readily so that virtually all combustibles including the fire retardant wood paneling on the telephone closet walls of the 10th and 12th floors were destroyed".

"High temperatures in the plenum radiated enough heat into the offices to the north of the fire to melt plastic phones and char papers on desk tops, but ignition did not take place".

"Cables passing from one closet to another closet on the same floor pass through the plenum above the hung ceiling. The exposed cable [1975 PE and PVC compounds] is combustible and constitutes a hazard because fire will be drawn into the plenum and the insulation will intensify the fire at this point".

"The central air conditioning system of itself is not a fire hazard. It is of importance because of its ability to spread smoke throughout a building, to carry the fire from one section to another, and to intensify the fire. Smoke can carry through a system even when it is shut down and create intolerable conditions on other than the fire floor. It is because of this that no combustibles should be allowed in the plenum to create additional heat and additional problems".

"It should be noted, that the mass of cables to supply communications equipment in many office occupancies is sufficient to sustain a substantial fire. While an individual cable is extremely difficult to ignite, a group of cables lying parallel will burn intensely, similar to the situation that exists with a group of logs in a fire".

The quotes listed here, and the One World Trade Center Fire report clearly reinforce the following two conclusions:

  1. Plenum fire safety must be strongly emphasized because the plenum is an excellent path for both flame and smoke transport throughout a building structure.
  2. Even in 1975 it was clear that cables placed in plenums were potentially a serious fire risk and selection and installation practices must be developed to manage that risk properly.

How to address these concerns, both in 1975 and today? Through the evolution of plenum cable fire safety practices.

The Evolution of Plenum Cable Fire Safety Practices

Prior to 1975, the National Electric Code (established by the National Fire Protection, NFPA, no. 70) required that all cables installed in building plenums be either encased in metal raceway or conduit, or have metal sheaths. Three types of metal sheaths were accepted - mineral-insulated metal, metal clad, and armored.

  • Although the mandate to place all cables inside of metal protection made fire safety professionals feel comfortable, was it the overall best solution?
  • Construction practices must be monitored closely to ensure compliance to the installation standard.
  • With the dramatic proliferation of voice and data cables, a huge cost and installation time penalty was required.
  • Inability to easily re configure installed voice and data cabling.
  • And perhaps the most important issue - placing flammable cables inside conduit does not necessarily prevent their contribution to the fuel load. Most plastics break down into flammable gaseous by products at high temperatures, which can easily vent from joints and fittings in the conduit or sheath.

In 1975 The National Electric Code added the following wording to the cabling standard: "Exceptions to the conduit requirement are provided for communications, power-limited, and fire alarm cables that are listed as having "adequate" fire resistant and low smoke producing characteristics".

Unlike the previous code that identified specific construction details, the added wording specifies a performance requirement. But how does one test for "adequate fire resistance and low smoke producing characteristics" of cables?

To answer this question, in 1979/1980 Underwriters Laboratories, Bell Laboratories, and E.I. du Pont de Nemours cooperated in developing a suitable test method. The apparatus chosen was The Steiner Tunnel; a well recognized large-scale burn apparatus routinely used to evaluate construction material fire safety (ASTM E84). Conditions of the test were established to accommodate cables and their unique flame spread and fire characteristics. Careful attention was paid to establishing a smoke scale that was directly related to human visibility. Work published by R.G. Silversides "measurement and Control of Smoke in Building Fires" and Gross, Loftus and Robertson's "Method for Measuring Smoke from Burning Materials" was used to ensure that smoke obscuration measured in the test related to real room fires.

A variety of cables insulated with PE (polyethylene) and PVC (polyvinyl chloride) compounds were tested both within metal conduit or sheathing to reflect the code practices required at that time. Cables with fluoropolymer primary and jacket insulation were tested without conduit or metal sheathing. The fluoropolymer cables were chosen because fluoropolymers are known to have excellent fire performance (difficult to ignite, low smoke generation, low heat of combustion).

The results of the work had the following conclusions:

  • A reproducible test was developed that can be used to quantify and to differentiate the fire and smoke performance of cables.
  • The highest smoke-generation and flame spread was generated by PE/PVC cables within metal conduit of metal sheathing, confirming that conduit does not always effectively eliminate the contribution to smoke and flame of the enclosed cable.
  • FEP (fluoropolymer) cables not in conduit showed flame spread and smoke generation characteristics comparable to, or less than, conventional PVC and PE cable in conduit, and have been classified by UL as to their fire resistance and smoke producing characteristics per the National Electrical Code.

Thus, from the Underwriters Laboratories/Bell Laboratories/DuPont cooperation came two significant achievements, an accepted test method to qualify cables for plenum service without metal protection, UL 910, and the first plenum rated cables.

THE ACCEPTANCE And EVOLUTION Of UL 910

In the early 1980s the National Fire Protection Agency (NFPA) recognized that the UL 910 test was appropriate and incorporated it into the 90A standard (Standard for the Installation of Air Conditioning and Ventilation Systems).

It is the 90A standard that identifies the fire and smoke properties required for materials to be used I plenums. During the 1980s the NFPA chose to establish its own test standard identified as NFPA 262. NFPA 262 was written directly from UL 910 with the intent of copying it exactly. By writing a general standard, NFPA opened the door to laboratories other than UL to characterize cables for plenum use. During the 1980s fluoropolymer insulated voice, data, and fire alarm cables became the standard in the industry due to their cost effective installation and rerouting with no compromise of flame spread and smoke generation compared to PVC and PE cables in conduit.

When NFPA 262 was first drafted, exact equivalence to UL 910 was not established. The differences between the documents were not intentional. This issue became apparent in the codes and standards arena in 1997 as companies pushed for cheaper, highly compounded PVCs and polyolefins (polyethylene and polypropylene), to be used in plenums. Because even heavily compounded PVCs and polyolefins (POs) have more marginal flame spread and smoke generation performance than fluoropolymers, there was pressure on the codes groups to weaken plenum standards. Fortunately, rational thought prevailed and the NFPA 262 and UL 910 standards were redrafted to be identical in 1998, and now match the protocol first established in 1979/1980 to characterize plenum cables.

During the effort to harmonize the UL 910/NFPA 262 protocols, and based on other events occurring in the industry, it became apparent that proper installation, maintenance, and operation of the equipment is crucial to achieve high reliability in the UL 910/NFPA 262 test. To assure proper results, a research program was established by the Fire Protection Research Foundation (FPRF) in 1998 to harmonize laboratories performing UL 910/ NFPA 262.

The project, entitled "International UL 910/ NFPA 262 Fire test Harmonization Project", is harmonizing five laboratories globally:

  • British Research Engineering, United Kingdom
  • Loss Prevention council, United Kingdom
  • Underwriters Laboratories, United States
  • Intertek Testing Services, United States
  • Japan Electric Cable technology Center, Inc., Japan

Additionally, FPRF started a Technical Advisory Council for tunnel operators to ensure continued consistency laboratory to laboratory by meeting regularly and providing a forum to discuss maintenance, calibration, and operational issues.

As recently as the mid-1990s, the validity of the UL 910/NFPA 262 protocol was further demonstrated. The Fluoropolymers Division of the Society of the Plastics Industry (SPI) sponsored a research program at British Research Engineering (BRE) in the UK. The objective was to compare the flame and smoke performance of a variety of cable constructions. Tests were performed both in the UL 910/NFPA 262 test, and also in a full room fire test constructed at BRE. At the completion of the program, DuPont carried out additional work in the same apparatus, and eventually the British Government sponsored a research project that is still ongoing. SPI was expected to publish a paper by the end of 1999. Dupont published results from this work that clearly demonstrate the following:

  • "All [UL 910/NFPA 262] results are comparable to the full-scale [full room] results."
  • "The fire performance of the exposed CMP [plenum rated] cable was comparable to CMX [non-plenum rated] cable in metal trunking in the BRE full-scale [full room] and Steiner tunnel [UL 910/NFPA 262] tests."

Establishing that even for newer cable designs with new compounds used for insulation, the UL 910/NFPA 262 test still has excellent correlation with a full room fire scenario.

Not All Plenum Rated Cables Are The Same

Despite the excellent fire history and cost effectiveness (compared with metal conduit) of fluoropolymer insulated cables, there has been an unfortunate evolution to lower quality PVC (polyvinylchloride) and PO (polyolefin) compounds for use as primary and jacket insulation on plenum cables. The motivation to use PVC and PO compounds is purely to reduce the cost of cable. No positive benefits ensue. In fact, there are many compromises, such as reduced physical properties, higher smoke generation, higher fuel load contribution, greater moisture permeation, and deterioration with age, that have led to several problems in the industry.

New York City Local Law 5, as mentioned earlier, is written to establish practices to ensure fire safety in commercial high-rise buildings in NYC. Part of this law requires fire monitoring and alarms. Although originally the cables used for fire monitoring required only plenum rating (as determined by UL 910/NFPA 262), a serious problem was encountered. Cables were being produced with highly compounded PVC that were able to pass the fire and smoke test. However, the physical properties of the PVC were so reduced due to large amounts of additives, that during installation the jacket and insulation was being torn, significantly deteriorating the performance of the cable.

To overcome this problem with PVC cables, in the mid 1990s, NYC Local Law 5 was re-written to require cables to have 150 C temperature rating as well as pass UL 910/NFPA 262. Only fluoropolymers are capable of meeting these requirements. Because fluoropolymers are not highly filled, the mechanical properties of the insulation are much higher and eliminate the installation problems. This strategy has successfully eliminated the problem and remains in effect today.

When PVC compounds are analyzed, it is not hard to see why this problem occurs. PVC, as a material, cannot pass the UL 910/NFPA 262 due to the evolution of thick black smoke. A large quantity of additives must be added to the PVC. In 1996 three commercial PVC cable compounds were analyzed at Ausimont. Results are shown in Table 1. In each case less than 50%by weight of the compound is plastic. The rest is inorganic fillers, fire retardants, plasticizers, and stabilizers. This composition is the explanation for the poor mechanical properties.

Quantified evidence of the poor properties of PVC is given in Underwriters Laboratories UL 444 standard ("Standard for Communications Cable"). UL 444 identifies the physical requirements of plastic materials used in communications cables. PVC compounds are identified as requiring only 2000 psi tensile strength, while a typical fluoropolymer jacket material, ECTFE, has 5000 psi minimum tensile strength. An even more dramatic detail is that the PVC can lose 50% of its properties after only 7 days aging at 136 C. The ECTFE will retain 75% or more of its tensile properties even when aged at 180 C. Therefore, the concern with regard to installation damage is supported by information contained in UL 444. Implied in this same data is the conclusion that damage during re configuration is even greater with compounded PVCs given the fact that they exhibit a significant loss of mechanical properties with age.

A more dramatic consequence of increased use of PVC and PO compounds is the potential of greater flame spread and smoke developed values of cables where these compounds are used. Additionally, although not specifically tested for in UL 910/NFPA 262, PVC and PO have 3 to 6 times the heat of combustion (fire load) as do fluoropolymers. Underwriters Laboratories realized the consequence of gradual transition to more PVC and PO in late 1996. The normal procedure for UL to list a plenum cable is to perform approval testing, and then continue to re-sample and test production samples of the cable on a regular basis to ensure continued compliance. This second activity is called the Follow Up Service (FUS).

Analysis showed that for the 9 years starting in 1988 and ending in 1996, the percentage of cables failing the UL 910/NFPA 262 FUS test increased from 10% to over 50%. Although some failure rate can be expected because of testing variation and cable manufacturing variation, a FUS failure rate of 50% is clearly an indication of a chronic problem. Certainly it is not coincidental that this deterioration in flame and smoke performance occurred during a period when more PVCs and PO compounds were being designed into cable solely for economic reasons. UL responded to the FUS failure problem by convening an industry advisory panel referred to as TAPCOM (Technical Advisory Panel for Communications Cable). Some critical observations/ conclusions from the meetings have been:

  • It was found that the UL 910 test apparatus (Steiner Tunnel) had a small leak due to corrosion in the exhaust duct. It is not known how long the leak was present, but certainly for some years. The impact of the leak was to reduce the smoke developed value. Again, it's most likely not a coincidence that the leak was occurring during a time when more PVC and Pos were being used. UL has fixed the leak and the test performance has returned to the more severe screening criteria that it was originally designed to be in 1979/1980.
  • UL has hired a statistical process control consultant to help establish and maintain the reproducibility of the test. Conclusions by the consultant are that the test itself does not have an unusually high degree of variation. Never the less, UL has continued to make improvements to further increase reproducibility.
  • It was discovered that in some cases cables jacketed with PVC compounds produced more variable smoke developed results depending on humidity (humid cables tended to generate more smoke). This was not true of the fluoropolymer-jacketed cables. Because of this, conditioning requirements have now been specified for cables prior to testing. Unfortunately, cable in a plenum cannot be conditioned before a real fire.
  • Cables made with complex compounds are much more sensitive to the conditions by which they are made. Processing of the polymer compounds can cause variation in the chemical reaction that is required to keep PVC and PO from generating dark smoke. Variations in flame spread and smoke generation results from changes in manufacturing speed, location, and tooling. Again, this is not the case when fluoropolymers are used.
  • Cables that are made with PVC and PO compounds may pass the requirements of the UL 910/NFPA test, but are typically closer to the failure point than "all" fluoropolymer cables. Because of this UL will be introducing a more severe FUS procedure for cables that perform marginally in smoke and flame. www.ausiusa.com
TABLE 1 - Composition Analysis of Commercial PVC
            Plenum Compounds Performed in 1996

Weight%
Sample 5C
Sample 7A
Sample 1A
Molybdenum Oxide (MoO3)
2.6
3.2
-
Antimony Oxide (Sb2O3)
0.2
0.3
-
Zinc Oxide (ZnO)
2.1
5.2
-
Lead Oxide (PbO)
2.0
1.3
2.3
Iron Oxide (Fe2O3)
0.3
0.2
-
Magnesium Oxide (MgO)
3.7
-
2.4
Calcium Oxide (CaO)
0.3
0.1
-
Titanium Dioxide (TiO2)
-
-
0.4
Aluminum Oxide (Al2O3)
5.0
13.3
17.0
Plasticizers (aliphatic esters)
2.0
14.0
3.0
Halogen Flame Retardant
(Tetrabromo benzene derivative)
7.0
7.0
14.0
Low-Smoke Plasticizer
(Alkyl-aryl-phosphate)
8.0
2.0
3.0
Unknowns
23
5.4
8 to 13
Polymer (either PVC or PVC/EVA)
44
48
45 to 50

 

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  Tuesday, May 04, 2004
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