Canadian Consulting Engineer

Exposed cable in plenum

May 1, 2001
By J. K. Richardson, P.Eng.

An article appeared in the August-September 2000 issue of Canadian Consulting Engineer (pp. 49-52) by author David Dykeman. He argued that we need to have more stringent fire and smoke testing standar...

An article appeared in the August-September 2000 issue of Canadian Consulting Engineer (pp. 49-52) by author David Dykeman. He argued that we need to have more stringent fire and smoke testing standards in Canada for exposed communications cables in plenums in high-rise buildings. He recommended that the CSA FT6 (equivalent to UL 910) flame and smoke test for such cable should be adopted by the National Building Code of Canada.

We received the following response from Kenneth Richardson, P.Eng., co-author of an article on the same subject that appeared 15 years ago in this magazine. Richardson was involved with National Building Code committees that requested the original research that led to the present cable fire test standard listed in the National Building Code of Canada, CSA FT4.

As Richardson notes at the end of his article, the National Research Council of Canada’s Institute for Research in Construction is currently doing new studies in this area with the American Society of Heating, Refrigerating and Air-Conditioning Engineers. The studies were called for in recognition that the amount of cable in plenums has greatly increased: 46% between 1991 and 1997, and another 20% between 1997 and last year.

The article entitled “Cable Pile-Up” rightly draws our attention to the growing problem of increasing numbers of cables in plenums as each generation of electronic data transmission technology passes. However, before adopting the solution recommended in the article — to use only cables meeting the CSA FT6/UL 910 test — it is important that readers understand the background of the current requirements for cables in plenums contained in the National Building Code of Canada (and the Canadian Electrical Code, Part 1).

With the development of low flame/smoke cables in the 1970s, it was recognized that there was an opportunity to install exposed cables in plenums and still meet the fire safety standard previously established for cables installed in metal conduit. At that time in Canada, note, the codes already permitted limited quantities of exposed communications cables in plenums and these did not have to meet the UL 910 test.

In the early 1980s, the Canadian building and electrical code committees were petitioned to permit cable that passed the UL 910 test to be used exposed in plenums, and not have to be enclosed in metal conduit. The results of an extensive study and NBC committee deliberations were reported in the August 1985 issue of Canadian Consulting Engineer in an article written by James Mehaffey, Ph.D and me entitled: “Electrical Cables — A Less Significant Factor in Fire.” The conclusions from that paper provide illumination to counteract the statements made in the August/September 2000 article. They include:

Statistics on fires involving electrical cables showed that fire spread on cables themselves was not the primary issue. The major fires were spread by the absence of firestopping where required. (Incidentally, the NRC and the NFPA reports on the Alexis Nihon fire cited in the August-September article make no mention of fire spreading over cables through the plenums as a factor in fire propagation. The major issue was the absence of required firestopping.)

Despite the fact that in Canada there were numerous plenum installations of exposed communication cables (as noted above), there were no reported cases of significant fire spread. In tests conducted at ULI in the late 1970s with exposed PVC jacketed cables, despite creating worst case conditions, uncontrolled fire spread from an ignition room to an exhaust room through a plenum did not occur.

To try to simulate a worst case fire in a plenum, NRC, on behalf of the NBC committees, used a computer model (Harvard Mark V, developed by Harvard University) which was the most advanced at the time. It was an internationally-respected fire growth model, not an obscure one as the August-September 2000 article suggests. The results presented to the committees demonstrated that the likelihood of flashover in the plenum was small. This was, however, not the only evidence that the committees reviewed but one piece of extensive evidence.

To ensure that “bad actors” were not used in plenums, the committees recommended that a vertical tray fire test be used. This method, which has become the FT4 test, was shown to be able to clearly delineate between cables which had a propensity for uncontrolled fire spread and those which did not. The vertical tray test (FT4) was also found to delineate between bad actors and acceptable performance on the same basis as the modified Steiner tunnel test (ULC S102.2) that was in use in Canada at that time.

The committees considered that the incremental smoke produced by cables would probably be small compared to that from the other combustibles in a building and that the fire safety measures already required by the NBC for preventing smoke spread, in general, would also prevent the spread of any incremental smoke from cables (e.g. fan shut down, smoke control).

This background shows that the FT4 requirements in the NBC were not the result of capricious actions by committees but were based on significant study, research, testing and eventually committee debate involving a balanced membership of all affected parties. I am unaware of additional research, study or testing conducted in Ontario and British Columbia when they opted for the more stringent FT6 requirements. The bases for those decisions are not known.

To address this new and growing problem of increasing numbers of cables in plenums, a major research project is underway at the National Research Council, co-sponsored by ASHRAE. That study, which should help the fire safety community come to grips with this cable fuel load issue, will examine the fire performance of quantities of both new and older cables in plenums, including FT6-rated cables.

I agree, then, that the issue of cable build-up in plenums needs to be addressed. I do not, however, believe that the solution put forward in the August-September 2000 article is the only solution, or that it is necessarily even a viable one. I prefer to wait for the results of unbiased, authoritative research.

J.K. Richardson, P.Eng. is president of Ken Richardson Fire Technologies Inc. in Gloucester, Ontario.


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