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Electrical cables: a less significant factor in fire
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Electrical Cables
-
A
Less Significant Fact in Fire
by J. Mehaffey and J.K. Richardson
Reprinted from
Canadian Consulting Engineer
Volume 27, No. 8, August 1985, 3p.
(DBR Paper No. 1346)
Price $2.00
NRCC 25296
ABSTRACT The p a p e r a d d r e s s e s t h e f l a m m a b i l i t y and c o m b u s t i b l e l o a d of e l e c t r i c a l c a b l e s i n b u i l d i n g s . S e v e r a l t e s t methods f o r a s s e s s i n g t h e f i r e p e r f o r m a n c e of e l e c t r i c a l c a b l e s a r e p r e s e n t e d . P r o p o s a l s b e f o r e t h e A s s o c i a t e Committee on t h e N a t i o n a l B u i l d i n g Code of Canada a r e o u t l i n e d .
SUM^
Cet a r t i c l e p o r t e s u r 1 1 i n f l a m m a b i l i t 6 e t l a c h a r g e c o m b u s t i b l e d e s c l b l e s e l e c t r i q u e s i n s t a l l & d a n s l e s b b t i m e n t s . On y d e c r i t p l u s i e u r s m6thodes d ' e s s a i p e r m e t t a n t d 1 6 v a l u e r l e comportement a u f e u d e s c l b l e s G l e c t r i q u e s , a i n s i que l e s p r o p o s i t i o n s q u i o n t 6 t 6 f a i t e s 3 Ce s u j e t a u Comit6 a s s o c i G d u Code n a t i o n a l d u bfitiment du Canada. -- - - 4 - - - - I
1
I - - --
-- -Technical Trends
Electrical cables
-
a less
significant factor in fire
By James
0
ver the past decade consider- able attention has been given to the performance of electri- cal cables in fires. Reports of fires, such as those in the United States at the Brown's Ferry Nuclear Plant, the New York Telephone Exchange, and the World Trade Center, have stressed the contribution of cables tofire development, the spread of fire, and smoke.
However, careful study shows that while electrical cables may have been a factor in these fires. their involve- ment was not as significant as one would expect. A major problem in all of these fires was the absence of proper firestopping between fire compartments.
To assess the potential flammabili- ty and smoke hazards of electrical cables, studies have been undertaken by both the National Building Code (NBC) Part 3 and the Canadian Electrical Code (CEC) Part I commit- tees. The associate committee on the National Building Code's standing committee on use and occupancy established a task group to review this subject and make recommendations for changes to the NBC. The CEC Part I committee has also been studying the problem. A joint liaison committee comprised of members knowledgeable about both codes is monitoring the two groups' work.
Electrical fire reports
Published statistics related to the propagation of fire by electrical cables are scarce. Only a detailed analysis of fire reports enables one to extract this specific information. Since the statistical data are not
I
Kenneth Richardson, P. Eng. is nzanager of the Jre research sfafion of the National Research Council of Canada. James MehaJey, Ph. D. is a research officer in the fire research sectiorl of
NRC.
Mehaffey, Ph.D. and Kenneth Richards
readily available, surveys, literature searches, and reviews of fire reports were undertaken.
Reports on fires involving electri- cal items, including cables, in the Province of Alberta for 1981 to 1983 show that approximately eight per- cent of the 2717 fires in those three years involving electrical wiring occurred in other than single-family dwellings. The 1983 reports show that no vertical fire-spread beyond the compartment of fire origin was attri- buted to electrical items (including cables).
These revorts also show that hori- zontal fire and smoke-spread from the compartment of fire origin occurred in 30 (four percent) of the 725 incidents involving electrical items. The spread occurred through air-handling ducts, attics, ceiling spaces or utility openings. The nupm: ber of these incidents that can be attributed to cables cannot be deter- mined from the reports.
A search of computer databases, including the Engineering Index, NTIS Data Base and RAPRA Data Base, for fires and electrical cables yielded only those major fires dis- cussed previously. It was not antici- pated that the search would yield much because details of fire incidents are rarely published.
Review of the often-cited report on the New York World Trade Center fire showed that vertical fire-spread was primarily through a 450 mm by 300 mm non-firestopped opening between floors. Horizontal fire- spread through the plenum was caused by a fully-developed fire in the office space below being drawn into the plenum when the heating, venti- lating and air-conditioning system was
laced
in the exhaust mode. The fire did not spread past fire separa- tions which penetrated the plenum.Another example of fire-spread via
electrical cables is the New York Telephone Exchange fire. This build- ing contained an extraordinary num- ber of cables and fire spread primari- ly through a 150 mm by 83 mm non-firestopped opening between a cable vault and the main distribution frame on the floor above. This quantity and arrangement of cables would be expected only in such industrial buildings as telephone exchanges and power plants.
Cables in plenums
In Canada, the Canadian Electrical Code Part I began to regulate the flammability of communication ca- bles in concealed spaces, primarily plenums, in the 1975 edition by requiring that such cables be installed in totally enclosed raceways. The same restriction for power cables was included in the 1982 edition. Before these dates cables with exposed plas- tic jackets could be installed in plenums for environmental air with- out additional protection.
The 1975 edition of the CEC Part I also included an exception for the final nine metres of telecommunica- tions cable whlch could be exposed in the plenum. Since the CEC Part I is not retroactive in application, there are a large number of installations where cables are exposed in con- cealed spaces and fire reports should show evidence of problems with those cables, if such problems exist. No reports have been found.
It is not entirely satisfying to conclude that. because there are no statistical data documenting the problem, there is no fire hazard resulting from the installation of electrical cables in plenums. For this reason, a thorough technical review of the problems one might encounter is necessary. Plenums, because of their air circulation functions, and because they are deemed the most
Technical Trends
critical concealed spaces in buildings are considered in detail.
To begin with. i t is crucial to
determine what quantities of cables are found in plenums and what levels of fire hazard these auantities entail. For instance. is there so much cable per unit area of the plenum that upon ignition there is likely to be a rapid fire build-up resulting in flashover (the entire plenum engulfed in flames)? Or, is the combustible load low enough to indicate that fire spread by the progression of flames along the cables should be the prima- ry concern? What quantity of smoke is likely to be released?
To obtain an estimate of the combustible loads found in the ple- nums of noncombustible buildings. a study was conducted on a specific plenum in a specific building. This plenum was deemed to represent an upper limi t to the quantity of cables for an office building. as it contains. in addition to telephone and power cables. cables for 86 computer termi- nals located on the 1634 m' floor below.
The functions and properties of the various cables installed in the plenum were provided by the electrical engi- neer involved in construction of the building. Using this information the cables were classified according to their diameter. The length of cable and an estimate of the mass of combustibles for each diameter clas-
sification are presented in Table I.
In computing the mass of combus- tibles. the cables were assumed to be comprised entirely of polyethylene
(density = 915 kg m ') as polyethyl-
ene has the largest heat of combus- tion of the materials found in the cables. In this manner. the quantity of combustibles and the heat they release on burning is overestimated. On the average. each square metre of plenum has 1.5 m or 0.16 kg of cable laying above the ceiling. This is equivalent to one cable laying along the diagonal of the square metre.
The heat of combustion of polyeth- ylene is 46.6 MJ/kg. Consequently.
the specific fire load (heat content of
combustibles/~loor area) of the
cables is 7.5 hfJ/m'. This is much
Table I. Quantities of cables in a plenum.
Cable Diameter Cable Length Combustible
Mass
(mm) (m)
(kg)
Total 2455 255
lower than specific fire loads encoun- tered in the inhabited spaces of buildings. For offices. the specific fire load is about 467 MJ/m2.
In addition to cables. there are other combustibles in the plenum. Except for 51 mm by 102 mm wood blocking. their combustible content is negligible. This wood is estimated to have a specific fire load contribution
of 6.2 MJ/m2.
It is known that for flashover to occur within an enclosure. the tem- peratureof the hot smokey layer must reach 500-600°C. Several computer simulations of fires in the plenum under what uere deemed to be the worst case conditions were con-
ducted. I t appears safe to assume
that. given the nature and quantities of materials to be found in present- day plenums. the likelihood of the plenum experiencing flashover as the result of a fire involving only the combustibles in the plenum is small.
I t has been argued that with the increasing use of computers the number of cables in a plenum will increase. On the 0the.r hand. the quantity of combustibles is more likely to decrease due to the move away from analog telecommunica- tion signals todigital. which results in the use of smaller cables.
Full-scale fire tests
Although flashover is unlikely in a plenum fire. there nlay be other problems resulting from the flame- spreading and smoke-producing characteristics of electrical cables. To investigate this possibility. Under- writers' Laboratories Inc.. in collabo-
ration with Bell Laboratories. con- ducted full-scale tests on a simulated plenum. Communication cables without conduit were run from an ignition room through a plenum to a room containing an exhaust fan.
In the ignition room a ceiling tile was removed and the cables permit- ted to drop down to rest on a wooden crib. The wooden crib was ignited and flame propagation along the cables in the plenum and smoke flowing through an exhaust duct uere monitored. Several tests were made to determine the most severe exposure. by varying the size of the ignition source (crib), the rate of forced ventilation. and the positioningof the cables.
Large quantities of smoke were generated during the tests from the burning crib and cables. In addition. flames were found to propagate along the cables to varying degrees depend- ing on the type of cable and level of ventilation. In no case. however. did - -
the flame propagate the six metres from the ignition room through the plenum to the exhaust room.
Fire performance tests
I t is thought that some control needs to be exercised on the use of electrical cables in plenums. To assess the fire performance of cables and set acceptance criteria to prevent the use of "bad actors". a standard test is required. There are a number of test methods from which to choose. Although none simulates the com- plete range of conditions to which cables may be subjected in a building fire. four methods are noteworthy.
Technical Trends
flame test is cited in the Canadian Electrical Code as an appropriate method for assessing the flame retar- dance of electrical cables. In the test, a single length of cable is mounted vertically and exposed at the bottom to the flame from a bunsen burner. The test gives a good indication of the possibility of ignition and propaga- tion of fire from small ignition sources, but performance in the test is not considered representative of behavior in full-scale fires.
Several versions of a vertical cable-tray test have been developed to address the need for more severe fire tests. In these tests a vertical tray of cables is exposed near the bottom to a large flame source (20.5 kW) for 20 minutes. Multiple lengths of cable are attached to the tray, which is 75
mm deep. 300 mm wide and about 2.4 m long. T h e extent of flame spread is determined by the length of charred cables.
Ontario Hydro has developed a version of this test which also has provisions for measuring the acid-gas by-products produced by burning cables. The CSA committee on test methods for electrical wires and cables has developed a similar test method. These vertical cable-tray tests are capable of distinguishing between the behavior of older polyvi- nyl chloride formulations and the
newer fire retardant treated PVC
formulations. Generally, cables which Dass these tests are resistant to ignition from small sources and require significant exposure to fire before they become involved.
Electrical cables may also be sub- jected to the floor-mounted version of
the tunnel test, CAN4-S102.2. In this
test, 7.3 m lengths of cable are
attached side by side to a steel ladder 0.45 m in width, and this complete assembly is mounted on the tunnel floor. At one end of the tunnel a 90 kW flame impinges on the cables for a 10-minute period. The flames from the burner and cables are forced along the tunnel by a draught of air. The propagation of flame along the cables and the obscuration of light in the exhaust duct due to the produc- tion of smoke are monitored.
From these data a flame-spread classification (FSC) and a smoke- developed classification are deter- mined. The results of conventional
tunnel tests on cables provide for a comparison of the fire performance of cables with those of common building materials. Severe fire expo- sure in an enclosed space is simu- lated; however, no correlation has been developed between convention- al tunnel test results and the perform- ance of cables in real-world fires in plenums.
Underwriters' Laboratories Inc., in collaboration with Bell Laboratories, has developed a variant of the tunnel test method, designated UL 910, for assessing the fire performance of communication cables intended for use in ducts and plenums. In the test, cables are mounted in a single layer. side by side, on a cable tray supported at approximately mid-height in the tunnel chamber. The distance of flame-advance beyond the burner flame after a 20 minute exposure is the measure of cable flammability. and the optical density of the smoke in the exhaust duct is the measure of smoke-producing propensity.
Comparisons of tests
Underwriters' Laboratories Inc. conducted tunnel tests in accordance
with U L 910 to compare the flame
spread characteristics of cables in these tests with their behavior in the simulated plenum tests cited earlier.
A correlation was found between the
distance of flame propagation in the tunnel and in the simulated ~ l e n u m , although conditions in the plenum test did not accentuate the difference between various cables to the same degree as in tunnel tests.
A study was conducted by the NBC task group to compare cable behavior in the vertical cable-tray test to its behavior in the floor-mounted tunnel test, CAN4-S102.2. InTable I1 the flame-spread classifications (FSC) of three cables measured in the tunnel test are compared with the length of charring of the cables measured in the Ontario Hvdro test. It is known that some cables designed to be fire resistant behave better than the cables listed.
Future directions
Regulatory authorities are looking seriously at methods whereby cables without raceways can be installed in concealed spaces, primarily plenums. In the U.S. and in some Canadian
Table II. Comparison of FSC with length of charring (Ontario Hydro test). FSC Length of char (mm) Cable A 86 450 Cable B 106 560 Cable C 177 >2100
provinces, the regulatory authorities have accepted cables that can meet specific limits when tested in confor- mance with UL 910. These limits are more stringent than those in the Ontario Hydro vertical cable-tray test and, unlike the Ontario Hydro test, include smoke-production limits.
The NBC task group on electrical cables has recommended that there be some limits on the flammability of electrical cables in all locations in buildings of noncombustible con- struction, but that the limits not be as stringent as those specified for use with U L 910. A vertical cable-tray test method was considered to be an appropriate means of eliminating potential "bad actors". The Ontario Hydro vertical cable-tray test meth- od, with a vertical-char distance limit of 1.2 m, was chosen.
The task group also suggested that there is no need to impose smoke- production limits on cables, since the
presently-required safeguards for
recirculation-fan shutdown and for smoke control systems are intended to minimize smoke movement, whether the smoke is produced by cables or by other materials in the building. The task group recognized that under fire test conditions, large quantities of smoke may be produced but concluded that the smoke pro- duced by electrical cables in real- world fires would probably not be significant in comparison to smoke produced by other building materials or furnishings.
The NBC proposals were circu- lated for public review and comment and the proposals were accepted, with slight modifications, in May
1985. The CEC Part I Committee has
examined the changes reconimended by the NBC committees and these
will be incorporated in the CEC Part I
1986 edition and the CSA product
T h i s p a p e r , w h i l e b e i n g d i s t r i b u t e d i n r e p r i n t form by t h e D i v i s i o n of B u i l d i n g R e s e a r c h , remains t h e c o p y r i g h t of t h e o r i g i n a l p u b l i s h e r . It s h o u l d n o t be r e p r o d u c e d i n whole o r i n p a r t w i t h o u t t h e p e r m i s s i o n of t h e p u b l i s h e r . A l i s t of a l l p u b l i c a t i o n s a v a i l a b l e from t h e D i v i s i o n may be o b t a i n e d by w r i t i n g t o t h e P u b l i c a t i o n s S e c t i o n , D i v i s i o n of B u i l d i n g R e s e a r c h , N a t i o n a l R e s e a r c h C o u n c i l of C a n a d a , O t t a w a , O n t a r i o ,
