Predicting Toxic Hazards of Cables

7/30/2019 Predicting Toxic Hazards of Cables

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F r e d e r i c k B . C l a r k e , 1 H e n r i v a n K u i j k , ~ a n d S h a r o n S t e e le ~

Predict ing the Toxic Hazard of Cable F i resR E F E R E N C E : Clarke , F . B. , van Kui jk , H. , and S tee le , S . , "P red ic t ing t h e T o x i c H a z a r d o fC a b l e F i r e s , " Characterization and Toxicity of Smoke, ASTM STP 1082, H. K . Hasegaua , Ed . ,A m er ic an Soc ie ty fo r Tes t in g and M ate r ia ls , Ph i lade lph ia , 1990 , pp . 46 -56 .A B S T R A C r : Des p i te a g row ing in te res t in cab le f ires , no sys temat ic eva lua t ion o f the tox icth r e a t f r o m c a b l e s u n d e r a c tu a l f i r e c o n d i t i o n s h a s b e e n a t t e m p te d . T h e p r e s e n t p a p e r d e m -o n s t r a te s t h e m e th o d o lo g y u s e d b y B e n j a m in /C la r k e A s s o c ia t e s t o e s t im a te h a z a r d o f ca b l es m o k e . T h e m e th o d m a k e s u s e o f s m a l l- s c al e f ir e p r o p e r ty t e st s a n d c o m p u te r m o d e l i n g . Ce r -ta in o f the p red ic t ions ha ve been te s ted a t fu l l scale.T h e s m a l l- s c al e t e st s p r o v id e d a t a o n t h e r e s p o n s e o f t h e c a b l e s to a r a d i a ti v e h e a t e n v i r o n -m e n t , p r e d i c t e d b y th e m a th e m a t i c a l m o d e l . I n t h i s w o r k , t h e N I S T c o n e c a lo r im e te r p r o v id e dmass loss da ta an d th e L IFrF appara tus w as used to me asure f lam e sp read pa ramete rs . Al l cab lef ir e d a t a w e r e o b t a in e d o v e r a r a n g e o f im p o s e d f lu x. Be n c h - s ca l e m e a s u r e m e n t o f t o x icp o t e n c y o f c a b le s m o k e w a s o b t a in e d ; i n s o m e c a se s, f u ll -s c a le s m o k e p o t e n c y d a t a w e r eo b t a in e d a s w el l. T h e m o d e l in g w a s d o n e b y m o d i f y in g a r o o m f i re c o d e ( H a r v a r d V ) t o a l lo wfor cab le f ires . Th is in vo lv ed speci f ic ca lcu la t ion o f the inc ide n t f lux to subs t ra tes in the ho tupp er laye r o f a roo m f ire. On ce the f lux to a c ab le bed cou ld b e p red ic ted , i t was poss ib le toc a l c u l a te t h e r a t e o f s m o k e e v o lu t i o n f r o m th e c ab le s, m a k in g u s e o f t h e s m a l l- s ca l e t e s t d a tad e s c r ib e d a b o v e . I n a f u ll - sc a le t e s t o f t h e m e th o d , g o o d a g r e e m e n t b e tw e e n p r e d i c t e d a n dobserved mass loss was ob ta ined .

T h e m e th o d o lo g y w a s t h e n e x t e n d e d t o a s e r i es o f f o u r c a b le s i n tw o f ir e s c e n a rio s s im i l a r t ob u r n s t u di e s c a rr i e d o u t b y t h e U .S . N a v y . T h e p r e d i c te d r a n k in g s o f t h e c a b l es i n t h e se tw oscenar ios were com par ed wi th th e rank ings o f the cab les by : (1 ) a tox ic po tency te s t ; (2) re su l tso f a t o x i c p o t e n c y t e s t a d ju st e d b y t h e a m o u n t o f c a b le p r e s e n t i n e a c h c as e; a n d ( 3) a n e ws m o k e t o x i c i ty te s t, w h ic h m a k e s s o m e a l l o w a n c e f o r t h e f i r e p r o p e r ti e s o f t h e m a te ri a l. I t w a sfound tha t th e bench-sca le tox ic po tency m easu rem ents w ere no t good ind ica to rs o f ful l- sca lep e r f o r m a n c e .K E Y W O R D S : f i re haza rd assessment , f i re mode l ing , cone ca lo r imete r , f lame sp read , smoketox ic i ty

T h e p a s t f i v e y e a r s h a v e s e e n g r e a t l y i n c r e a s e d i n t e r e s t i n t h e f ir e h a z a r d s p o s e d b y e l e c-t r ic a l c ab l es . T h e d e c i s i o n o f N e w Y o r k S t a t e t o r e q u i r e l is t i n g o f w i r e a n d c a b l e p r o d u c t s b ys m o k e t o x i c i t y d a ta , c o u p l e d w i t h t h e d e v e l o p m e n t o f n e w i n s u l a t i o n a n d j a c k e t i n g m a t e r i -a ls , h a v e p r o m p t e d t h e f i re c o m m u n i t y t o e x a m i n e h o w m o d e m t o ol s o f f i re p r o te c t io n a n dh a z a r d a s s e s s m e n t c a n b e a p p l i e d s p e c i fi c a ll y t o c a bl e s. R e c e n t a p p l i c a t i o n s h a v e i n c l u d e da n a n a l y s i s o f t h e t h r e a t p o s e d b y p l e n u m c a b l e [ 1] , t h e b e h a v i o r o f c a b l e s i n a d e v e l o p i n gr o o m f ir e [ 2] , a n d , i n a s l i g ht l y d i f f e re n t v e i n , t h e h a z a r d p o s e d b y n o n m e t a l l i c t u b i n g u s e da s e l e c t ri c a l c o n d u i t [ 3] . A l l o f t h e s e a n a ly s e s h a v e , i n o n e s e n s e o r a n o t h e r , e m p l o y e d m o d -e r n f i re sc i en c e ; t w o h a v e u s e d m a t h e m a t i c a l r o o m f ir e m o d e l s . T h i s p a p e r w i l l p r o v i d e a nu p d a t e o n t h e c a p a b i li t ie s o f t h e m o d e l - b a s e d m e t h o d s o f p r e d i c ti n g t o x i c fi re h a z a r d f r o mc a b l e s .

P res iden t , Ben ja m in /C la rke Associates , Inc . , Kens ing ton , M D 20879 .

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C L AR K E ET AL . ON C ABL E F IR ES 4 7

A s th e c o m p u t a t i o n a l a p p r o a c h t o f ir e h a z a r d h a s a d v a n c e d , s o a l so h a s a la b o r a t o r y te s tf o r s m o k e t o x i c i t y h a z a r d . T h i s t e st , w h i c h is b e i n g d e v e l o p e d u n d e r t h e a u s p ic e s o f t h eN a t i o n a l I n s t i t u t e o f B u i l d i n g S c i e n c e s ( N I B S ) , i s c o n c e i v e d a s a p r e f e r r e d a l t e r n a t iv e t o t h es i m p l e m e a s u r e o f s m o k e ' s t o x i c p o t e n c y . I t ta k e s s o m e a c c o u n t o f a m a t e r i a l 's fire p r o p e r ti e si n t h e c o u r s e o f d e t e r m i n i n g s m o k e ' s e ff ec t o n t e st a n i m a l s .T h i s p a p e r h a s a t w o f o l d p u r p o s e : f irs t, t o p r o v i d e a n u p d a t e o n p r e d i c t i n g c a b l e t o x i ch a z a r d u s i n g a n u m e r i c a l a p p r o a c h ( la b o r a to r y te s t d a t a i n a n d i n c o m b i n a t i o n w i t h m a t h -e m a t i c a l f ir e m o d e l s ) ; a n d , s e c o n d , t o s h o w h o w l a b o r a t o r y t o x i c i t y t e st s , i.e ., t o x i c p o t e n c ya n d t h e n e w l y e s ta b l is h e d N I B S te s t, c o m p a r e w i t h t h e n u m e r i c a l a p p r o a c h i n th e i r a b il it yt o i d e n t if y t h e t o x i c h a z a r d .

T o x i c H a z a r d A s s e s s m e n t o f C a b l e sG e ne r a l

T h e t e r m " t o x i c h a z a r d " is u s e d i n th i s p a p e r t o m e a n t h e t h r e a t p o s e d b y th e p o t e n t i a ll e th a l e f fe c ts o ff e r e d b y s m o k e f r o m a f ire . T o x i c h a z a r d i s o n l y a p a r t o f fire h a z a r d , w h e r et h e t h r e a t s f r o m t e m p e r a t u r e , h i n d e r e d e s c a p e , a n d t h e l i k e a r e a l s o c o n s i d e r e d . T h e u s u a la p p r o a c h t o f ir e h a z a r d a s s e s s m e n t is t o i d e n t if y th e t h r e a t, w h e t h e r t h e r m a l o r f r o m s m o k e ,w h i c h i s f i r s t e n c o u n t e r e d . A s a g e n e r a l r u l e , p o t e n t i a l f i r e v i c t i m s l o c a t e d c l o s e t o t h e f i r ea r e t h r e a t e n e d f ir s t b y h e a t , i n w h i c h c a s e it m a t t e r s l i t tl e w h a t t h e d e t a i ls o f t h e s m o k e t o x -i c i ty m i g h t b e . W h e n t h e p o t e n t i a l v i c t i m s a r e s u f f ic i e n tl y fa r f r o m t h e f ir e, h o w e v e r , th e h e a ti s d i s si p a te d a n d t h e m a i n t h r e a t i s t h e e f fe c t o f t h e s m o k e . G e n e r a l ly , b o t h k i n d s o f h a z a r dc a n b e e v a l u a t e d w i t h n u m e r i c a l h a z a r d a s s e s sm e n t ; t h e t e c h n i q u e s w h i c h g i v e r is e t o e s ti -m a t e s o f th e r m a l h a z a r d c a n b e u s e d t o e s t im a t e t o x i c h a z a r d a s w e ll, p r o v i d e d s u i ta b l es m o k e d a t a a r e a t h a n d .

H a z a r d a s s e s s m e n t u s i n g a n a n a l y t ic a l a p p r o a c h c o n s i st s o f t h r e e s e p a r a te s t ep s :1. E valuat ing how the produc t o f in teres t behaves over a range o f therm al env i ronments.

T h e e n v i r o n m e n t i s u s u a l ly d e s c r ib e d i n t e r m s o f t h e t o t a l h e a t fl ux it im p o s e s o n t h e p r o d -u c t . R e c e n t l y , s e v e r a l i n s t r u m e n t s h a v e b e e n d e v e l o p e d w h i c h m e a s u r e m a t e r i a l f i r e p r o p -e r ti e s a s a f u n c t i o n o f h e a t fl u x. A t p r e s e n t, d a t a f r o m t w o i n s t r u m e n t s a r e re q u i r e d . E i t h e rt h e c o n e c a l o r i m e t e r [ 4 ], d e v e l o p e d a t t h e N a t i o n a l I n s t it u t e o f S t a n d a r d a n d T e c h n o l o g y( N I S T ) , o r t h e f a c t o r y m u t u a l f l a m m a b i l i ty a p p a r a t u s [ 5 ] a r e u s e d t o p r o v i d e m a s s lo s s r a te ,h e a t r e l e a s e r a t e , a n d s m o k e y i e l d . T h e l a t e r a l i g n i t i o n a n d f l a m m a b i l i t y t e s t e r ( L I F T ) [ 6 ] ,a l s o d e v e l o p e d a t N I S T , i s u s e d t o d e t e r m i n e t h e m i n i m u m f l u x f o r f l a m e s p r e a d , t h e f l u xi n t e rv a l o v e r w h i c h f la m e s p r e a d o c c u r s a t a f in i te r a te , a n d t h e d e p e n d e n c e o f s p r e a d r a teo n f l u x w i t h i n t h a t i n t e r v a l .2. A m e tho d i s fo un d to descr ibe the f lu x the produc t wi l l r ece ive in an ac tua l f ir e . W h e r es u i t a b l e f u l l - s c a l e d a t a a r e a v a i l a b l e , t h e y c a n b e u s e d . C o m p u t a t i o n a l a p p r o a c h e s , s u c h a sr o o m fire m o d e l s , a re , h o w e v e r, m o r e c o m m o n a n d h a v e b e e n d e v e l o p e d t o th e p o i n t w h e ret h e f lu x r e c e i v e d b y t h e w a l l s u rf a c e s , f lo o r s , a n d c e i li n g s c a n b e p r e d i c t e d i n a v a r i e t y o f fir es .W h e n p r o d u c t s u n d e r s t u d y a r e l o c a t e d i n t h e se l o c a t io n s , t h e f lu x t h e y r e c ei v e i n t h e f ir e ist h u s e s t i m a t e d , s o t h a t t h e a c t u a l m a s s l o ss a n d h e a t r e le a se r a t e o f th e p r o d u c t s c a n b ep r e d i c t e d u s i n g t h e d a t a o b t a i n e d i n t h e s m a l l - s c a l e t e s t s d e s c r i b e d a b o v e .3. Fo r es t imate s o f tox ic hazard , the ra te o f m ass los s f ro m the produc t, ca lcu la ted as par to f S tep 2 , i s com bine d w i th the tox ic po tency o f the smoke , which i s me asu red in a laboratorysmoke tox ic i t y t e s t . T h e r e s u l t is u s e d t o e s t i m a t e t h e b u i l d u p o f a l e t h a l ( o r i n c a p a c i t a t i n g )d o s e o f t h e s m o k e i n s o m e o n e e x p o s e d t o t h e f ire . T h i s i n f o r m a t i o n c a n b e u s e d t o d e s c ri b et o x i c c o n d i t i o n s i n t h e d e v e l o p i n g f ir e a n d t o c h o o s e p r o d u c t s w h i c h o f f e r t h e l o w e st to x i ch a z a r d .Copyright by ASTM Int'l (all rights reserved); Thu Apr 4 14:37:49 EDT 2013




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4 8 T O X IC IT Y O F S M O K EProducts are usually compared on the basis o f the time required for conditions to reach agiven level of thermal or toxic severity. The shorter the time required, the comparativelypoorer the product's performance.

F u l l - S c a le C o n f ir m a t i o nTo date, only predictions of mass loss have been checked by full-scale experiments. Ben-jamin has predicted the decomposition of electrical nonmetallic tubing in room fires, andthose predictions have beend found to be valid at full scale [3].Recently, a series of full-scale fire studies on fluoropolymer-insulated cables have beenconcluded at the University of Ghent in Belgium [ 7]. As part of this work, sizable amountsof cable in a burn room were exposed to wood crib fires with peak energy release rates around1 MW. From the heat release profile obtained from burning the crib alone, a modified Har-vard V room fire code [ 8 - 1 0 ] can predict the corresponding profile of heat flux on a cable

bed suspended in the hot layer. Making use of measured cable mass loss rate data obtainedfrom the cone calorimeter, the weight loss of cable in the tray could be predicted and com-pared with experiment. The results of the comparison are shown in Fig. 1. Agreementbetween theory and experiment was found to be very good, as was the case in the workpreviously reported by Benjamin. The real utility of these results is that accurate predictionof mass loss rates paves the way for assessment of toxic hazard since the smoke productionrate, in combination with the toxic potency, determines the effects on those exposed.A more difficult class of problem is one in which the fire is spreading on the cables. In thiscase, the area burning is changing with time, so the cables' mass loss rate is affected not onlyby the heat flux which reaches them but also by the amount of cable burning. Van Kuijk[ 10] has modified the Harvard V fire code to accommodate a simple flame spread algorithm.

2 5 . 0

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1 0 , 0 - ' " - \ FI I- S c a I e T e s tC a l c u l a t e d / ~ ' \ \ . ,l c u l a t e d / " \ ~ _5 . 0 ~ , l0 . 0 8 0 0 . 0 1 6 0 0 . 0

T i m e ( s )FIG. l--Weight loss o f cables in ful l-scale room f ire.

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C L A R K E E T A L . O N C A B L E F IR E S 49

F u r t h e r r e f in e m e n t s , b a s e d o n t h e f l a m e s p r e ad w o r k o f Q u i n t i e r e a n d H a r k l e r o a d [ 6 ], h a v er e c e n t ly b e e n i n c o r p o r a t e d . F o r p r e d i c t i o n s o f f l a m e s p r e a d, a n a p p a r a t u s l i k e L I F T m u s t b eu s e d t o d e s c r i b e f l a m e s p r e a d r a t e a s a f u n c t i o n o f f lu x .

S h ip b o a r d C a b le T o x i c H a z a r d A s s e s s m e n tM e t h o d o l o g y

I n 1 9 87 , t h e N a v a l R e s e a r c h L a b o r a t o r y c a r r i e d o u t a s e ri e s o f f u ll - s c al e s h i p b o a r d f i ret e s t s d e s i g n e d t o e v a l u a t e t h e e f fe c t o f n e w s h i p b o a r d c a b l e s p e c i f ic a t i o n s [ I I ] . T h e s a m es h i p b o a r d e n v i r o n m e n t w a s u s e d a s a s e tt i n g t o e x a m i n e t h e f i r e h a z a r d o f f e r ed b y a v a r i e t yo f s h i p b o a r d c a b l e d e s ig n s . N o f u l l -s c a l e e x p e r i m e n t s w e r e d o n e ; r a t h e r a n a n a l y t i c a l h a z a r da s s e s sm e n t o f c a b l e s i n t h e s h i p b o a r d e n v i r o n m e n t w a s c o n d u c t e d . A p o r t i o n o f t h e w o r k i sp r e s e n t e d h e r e a s a n i l l u s t r a t i o n o f t h e u t i l i t y o f t h e a p p r o a c h a s a t o o l f o r e v a l u a t i n g c a b l ep e r f o r m a n c e i n r e a l fi re s i t u a t i o n s a n d t o p r o v i d e a b e n c h m a r k f o r c o m p a r i s o n w i t h o t h e rs u g g e s te d m e t h o d s o f a s s e ss i n g t o x i c h a z a r d .

T h e b a s i c f ir e m o d e l u s e d w a s t h e H a r v a r d F i r e C o d e ( V e r s io n 5 .2 ) d es i g n e d f o r n o r m a lb u i l di n g s . A s o r i g i n a ll y d e s ig n e d , i t f a il e d t o p r o v i d e s o l u ti o n s f o r a s h i p b o a r d c o m p a r t m e n th a v i n g s t ee l w a l ls : n o r m a l b u i l d i n g w a l ls c o n d u c t f a r le ss h e a t f r o m t h e r o o m . T h i s p r o b l e mw a s s o l v e d b y u s i n g a m o r e r o b u s t a l g o r i t h m f o r so l v i n g t h e h e a t t r a n s f e r e q u a ti o n s i n t h em o d e l . G o o d a g r e e m e n t w i t h e x p e r i m e n t a l w a s f o u n d w h e n t h e m o d i f i e d c o d e re s u lt s w e r eu s e d t o r e p r o d u c e t h e t e m p e r a t u r e p r o f il e f o r a b l a n k t e s t in w h i c h n o c a b l es w e r e b u r n e d .

T h e t e s t c o m p a r t m e n t , F i g . 2 , w a s 5 .2 b y 2 . 8 b y 2 . 4 m h i g h ; t h e w a l l s w e r e s te e l. T h ed o o r w a y o p e n i n g w a s 1 .4 b y 1 .8 3 m h i g h w i t h a s i ll o f 0. 1 8 m . T h e s o u r c e f ir e w a s a 2 b y 2 -f i p a n o f e th a n o l . A c a b l e t r a y s u s p e n d e d 0 . 2 m f r o m t h e c e il in g r a n t h e l e n gt h o f t h e r o o m .F o r p u r p o s e s o f a ss e ss in g t o x i c h a z a r d , i t w a s a s s u m e d t h a t t h e d o o r o p e n e d t o a c o r r i d o rw h i c h l e d t o a r e m o t e r o o m ( F ig . 3 ). T h e d i m e n s i o n s o f t h e r e m o t e r o o m w e r e t h e s a m e a st h e b u r n r o o m .

F o u r d i f f e r e n t t y p e s o f c a b l e w e r e e v a l u a t e d i n e a c h o f t w o s c e n a r i o s . I n S c e n a r i o 1, a 7 5 0 -k W f ir e w a s l o c a t e d 1 m a w a y f r o m t h e c a b l e s; i n S c e n a r i o 2, t h e f i re w a s 1 0 00 k W a n d 2 ma w a y . T h e f ir st 0 .1 m ( 4 i n .) o f t h e c a b l e s w e r e a s s u m e d t o b e i g n i t e d b y t h e f ir e a t t h e s t a r to f th e b u r n . T h e m o d e l e d c a b l e c o n f ig u r a t i o n w as a t r a y c o n t a i n i n g 8 2 c a b le s a r r a n g e d t w ol a y e r s d e e p . E a c h o f th e f o u r c a b l e t y p e s w a s e v a l u a t e d i n b o t h s c e n a r i o s . T h e m a s s lo s s r a t es ,f l a m e p r o g r e s s i o n o n t h e c a bl e s, v i s ib i li ty , t o x i ci t y , a n d t e m p e r a t u r e i n t h e r o o m o f o r ig i na n d r e m o t e r o o m w e r e c a l c u l a t e d f o r e a c h c a b l e i n e a c h s c e n a r i o u s i n g it s m e a s u r e d p r o p -e r ti e s i n c o n j u n c t i o n w i t h t h e m o d i f i e d m o d e l .

P r o p e r t i e s o f t h e f o u r d i f f e r e n t e x p e r i m e n t a l e l e c t r i c a l c a b l e s e v a l u a t e d i n t h i s a s s e s s m e n ta r e l i st e d i n T a b l e 1. T h e h e a t o f g a si f ic a ti o n a n d t h e h e a t o f c o m b u s t i o n w e r e m e a s u r e d o nt h e N I S T c o n e c a l o r i m e t e r o v e r a r a n g e o f e x t e r n a l h e a t fl ux f r o m 15 t o 5 0 k W / m 2.

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5/11

5 0 T O X I C I T Y O F S M O K E

r e m o t e 1' x ) rO O m ,

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The minimum flame spread flux, ~/~,, was measured using the lateral ignition and flametravel (LIFT) apparatus. The minimum ignition flux i/'f was determined in the flame spreadand ignition modes of the L IFr apparatus, and also using the cone calorimeter. The resultsof the three methods were averaged to yield the final value. For Cable D, the ignition fluxvalue was obtained using only the LIFT apparatus in the flame spread mode. The combus-tible content of the cables was measured by irradiation to constant weight in the cone calo-rimeter at 50 kW/m 1.Toxic potency of cable smoke was determined using the NIST toxicity test [12] modifiedto accept a radiant heat source as suggested by Packham [13]. LCs0s for each cable, deter-mined at an imposed flux of 50 kW/m 2, are for a 30-min animal exposure followed by a 14-

day postexposure observation period. The lethal dose, L(Ct)s0 (shown in Table 1), is takento be the product of the measured LC50 and the 30-min exposure period.In addition, the NIST test apparatus was operated in a mode analogous to that prescribedby the newly developed NIBS test [14]. In the NIBS procedure, the sample is irradiated witha fixed heat flux (here 50 kW/mZ), but instead of varying the amount of sample needed toproduce a lethal atmosphere, the t i m e necessary for a fixed sample area to generate a lethalatmosphere is measured and reported. This parameter, the irradiation time needed to pro-duce enough smoke to cause death in 50% of the animals --the so-called ITs0--was recorded.The most important assumptions in the assessment were the following:1. Smoke movement occurred in only one direction, that is, from the burn room into thecorridor and the remote room. No smoke flow from the corridor back into the burn room

was permitted.2. Mass loss was experienced only by the part of the cable which was burning. Mass lossof the cables began when i/r, was reached, regardless of how long the cables had soaked inthe hot layer.3. Heat of gasification and heat of combustion of the cables were independent of imposedflux. This is probably the weakest assumption, since it is strictly true only when the chemistryof decomposition and combustion is invariant over the flux range studied.

R e s u l t sThe buildup o f cable smoke in the remote room for Scenarios 1 and 2 are shown in Figs.4 and 5, respectively. The vertical axis is the fraction of"tolerable" smoke dose--"tolerable"being defined here as one quarter of the lethal dose. When the fraction of tolerable doseapproaches unity, one may expect smoke toxicity to be important in inhibiting escape from





6/11

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CLARKE ET AL. ON CABLE FIRES 53

t h e s m o k e . ( N o t s h o w n i s t h e b u i l d u p o f t o x i c it y f ro m t h e e x p o s u r e f ir e a l o n e , w h i c h , i nt h e s e s c e n a r i o s , w a s n e g l i g i b l e . )

T h e t h r e a t f r o m s m o k e t o x i c i t y w a s n o t f o u n d t o b e a n i m p o r t a n t f a c t o r o v e r t h e 15 m i ns t u d ie d : t h e t o l e r a b le d o s e w a s n o t a p p r o a c h e d b y a n y o f t h e m o d e l e d c a b l e f ir es . C a b l e Bp e r f o r m e d t h e b e s t i n b o t h s c e n a r i o s . T h i s i s d u e t o t h e f a c t t h a t i t s f l a m e - s p r e a d a n d p y r o l -y s is p r o p e r t i e s w e r e q u i t e g o o d , s o t h a t w h i l e t h e f ir e w a s s p r e a d i n g t h e a r e a a f l a m e w a s l e s st h a n s o m e o f t h e o t h e r s . E v e n w h e r e t h e c a b l e w a s f u l l y a f l a m e , i t s h i g h h e a t o f g as i f ic a t i o n( A H v) m e a n t t h a t t h e m a s s l o s s r a t e w a s r e l a t i v e l y l o w . F i n a l l y , t h e s m o k e , o n c e p r o d u c e d ,w a s , b y t h e t e s t u s e d , s o m e w h a t l e ss t o x i c .

T h e d i f fe r e n t f ir e c o n d i t i o n s i n t h e t w o s c e n a ri o s c h a n g e d t h e o r d e r o f c a b le p e r f o r m a n c ei n t h e t w o s c e n a ri o s . C a b l e D , w h i c h h a d t h e b e s t f l a m e - s p r e a d p r o p e r ti e s , b u r n e d c o m p a r -a t i v el y l es s i n S c e n a r i o 2 t h a n t h e o t h e r c a b l e s, th u s r e d u c i n g t h e a m o u n t o f sm o k e p r o d u c e da n d i m p r o v i n g i t s r e l a ti v e p e r f o r m a n c e .

Com par ison of Met hod s to Pred ic t To xic HazardN u m e r i c a l l y b a s e d h a z a r d a s s e ss m e n t m a k e s u s e o f d a t a f r o m l a b o r a t o r y t e st s a n d m a t h -

e m a t i c a l r o o m f ir e m o d e l s t o p r e d i c t , i n t h i s ca s e, t h e o r d e r o f t o x i c h a z a r d o f a s er i e s o fc a b le s . H o w e v e r , i n t h e c o u r s e o f th i s w o r k , t h r e e l e s s s o p h i s t i c a t e d p r e d i c t o r s o f c a b l e p e r -f o r m a n c e h a v e a l r e a d y b e e n e n c o u n t e r e d :

Predictor/--First a m o n g t h e se i s t h e t o x i c p o t e n c y m e a s u r e m e n t w h i c h w a s u se d in p a r to f t h e h a z a r d a s s es s m e n t . T o x i c p o t e n c y d a t a s u c h a s t h e s e a r e s o m e t i m e s u s e d b y t h e m s e l v e si n a n a t t e m p t t o e s t i m a t e t h e r e l a t i v e t o x i c h a z a r d o f a s e t o f m a t e r i a ls .

Predictor 2 - - I n t h e s c e n a r i o s u n d e r d i s c u s si o n , t h e s a m e n u m b e r o f c a b l es w e r e u se d ine v e r y c a se , b u t o w i n g t o d i f f e re n c e s i n c o n s t r u c t i o n t h e w e i g h t o f e a c h c a b l e in t h e r o o m w a sd i ff e re n t. T h e r e f o r e o n e c a n c o n s t r u c t a n e s t i m a t e o f t o x i c h a z a r d b a s e d o n t h e a m o u n t o fc a b l e u s e d d i v i d e d b y t h e t o x i c p o t e n c y o f t h e s m o k e f r o m t h a t c a b l e. T h i s w o u l d c o n s t it u t ea s o m e w h a t m o r e s o p h i s t ic a t e d e s t i m a t e o f h a z a r d t h a n t h e u s e o f s i m p l e to x i c p o t e n c y d a t aa l o n e .

Predictor 3--A t h i r d e s t i m a t e o f t o x i c h a z a r d i s a f f o r d ed b y t h e d a t a u s i n g t h e m o d i f i e dN I B S a p p r o a c h t o t o x i c it y , t h e I Ts0 m e a s u r e m e n t . B e c a u s e t h e r a d i a t i o n t i m e n e e d e d t o p r o -d u c e a t o x i c l e v e l o f s m o k e i m p l i c i t l y r e f le c t s t h e e a s e w i t h w h i c h t h e m a t e r i a l s a re i g n i t e da n d t h e r a t e a t w h i c h t h e y p r o d u c e s m o k e o n c e i g n i te d , t h e I Tso i s a r u d i m e n t a r y a t t e m p t t oi n c o r p o r a t e s o m e n o t i o n o f a m a t e r i a l ' s b u r n i n g b e h a v i o r i n t o a n e s t i m a t e o f i ts t o x i c h a z a r d .

F i n a l l y , a s a b a s is f o r c o m p a r i s o n w i t h t h e s e t h r e e m e t h o d s , w e h a v e P r e d i c t o r 4 , w h i chg i v es t h e r e s u l ts o f t h e h a z a r d a s s e s s m e n t d i s c u s s e d i n t h e p r e v i o u s s e c t i o n .

T h e r e su l ts o f t h e t o x i c p o t e n c y m e a s u r e m e n t s , t h e b a s i s o f P r e d i c t i o n 1, a r e s h o w n i nT a b l e 2 . A l s o s h o w n i s t h e r a n g e o f t o x i c p o t e n c y m e a s u r e m e n t s f o u n d f o r e a c h d e t e r m i -

T A B L E 2--Prediction 1: Toxic hazard based on toxic potency.Cable LCso, m g/L a Range

A 36 15-57B 67 63-75C 72 33-76D 29 23-37Resu lt : C ( - - B) > D (= A) (> = be t te r than) .a NB S B o x - -P a c k h a m h e a tin g sy stem.





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54 T O X IG IT Y O F S M O K E

T A B L E 3--Prediction 2." Toxic hazard based on cable weight.T o ta l L e n g th (m)o f C a b le inCable LCs0 Ro om a kg /m Mass Used, kg H b

A 36 426 0,291 124 3.4B 67 426 0.248 106 1.6C 72 426 0.558 238 3.3D 29 426 0.204 87 3.0?Result: B > D > A ( = C).a 82 pieces, each 5.2 m long.b Hazard ( / / ) = Cab le mass used /LCso .

n a t i o n . C a b l e s B a n d C s h a r e e s s e n t ia l l y t h e s a m e L C s0 , a s d o C a b l e s A a n d D . T h u s , i f o n ew e r e b a s in g a h a z a r d p r e d i c t i o n o n t h e t o x i c p o t e n c y , o n e w o u l d c o n c l u d e t h a t C a n d B a r ee s s e n t i a l l y e q u i v a l e n t a n d o f fe r a b e t t e r c h o i c e t h a n d o C a b l e s D a n d A , w h i c h a r e a l s oe q u i v a l e n t .

I f t h e t o x i c p o t e n c y n u m b e r s a r e w e i g h t e d b a s e d o n t h e a m o u n t o f c a b l e a c t u a ll y u s e d i ne a c h c a s e, t h e r e s u l t s c o n s t i t u t e P r e d i c t i o n 2 , s h o w n i n T a b l e 3 . T h e h a z a r d p a r a m e t e r , H ,i s s i m p l y t h e q u o t i e n t o f t h e m a s s u s e d i n e a c h c a s e d i v i d e d b y t h e r e s p e c t i v e L C~ 0. O n t h i sb a s is , o n e w o u l d c o n c l u d e t h a t B i s s u p e r i o r t o t h e o t h e r t h r e e , w h i c h a r e n e a r l y e q u i v a l e n t.

T h e e f fe c t o f g o i n g f r o m t h e s i m p l e s t p r e d i c t i v e m e t h o d , i .e ., t o x i c p o t e n c y , t o o n e w h i c ht a k e s a c c o u n t o f t h e a m o u n t o f c a b le p r e s e n t w o u l d d e f i n i te l y i n f lu e n c e w h a t c a b l e w a s c h o -s e n. A l t h o u g h t h e f ir st m e t h o d o f p r e d i c t i o n w o u l d s h o w n o d i ff e re n c e b e t w e e n C a b l e B a n dC , t a k i n g a c c o u n t o f c a b l e w e i gh t w o u l d d e f in i te l y d i s t in g u i s h b e tw e e n t h e t w o .

T h e p r e d i c t i o n o f a N I B S - t y p e t e st p a r a m e t e r i s s h o w n i n T a b l e 4. U s i n g t h i s d e t e r m i n a n t ,B o f f e r s t h e l o w e s t t o x i c h a z a r d ( i . e ., t h e l o n g e s t IT s0 ) f o l l o w e d b y C a b l e s C a n d D , w h i c h a r ee s s e n t i a l l y e q u i v a l e n t b a s e d o n t h e o v e r l a p o f t h e r a n g e s o f t h e I T s0 . F i n a l l y , C a b l e A i s p r e -d i c t e d t o b e t h e p o o r e s t o f t h e f o u r.

T h e r e s u l t s o f t h e h a z a r d a s s e s s m e n t c a r r ie d o u t o n t h e p r e v i o u s s e c t io n a r e p r e s e n t e d a sP r e d i c t i o n 4 , T a b l e 5 . T h e o r d e r o f p e r f o r m a n c e v a r ie s s o m e w h a t b e t w e e n S c e n a ri o 1 a n dS c e n a r i o 2 . I n b o t h c a se s , h o w e v e r , C a b l e B i s s h o w n t o b e s u p e r i o r . T h e s a m e r e s u l t w a ss h o w n b y P r e d i c t i o n s 2 a n d 3 , b u t w a s n o t d i s c l o s ed b y t h e t o x ic p o t e n c y m e t h o d a l o n e .

T a b l e 6 t a b u l a t es t h e p r e d i c t e d o r d e r o f c a b le t o x ic h a z a r d f o r e a ch o f th e p r e d i c t io n t e c h -n i q u e s u s e d . P r e d i c t i o n s b a se d o n t o x i c p o t e n c y ( ! a n d 2 ) f ai l t o p r e d i c t t h e o r d e r o f c a b leh a z a r d p e r f o r m a n c e i n e i t h e r o f t h e t w o s c e n ar i o s a n a l y z e d . T h e o r d e r o f c a b l e h a z a r d p e r -f o r m a n c e i n S c e n a r i o 2 i s t h e s a m e a s p r e d i c t e d b y t h e N I B S t es t.

A l t h o u g h to x i c p o t e n c y m e a s u r e m e n t s h a v e b e e n e m b r a c e d b y s o m e a s a n i m p r o v e m e n ti n o u r a b i l i t y t o p r e d i c t t o x i c h a z a rd , t h e w o r k p r e s e n t e d h e r e s h o u l d s h o w t h a t s u c h m e a -

T A B L E 4--Prediction 3: Toxic hazard based on modified "NIBS" test.Cab le ITs0, ra in a Range

A 2,1 0 .9-2 .9B 4.6 4 .0-5 .6C 3.7 3.4-4.1D 4.0 3 .0-4 .6Result: B > D ( = C ) > A .a Equ al lengths (0 .09 m) of cable; the presen t NIBS tes t calls for a f ixed area of exposed sample .





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C L A R K E E T A L . O N C A B L E F IR E S 5 5

T A B L E 5--P rediction 4: Toxic haza rd based on analytical assessment. (Fraction o f Tolerance Do seRea ched After 900 s .)Cable Scenar io 1 Scenar io 2

A 0.30 0.18B 0.15 0.10C 0.26 0.14D 0.35 0.14?R e s u l t : B > C > A > D B > C ( = D ) > A .

T A B L E 6--Pred ic t ion o f tox ic hazard.Too l Pred ic ted Order

Be t te r Pred ic t ions 1. Toxic pote ncy2. Pote ncy by weight3 . "N IBS " te s t4 . An alytica l haza rd assessment

(C o r B) > (A o r D)B > ( D o r A o r C )B > ( C o r D ) > AScenario 1: B > (A or C) > DScenario 2: B > (C or D) > A

s u r e m e n t s h a v e d e f i n i t e l i m i t a t i o n s . S i n c e t h e c o m m o n t o x i c i t y t e st s ( e. g. , t h e N I S T t e s t [12]a n d t h e U P I T T t e s t [ 1 5] ) d e c o m p o s e t h e s a m p l e w i t h a n e x t e r n a l h e a t s o u rc e , r a t h e r t h a na l l o w i t t o b u r n o n i t s o w n , o n e w o u l d e x p e c t s u c h t e s t s t o p r e d i c t h a z a r d b e s t f o r a s e v e r ef ir e, w h e r e a n y d i f f e r e n c e s a m o n g p r o d u c t s i n i g n i t a b i l i t y a n d f l a m e s p r e a d a r e o v e r p o w e r e db y t h e i n t e n s i ty o f t h e e x p o s u r e f ir e. P u t a n o t h e r w a y , i f t o x i c p o t e n c y i s a v a l i d m e a s u r e o fh a z a r d , t h e n u m e r i c a l h a z a r d a s s e s s m e n t o f th e f o u r c a b l e s i n p r o g r e s s i v e l y l a r g e r f ir e s w o u l dm o v e t h e p e r f o r m a n c e o f C a b le s C a n d B c l o se r t o g e th e r , a s i t w o u l d C a b l e A a n d C a b l e D .

O n e w o u l d n o t e x p e c t to x i c p o t e n c y a l o n e t o b e t h e t r u e i n d i c a t o r o f h a z a r d , h o w e v er ,s i m p l y b e c a u s e t h e r e a r e d i ff e r en t a m o u n t s o f e ac h o f t h e f o u r c a b l es i n t h e r o o m . T h i s i sa d d r e s s e d b y P r e d i c t i o n 2 , w h e r e t o x i c p o t e n c y i s c o r r e c t e d f o r th e w e i g h t o f t h e c a b l e s p r e s -e n t . I n s u f f ic i e n t ly l a r g e fi re s, o n e w o u l d e x p e c t P r e d i c t i o n 2 t o m i r r o r t h e o r d e r o f p e r f o r -m a n c e d e t e r m i n e d b y h a z a r d a s s es sm e n t .

C o n v e r s e l y , t h e IT s0 m e a s u r e d b y t h e N I B S t e s t d i r e c t l y r e f le c t s t h e t i m e r e q u i r e d f o r i g n i-t i o n a n d s m o k e p r o d u c t i o n . E v e n t h o u g h t h e r a d i a n t e x p o s u r e o f t h e s a m p l e is r e l at i ve l ys e v e re , i t i s n o t a f a c t o r u n t i l a f te r t h e s a m p l e h a s b e e n i g n i t e d . H e n c e , o n e w o u l d i n t u i t i v e l ye x p e c t t h e N I B S t e s t t o b e a b e t t e r i n d i c a t o r o f t r u e p e r f o r m a n c e u n d e r l e ss s e v er e f ir e c o n -d i t io n s , a n d t h i s p e r h a p s i s w h y i t c o r r e c t l y p r e d i c t s t h e o r d e r o f p e r f o r m a n c e i n S c e n a r i o 2 ,w h i c h i s a l e s s s e v e r e f i r e e x p o s u r e .

H o w e v e r w e l l a s i n g le l a b o r a t o r y t e st b y i t s e l f c a n f o r e s ee o n e s e t o f fi re c o n d i t i o n s , i t c a no n l y f o r e se e t h a t s e t. H e n c e , a h a z a r d a s s e s sm e n t , b a s e d o n w h a t e v e r f ir e c o n d i t i o n s a r e e n v i -s i o n e d f o r a p a r t i c u l a r u se , r e m a i n s t h e m o s t r e l ia b l e m e t h o d o f d e t e r m i n i n g r e l a ti v e c a b l ep e r f o r m a n c e . T h i s i s t r u e w h e t h e r i t b e d o n e b y m a t h e m a t i c a l m o d e l l i n g a n d s m a l l -s c a let e s t i n g o r b y f u l l -s c a l e f ir e e x p e r i m e n t . T h e o t h e r a d v a n t a g e o f c o n d i t i o n - b a s e d h a z a r d a s se s s-m e n t i s t h a t i t p r o v i d e s a n e s t i m a t e o f t h e a b s o l u t e i m p o r t a n c e o f a m a t e r i a l ' s c o n t r i b u t i o nt o t h e o v e r a l l f ir e . H e r e , f o r e x a m p l e , n o n e o f t h e c a b l e s sh o w e d s m o k e t o x i c i t y t o b e a s ig -n i f i ca n t c o n t r i b u t o r t o i m m e d i a t e f ir e h a z a r d i n t h e s c e n a r i o s e x a m i n e d , a f a ct w h i ch w o u l dn o t h a v e b e e n u n c o v e r e d u s i ng a n y o f t h e o t h e r t e c h n iq u e s . T h i s a l lo w s o n e t o d i s ti n g u is hb e t w e e n s c e n a r io s w h e r e a d e t a i l e d c o m p a r i s o n o f a p r o d u c t ' s s m o k e t o x i c i t y i s u s ef u l a n dw h e r e s u c h a c o m p a r i s o n w o u l d h a v e r e l a ti v e l y l i tt l e i m p a c t o n p r o d u c t s e l ec t io n .





11/11

56 TOXICITYOF SMOKE

A c k n o w l e d g m e n tThe cone calorimetry and LIFT measureme nts were done by the Fire Technology Center

Polymer Products Department, E. I. du Pont de Nemour s & Company, under the directionof Drs. Al mu t Breazeale and John Ryan. Th e smo ke toxicity measurements were carried outat D uPo nt' s Haskell Laboratory under the direction of Dr. Rud olph Valentine.

Re f e r e n c e s[1] Bukowski, R., "Toxic Hazard Evaluation of Plenum Cables," Fire Technology, Vol. 21, No. 4,1986, p. 252.[2] Clarke, F., Benjamin, I., and DiNenno, P., "Overall Fire Safety of Wire and Cable Materials,"Proceed ings o f the 32nd ln terna tiona l Wire and Cable Sym pos ium, 1983, p. 390.[3] Benjamin, I., "Toxic Hazard Analysis: Electrical Non-Metallic Tubing," Journal F ire Science, Vol.5, No. 1, 1987, p. 25.[4] Barauskas, V., "Development of the Cone Calorimeter--a Bench-scale Heat Release ApparatusBased on Oxygen Consumption," Fire a nd Materials, Vol. 8, 1984, p. 81.[5] Tewarson, A., "Experimental Evaluation of Flammability Parameters of Polymeric Materials,"Chap. 3, in Fl am e Retardant Polymeric Materials, Vol. 3, M. Lewin, S. M. Atlas, and E. M. Pearce,Eds., Plenum Press, New York, 1982, p. 97.[6] Quintiere, J., "A Simplified Theory for Generalizing Results from a Radiant Panel Flame Spread9Apparatus," Fire a nd Materials, Vol. 6, 1982, p. 52.[7] Clarke, F. B., et al., "Full-Scale Fire Experiments on Cables with Fluoropolymer Insulation,"abstract of presentation at NBS-CFR annual conference, preprint for combined meeting, Com-bustion Institute~Eastern States Section, Chemical and Physical Processes in Combustion, 20thFal l Techn ica l M ee t ing and NB S/Ce nter for F ire Research , annual conference on fire research,combined technical meeting, 2-5 Nov. t987, Gaithersburg, MD.[8] Emmons, H., "Prediction of Fire in Buildings," 17th Sym posiu m on Combustion, The CombustionInstitute, Pittsburgh, PA, 1979, p. 1101.[9] Mitler, H., "Documentation of CFC-V (The Harvard Fire Code)," NBS-GCR-87-344, NationalBureau of Standards, Gaithersburg, MD, 1987.[10] van Kuijk, H., "Modelling of Cable Fires," Fi re Safet y Journal, in press.[11] "Full Scale Cable Fire Tests at U.S. Coast Guard Fire and Safety Detachment, Mobile, Alabama,"360/NRL Prob: 61-2296-0-5, Ser 6180-699, Naval Research Laboratory, Washington, DC, Sep-tember 1985.[12] Levin, B., et al., "Further Development of a Test Method for the Assessment of Acute InhalationToxicity of Combustion Products," NBSIR 87-2532, National Bureau of Standards, Gaithersburg,MD, June 1987.[13] Packham, S. and Alexeeff, G., "Use of a Radiant Furnace to Evaluate Acute Toxicity of Smoke,"Journa l Fire Science, Vol. 2, 1984, p. 306.[14] Roux, H., "The NIBS SMOTOX WG Program," Proceedings o f the 37th International W ire andC a b le S y mp o s iu m, 1988, p. 543.[15] Alarie, Y. and Anderson, R., "Toxicologic Classification of Thermal Decomposition Products ofSynthetic and Natural Polymers," Toxicology and Applied Pharmacology, Vol. 57, 1981, p. 18 I.




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Predicting Toxic Hazards of Cables