CAN THE CONE CALORIMETER BE USED TO PREDICT FULL SCALE HEAT AND SMOKE RELEASE CABLE TRAY RESULTS FROM A
FULL SCALE TEST PROTOCOL?
Marcelo M. Hirschler GBH International, USA
ABSTRACT
The results of three projects were combined to attempt to obtain simple correlations between cone calorimeter test results (at an incident heat flux of 50 kW/m
2) and the modified IEC 60332-3 cable tray test results. The tools used for attempting these correlations were: (a) the equations developed to predict flashover from burning wall linings in a room-corner test, with the same cone calorimeter data, (b) the correlations obtained from a different cable tray test, with cone data at a lower incident heat flux and (c) classification into different categories. The analysis in this work indicates that, although the cone calorimeter is an adequate tool to predict the vast majority of the cable tray heat and smoke release results, correlations which give full details are not a simple matter, and require low incident heat fluxes. However, it is important to note that the cone calorimeter is able to avoid unsafe predictions in > 90% of the cables studied.
INTRODUCTION
Limits have been developed, in specifications and regulations, for heat release rate of individual products in full-scale tests, the most frequent products being interior finish or items of upholstered furniture or mattress. That concept is now likely to start reaching the world of electrical or optical cables. Some of the criteria being used are based on fire hazard assessment and fire modeling, while others are based solely on expert judgment. This work investigates the concept of self-propagating fire, often ignored when setting arbitrary limits. It does that by comparing some results obtained using a small scale heat release technique, the cone calorimeter, and its efficacy in predicting actual full scale test results, mainly in terms of fire hazard, rather than in terms of the satisfaction of individual test requirements.
A recent European Community research project, the Fire Performance of Electrical Cables (FIPEC) project
1, developed a pair of new full scale cable tray fire tests, based on modifications of the IEC 60332-3
2test: scenarios 1 and 2. The authors, who came from laboratories in four different European countries (England, Sweden, Belgium and Italy), tested 43 cables, all of them in "full scale scenario 1" (which is very similar to the standard IEC 60332-3 test), and most of them also were also tested in the more severe "full scale scenario 2" and in the cone calorimeter
3
, at three incident heat fluxes (35, 50 and 75 kW/m
2). The results have been discussed
extensively and incorporated into a book. Cables tested can be subdivided: those that cause self-
propagating fires (considered here as those cables that burn all the way) and those that do not.
More recently, a screening tool equation was developed to predict full scale room-corner test results, in terms of flashover (an extreme case of a self-propagating fire), heat release and smoke release from cone calorimeter test results
4. The full scale tests were conducted by North American laboratories on interior finish materials using either the North American NFPA 265
5or NFPA 286
6room-corner protocols (very similar to each other), and the cone calorimeter data was obtained at 50 kW/m
2. Equations were developed which were used for that prediction.
Earlier, a number of cables were assessed
7using a different full scale cable tray fire test, the North American ASTM D5424/D5537, in the CSA FT4 protocol
8. Preliminary predictions were also made of the full scale cable tray test results using other cone calorimeter test data. In this case, the results were found to be dependent on whether the cable caused a self-propagating fire or whether the cable stopped burning on its own: a critical peak rate of heat release was found in the cone calorimeter to separate the self-propagating cables from the safer ones.
The present work combines results the three projects. It applies both the concept of the new screening tool equation and of the more traditional critical peak rate of heat release to the new European and to the older North American cable fire test data.
EXPERIMENTAL
In the FIPEC project, 43 cables were tested using the cone calorimeter, at incident heat fluxes of 35, 50 and 75 kW/m
2, with all cables tested in the horizontal orientation, with an edge frame and wire grid, and with the cable ends sealed, as recommended by cone calorimeter cable test protocols (e.g. ASTM D 6113
9). All cone calorimeter properties were measured. One rate of heat release curve as a function of time is shown in the first six attached Figures (1-6). The Figures are subdivided as a function of the peak rate of heat release, in the following sets: > 400, 300-400, 230-300, 200-230, 160 to 200 and < 150 kW/m
2. Table 1 contains the peak rate of heat release, total smoke production and SMOGRA data for the cables at 50 kW/m
2.
In the same project, cables were tested using the FIPEC protocols. Thus, cables were tested in
a thermally insulated vertical test chamber (1 m x 2 m x 4 m), with floor air inflow and ceiling
air and smoke outlet. The test chamber also has an observation door (to view the tests), and is
connected to a hood and duct system, where heat and smoke release information is determined
and assessed. The cables were mounted on a cable tray attached to the rear wall of the test
chamber, with multiple 3.5 m lengths of electrical or optical fiber cable as test specimens. The
lower part of the cables extended 0.2 m below the lower edge of the burner, and the normal
distance between the burner and the cables was 75 mm. The cables were centered along the
width of the cable tray, with each cable attached individually to each rung of the tray, by means
of a metal wire. The cables were exposed for a period of 40 min to a 20 kW flame (Protocol 1)
or a 30 kW flame (Protocol 2), from a gas burner. The two protocols also differ in the presence
of a non combustible board, made of calcium silicate behind the cables, at the back of the cable
tray in Protocol 2. Protocol 1 was applied to all cables, but Protocol 2 was not applied to those
cables that performed least well when tested in Protocol 1. Measurements made were cable char
length (damage), and all relevant heat and smoke release parameters. The six Figures mentioned
above also include the values of the peak rate of heat release measured for every cable in the
Protocol 1 cable tray test. Figure 7 shows the comparison between the cable damage, both in
Protocol 1 and in Protocol 2, as a function of peak rate of heat release: clearly the correlation is
excellent. Figure 8, on the other hand, shows the comparison between the peak rate of heat release in the FIPEC cable tray test (protocol 1) and in the cone at 50 kW/m
2, where the correlation is very poor indeed. Table 1 contains a significant amount of data from the FIPEC cable tray tests (with both protocols) and the classification category, as recommended by the FIPEC project. It is important to note that the smoke and heat classifications are independent.
Thus, low heat release and smoke release cables will be those in categories 1.1 or 1.2 and A In a separate project, a set of 21 cables, built with the same cable construction, but differing in the materials used for cable construction, were tested in the cone calorimeter and in the ASTM D 5537/ASTM D 5424 (Protocol B) test, namely the Canadian cable tray test (CSA FT4) modified to include heat and smoke release measurements
10. The same cables were also tested in the cone calorimeter, at incident heat fluxes of 20, 40 and 70 kW/m
2, also in the horizontal orientation and with the cable ends sealed. The data were further analyzed, in conjunction with an analysis of various other products, in a correlations paper
7. It was found that the cone calorimeter data, at a heat flux of 20 kW/m
2, were very suitable for correlation, by being able to predict whether a cable will or will not cause a self-propagating fire (with increasing flame spread rate) as a function of the peak rate of heat release. It was also seen that there was excellent correlation between the char length (or damage) in the cable tray test and the peak rate of heat release measured in the same test. This information is presented in Figure 9. On the other hand, Figure 10 shows that there is very poor correlation between cable tray test data and cone calorimeter test data at incident heat fluxes of 40 or 70 kW/m
2. Smoke release could also be predicted to some extent, from the lower heat flux data.
In another project, wall lining materials were tested using the NFPA 265 room-corner test
11, 12. The data were later analyzed, together with the data from wall lining materials tested in the NFPA 286 room-corner test, in a recent wall linings correlations paper
4, by comparison with data from tests conducted on the cone calorimeter, in the horizontal orientation, at an incident heat flux of 50 kW/m
2. In this work it was found that the cone calorimeter data, at a heat flux of 50 kW/m
2, were very suitable for correlation, for both heat and smoke release (and to assess whether flashover would be achieved) by using the correlation equations 1 and 2, where b is the parameter indicating whether flashover will occur (if b $ - 0.5) and where 8 is the cone calorimeter rate of heat release decay parameter (for the first peak):
b = [RHR (3 min Avg) * 0.01] - 1 -( 8 * Time to Ignition) (Eq. 1) RHR (t - Time to Ignition) = Pk RHR * exp [- 8 *(t - Time to Ignition)] (Eq. 2) A corresponding equation was also developed for smoke release, based on total heat released.
The correlation coefficients found in that work were very satisfactory.
RESULTS AND DISCUSSION
Using Equation 2 , the rate of heat release decay parameter, 8 , was calculated for all cables,
from the cone calorimeter results at 50 kW/m
2. With that information, the b parameter (expected
to be a flashover prediction parameter, as shown in room-corner tests) was calculated using
Equation 2. Both sets of data are contained in Table 1, for all cables. These analyses were
conducted with the expectation that the same parameter that can be used to predict whether
flashover will occur in a room-corner test might also predict whether a cable will cause a self- propagating fire in the FIPEC cable tray test protocol. The results shown indicate that there is no valid correlation between the values of b and the probability of a cable causing a self propagating fire. Clearly, the attempt to use room-corner flashover correlations to predict cable tray test results was unsuccessful.
The results in Figure 8 indicate that a simple correlation based on peak rate of heat release at that cone calorimeter heat flux is not successful either, irrespective of whether protocol 1 or protocol 2 is considered. An analysis similar to this has been conducted for the cone calorimeter peak rate of heat release data at 35 kW/m
2, with similar type of results. Analogously, smoke release could also not be predicted or correlated by using either of the techniques suggested by the earlier work, from either 35 or 50 kW/m
2cone data.
If this information is compared with the data presented in Figures 9 and 10, the analysis suggests that one explanation may be that the cone calorimeter tests should be conducted at lower heat fluxes in order to properly be able to predict cable tray test results.
It has been shown repeatedly that the cone calorimeter is usually a reasonable predictor of larger scale fire performance, for a variety of materials or products. Thus, a different type of analysis was conducted, by subdividing the cables into two classes, on heat release/flame spread:
"passing" cables fall into FIPEC classes 1.1, 1.2 and 2, and "failing" cables fall into FIPEC classes 3 and 4. The cables were also subdivided into two classes based on smoke: "passing"
cables fall into FIPEC smoke class A and heat classes 1.1, 1.2 or 2, and "failing" cables are those that fall into FIPEC smoke classes B or C and those falling into heat classes 3 and 4. The concept here is that a cable cannot be considered to "pass" on smoke if it fails on heat release or flame spread. The class limits are as follows:
Pk RHR - 1 THR - 1 Pk RHR - 2 THR - 2
kW MJ kW MJ
Class 1.1 # 80 # 30 # 45 # 10
Class 1.2 # 80 # 30 # 70 # 60
Class 2 # 80 # 30 > 70 > 60
Class 3 # 180 # 100 Not applicable
Class 4 > 180 > 100 Not applicable
Pk Smoke Production Rate - 1 Total Smoke Production - 1
m
2/s m
2Class A # 0.25 # 100
Class B # 1.00 # 400
Class C > 1.00 > 400
If an analysis considers whether the cone calorimeter peak rate of heat release exceeds 150 kW/m
2, at an incident heat flux of 50 kW/m
2, this analysis correctly predicts the results for 65%
of the 43 cables and safely predicts the results of 95% of the cables. (A safe prediction is one
where no failing cable is predicted to pass the large scale test). An unsafe prediction was
obtained only for two cables: # 18 and 21. If an analysis considers whether the cone calorimeter
peak rate of heat release exceeds 120 kW/m
2, at an incident heat flux of 35 kW/m
2, this analysis
correctly predicts the results for 74% of the 43 cables and safely predicts the results of 93% of the cables. An unsafe prediction was obtained only for three cables: # 18, 21 and 51.
If an analysis considers whether the cone calorimeter peak rate of heat release exceeds 150 kW/m
2, and the cone SMOGRA value exceeds 10, at an incident heat flux of 50 kW/m
2, this analysis correctly predicts the results for 88% of the 43 cables and safely predicts the results of 95% of the cables. An unsafe prediction was obtained only for two cables: # 18 and 21.
CONCLUSIONS
In conclusion, the cone calorimeter, at an incident heat flux of 50 kW/m
2is able to roughly predict whether cables will pass heat or smoke release requirements in FIPEC cable tray tests, and it will usually (in > 90% of cases) give a safe prediction. Better predictions are probably obtainable if other incident heat fluxes are used, or following detailed modeling.
REFERENCES
1. Grayson, S.J., van Hees, P., Vercellotti, U., Breuler, H. and Green, A., "Fire Performance of Electric Cables - New test Methods and Measurement Techniques", Final Report on the European Commission SMT Programme Sponsored Research Project SMT4-CT96- 2059, Interscience Communications, London, UK, 2000.
2. IEC 60332-3: Tests on Electric cables under Fire Conditions - Part 3: Test for Vertical Flame Spread of Vertically Mounted Bunched Wires or Cables, International Electrotechnical Commission, Geneva, Switzerland.
3. ASTM E 1354, Standard Test Method for Heat and Visible Smoke Release Rates for Materials and Products Using an Oxygen Consumption Calorimeter (cone calorimeter), Amer. Soc. Testing Materials, West Conshohocken, PA, Volume 04.07.
4. Janssens, M.L., Dillon, S.E. and Hirschler, M.M., "Using the Cone Calorimeter as a Screening Tool for the NFPA 265 and NFPA 286 Room Test Procedures", Fire and Materials Conf., San Francisco, CA, Jan. 22-24, 2001, Interscience Communications, London, UK, pp. 529-540.
5. NFPA 265, "Standard Methods of Fire Test for Evaluating Room Fire Growth Contribution of Textile Wall Coverings", Natl Fire Protection Association, Quincy, MA.
6. NFPA 286, "Standard Methods of Fire Test for Evaluating Room Fire Growth Contribution of Interior Finish", Natl Fire Protection Association, Quincy, MA.
7. Hirschler, M.M., "Use of Heat Release Rate to Predict Whether Individual Furnishings Would Cause Self Propagating Fires", Fire Safety J., 32, 273-296 (1999).
8. ASTM D 5424, Standard Test Method for Smoke Obscuration of Insulating Materials Contained in Electrical or Optical Fiber Cables When Burning in a Vertical Cable Tray Configuration and ASTM D 5537, Standard Test Method for Heat Release, Flame Spread and Mass Loss Testing of Insulating Materials Contained in Electrical or Optical Fiber Cables When Burning in a Vertical Cable Tray Configuration (Protocol B), Amer.
Soc. Testing Materials, West Conshohocken, PA, Volume 10.02.
9. ASTM D 6113, Standard Test Method for Using a Cone Calorimeter to Determine Fire-
Test-Response Characteristics of Insulating Materials Contained in Electrical or Optical
Fiber Cables , Amer. Soc. Testing Materials, West Conshohocken, PA, Volume 10.02.
10. Hirschler, M.M., "Analysis of and Potential Correlations Between Fire Tests for Electrical Cables, and How to Use This Information for Fire Hazard Assessment", Fire Technology, 33, 291-315, (1997).
11. Hirschler, M.M. and Janssens, M.L., "Heat and Smoke Measurements of Construction Materials Tested in a Room-Corner Configuration According to NFPA 265", 27th Int.
Conf. Fire Safety, Jan. 11-15, 1999, Product Safety Corp., San Francisco (CA, U.S.A.), Ed. C.J. Hilado, pp. 70-93 (1999), San Francisco, CA.
12. Hirschler, M.M. and Janssens, M.L., "Smoke Obscuration Measurements in the NFPA
265 Room-Corner Test", Fire and Materials Conf., San Antonio, TX, Feb. 22-23, 1999,
Interscience Communications, London, UK, pp. 179-198.
RHR Cables > 400 kW/m^2
0 100 200 300 400 500 600 700 800
0 500 1000 1500 2000 2500
Time (s)
Cable 3 Cable 31 Cable 11 Cable 35 Cable 39
320 414
373
273 382
Numbers are Pk RHR Prot. 1
RHR Cables 300-400 kW/m^2
0 50 100 150 200 250 300 350 400
0 200 400 600 800 1000 1200 1400 1600 1800
Time (s)
RHR (kW/m^2)
Cable 14 Cable 12 Cable 29 Cable 37 Cable 27
29 108457 396
273
Numbers are Pk RHR in Prot. 1
Figure 1: Cone rate of heat release of those cables releasing > 400 kW/m
2Figure 2: Cone rate of heat release of those cables releasing between 300 and 400 kW/m
2RHR Cables 230-300 kW/m^2
0 50 100 150 200 250 300
0 500 1000 1500 2000 2500 3000 3500
Time (s)
Cable 5 Cable 45 Cable 40 Cable 15 Cable 36 Cable 14 Cable 19 Cable 13
392 14412 45 99
79
584 Numbers are Pk RHR in Prot. 1
108
RHR Cables 200-230 kW/m^2
0 50 100 150 200 250
0 200 400 600 800 1000 1200 1400 1600 1800 2000
Time (s)
Cable 42 Cable 22 Cable 30 Cable 23 Cable 41 Cable 33
131
566
22 32
283 376
Numbers are Pk RHR in Prot. 1
Figure 3: Cone rate of heat release of those cables releasing between 230 and 300 kW/m
2Figure 4: Cone rate of heat release of those cables releasing between 200 and 230 kW/m
2RHR Cables 160-200 kW/m^2
0 20 40 60 80 100 120 140 160 180 200
0 200 400 600 800 1000 1200 1400 1600 1800 2000
Time (s)
Cable 24 Cable 26 Cable 25 Cable 32 Cable 6 Cable 17
250 407
704
367
20 28
Numbers are Pk RHR in Prot. 1
RHR Cables Less 150 kW/m2
0 20 40 60 80 100 120 140 160
0 500 1000 1500 2000 2500 3000 3500
Time (s)
Cable 41 Cable 8 Cable 10 Cable 21 Cable 9 Cable 18 Cable 38
28316
9 206
7
118 6
Numbers are Pk RHR Prot. 1
Figure 5: Cone rate of heat release of those cables releasing between 160 and 200 kW/m
2Figure 6: Cone rate of heat release of those cables releasing less than 150 kW/m
2Char Length vs Pk RHR in FIPEC Cable Tray Tests
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5
0 100 200 300 400 500 600 700 800 900
Pk RHR in Cable Tray Test (kW)
Protocol 1 Protocol 2
Pk RHR FIPEC Cable Tray Test vs Cone Test Data
0 100 200 300 400 500 600 700 800 900
0 100 200 300 400 500 600 700 800
Pk RHR Cone @ 50 (kW/m^2)
Pk RHR Protocol 1 Pk RHR Protocol 2
Figure 7: Char length versus peak rate of heat release of cables in FIPEC cable tray test
Figure 8: Peak rate of heat release in FIPEC cable tray tests (both protocols) and in cone
calorimeter at 50 kW/m
2incident heat flux tests
C S A FT4 C har Length & N on C orrelations w ith H igh C one Fluxes
0 200 400 600 800 1000 1200 1400
0 100 200 300 400 500 600
P k R H R Tray Test (kW )
Pk RHR Cone @ 40 & 70 (kW/m^2)
0.6 0.8 1 1.2 1.4 1.6 1.8 2 2.2 2.4 2.6
Tray Char Length (m)
C one R H R @ 40 C one R H R @ 70 C har
CSA FT4 Char Length & Correlations
0.6 0.8 1 1.2 1.4 1.6 1.8 2 2.2 2.4 2.6
0 100 200 300 400 500 600
P k R H R Tray Test (kW )
T ray C h ar Lengt h ( m )
0 50 100 150 200 250 300 350 400 450
Pk RHR Cone (kW/m^2)
C har C one R H R @ 20
