38.3 Electrical Cable Fire Toxicity

7/30/2019 38.3 Electrical Cable Fire Toxicity

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Energy and Resources Research InstituteFire and Explosion Research Group

Electrical Cable Fires using the ConeCalorimeter for Toxic Gas

Investigations

Al- Sayegh, W.A., Aljumaiah, O., Andrews, G.E.and Phylaktou, H.N.

Energy and Resources Research Institute,

School of Process, Environment and MaterialsEngineering

The University of Leeds


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INTRODUCTION - 1

The majority of deaths in fires (66%) are due to theeffects of toxic gases rather than flames [1]. PVCis the second largest contribution after wood tofires in non-domestic premises [2] and its use inelectrical cable insulation is one of the mainsources of PVC in fires. It has been estimated thatthere are 250m of electrical cable per house (50mper room) and 140 fires/million km cable/year [3].Electrical cable fires may occur as part of a fireload that has started elsewhere and spread to

ignite the cables externally. This is simulated inthe present work using the cone calorimeter torepresent radiant heating from a fire that engulfsthe cables.


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INTRODUCTION - 2

Sundstrom [4] has reviewed methods oftesting of cables for their fire resistanceand concluded that current testsconcentrate on fire propagation rates andnot on toxicity. However, he concludesthat the future must address the toxicityissue and the measurement requirements,as reliable test methods are lacking.

ISO TC 92 is working on this issue [4].

[4] Sundstrom, B., Flammability Tests for Cables Flammability Testing of

Materials used in Construction, Transport and Mining. Ed. Apte, V.B.,Woodhead Publishing Ltd., 2006, p. 187-199.


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INTRODUCTION - 3

Hirschler [5, 6] has compared the large scalecable tray fire tests of ASTM D9.21 and IEC 332-3with the cone calorimeter and concluded that theHRR and smoke production measured in the

cone calorimeter can be used to predict theresults of the larger scale tests.

This was the reason for the use of the conecalorimeter in the present work, which usedheated FTIR analysis for full acidic and irritant

toxic gas production measurements for PVC andpolypropylene cable sheaths.[5] Hirschler, M.M., Comparison of large and small-scale heat release tests with electrical cables. Fires and

Materials,18: 2, 61-76, 1994.[6] Hirschler, M.M., Can the Cone Calorimeter be Used to Predict Full Scale Heat and Smoke Release Cable

Tray Results from a Full Scale Test Protocol, Proc. Interflam, Interscience Communications, London,2001.


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Author Ref. Method COg/g HClg/g HCl%

Theoretical PVC C2H

3Cl 0.58

Babrauskas 7 NBS Cup Furnace 0.09 0.11 0.2 0.3 0.21 0.37

Babrauskas 7 SwRI/NIST 0.086

0.090

0.21

0.26

0.31 0.57

Babrauskas 7 Cone Calorimeter

35, 50, 75 kW/m20.066

0.076

0.3

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Babrauskas 7 Real Scale Fires

PVC board

0.19 - 0.47 0.09-0.22 0.04-0.14

Persson and

Simonson

2 Large scale fire 0.116 0.32

Bowes 8 Pyrolysis of 1kg of rigid

PVC in 1m3 furnace

0.09 3.38

Blomqvist 9 Vertical cable fire test with

FTIR

0.04-0.13 0.045-

0.13

Hull et al. 10 ISO Furnace Test

Purser Furnace with FTIR

0.025 -0.12 0.11-0.15

Hull et al.

Purser

11

12

Purser Furnace Test

Purser Furnace Test

0.23-0.46

0.01-0.17

0.15

0.16-0.56

This work Cone Calorimeter + heated

FTIR, electrical cables

0.09-0.12 0.2 0.5 0.025


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TOXIC GAS ASSESSEMENTS

Toxic Gas 15 MinLimit

COSHH[17]

LC5030 min

[18,19]

COSHHTo CO

Ratio

LC50Ratio to CO

limit

Carbon

Monoxide

CO 200 3,000 1 1

Nitrogen

Dioxide

NO2 5 500 40 6

Hydrogen

Cyanide

HCN 10 135 20 22

Benzene C6H6 3 67

Formaldehyde CH2O 2 250 100 12

Acrolein C3H5O 0.3 300 667 10

Formic Acid CH2O2 5 40

SO2 SO2 5 500 40 6

HCl HCl 5 3,700 40 0.8


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EXPERIMENTAL METHODS

Samples of polypropylene (PP) and PVC sheathedelectric cables were burned on the conecalorimeter at three different heat fluxes (20, 30and 40 Kw/m2).

Prior to burning the samples the heat releasemeasure by the apparatus was calibrated usingnatural gas.

Strips of electrical cable 9cm long were place sideby side until the whole of the cone calorimetersquare test section was full with one layer deep ofelectrical cable. The copper core was retained inthe wires as they may contribute to thetemperature rise, as a thermal heat sink.


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A Temet FTIR (Gasmet) was used for the toxic gas analysis. TheFTIR had a multi-pass cell with gold-coated mirrors with a 2mpath length and volume of 0.22l. This had an 8 cm-1 wavelengthresolution and a 900 4250 cm-1 scanning range. This lowresolution was used as this has a much better signal to noiseratio for complex multi-component gas analysis. A liquid

nitrogen cooled MCT detector was used that scans 10 spectraper second and several scans are used to produce a time-averaged spectrum.The response time of the instrument is 2s to reliably resolve all51 species that it can measure. The Temet FTIR gives a typical2ppm resolution with an accuracy of 2% and a precision that is0.1% of the measurement range. The FTIR has been calibrated

for all the significant species that are present in the gas samplefrom fires. The only calibration necessary prior to the test wasto zero the instrument on nitrogen. The calibration was checkedfor some gases, CO, CO2, benzene and methane using certifiedspan gases and the agreement was satisfactory.


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HEATED SAMPLING SYSTEMS AND HEATEDANALYSERS

Samples were taken from the cone calorimeterexhaust duct and transferred to the FTIR using a190oC heated sample line, heated pump and filterand a second 190oC heated sample line between

the pump and the FTIR. The FTIR measurementzone was also heated at 190oC so that no loss ofHCl and other condensable gases occurred.

There are two reasons to use completely heatedsampling and analysis systems when sampling

directly from fire product gases: loss ofcondensable species, mainly hydrocarbons andaldehydes and loss of species by solution in thewater that condenses in unheated systems.


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THE PROBLEM OF SAMPLE LOSSES INUNHEATED GAS SAMPLING AND ANALYSISSYSTEMS

Species Boiling Point Solubility in Water

at 20oC

Heated Sampling

System?

CO -192oC 0.006 g/litre No

Benzene 80oC 0.8 g/litre Yes >80oC

Naphthalene 218o

C 0.03 g/litre Yes > 220o

CFormaldehyde 96oC Extremely Soluble Yes > 100oC

Acrolein 53oC Very Soluble Yes > 100oC

NO2 21oC Hydrolyses to HNO3 Yes > 100

oC

HCN 25.6oC Completely Miscible Yes > 100oC

HCl -85oC Extremely soluble720g/litre

Yes > 100oC

SO2 -10oC Extremely soluble

80 vol/vol

Yes > 100oC


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THERMAL GRAVIMETRIC ANALYSIS

The PVC and PP samples and their residues from the cone testwere analysed using thermal gravimetric analysis (TGA). Asample weight of about 5 mg was used and the weightresolution was 1 g. The sample was heated in nitrogen toevaporate any volatile components followed by a phase ofcombustion in air to burn the remaining fuel. The sample was

heated at 20o

C per minute until a temperature of 100o

C wasachieved. The temperature was kept at 100oC for five minuteswhich allowed the sample to dry out and any water content to bedetermined. The sample was then heated to 550oC at a rate of 20oC per minute for 10 minutes this allows the volatile componentsto evaporate but not to combust as no oxygen is present. Thisdetermines the HCl released as this is the only volatile product

for PVC. Finally, the air combustion was introduced and theduration of this phase is 20 minutes where the sample is heatedto 560 oC at a rate of 10 C per minute. This determines thecarbon proportion of the sample, as any hydrocarbons orhydrogen would have been devolatilised by 550oC.


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Two types of electrical cable covering wereused.

PVC electrical cable covering: - This had a

thickness of 2.9mm. It was intended for usewith voltages between 300 and 500 volts.The calorific value for PVC is 19.9 MJ/Kg.

PP electrical cable covering: - This had athickness of 2.6mm and was used forelectric control circuit cables. The calorificvalue for PP is 46.4 MJ/Kg.


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ELEMENTAL ANALYSIS

Fuel Nitrogen Chlorine Carbon Hydrogen H/C Total

PVC N/A 55.26 39.67 5.07 1.53 100

PVC [7] 0.004 43.08 38.05 4.93 1.55 86.1

PVC, C2H3Cl 0 56.8 38.4 4.8 1.5 100

Polypropylene 0.69 N/A 65.97 8.76 1.59 75.42

Polypropylene

n(C3H6)

0 0 85.7 14.3 2.0 100


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TGA ANALYSIS

Normalised values for TGA of PVC electricalCable

0.2

0.4

0.6

0.8

1

0 500 1000 1500

Temperature (oC)

mass/(originalmass)

(mg/mg)

Combustion

Pyrolysis

Normalised Value for TGA of PVC Residue

0.4

0.5

0.6

0.7

0.8

0.9

1

0 500 1000 1500

Temperature (oC)

weight/originalweight

(mg/mg)

Pyrolysis

Combustion

The weight loss when heating the sample in nitrogen was due to therelease of HCl, this occurs between 250 and 300oC and is identical forheating in nitrogen and air. The elemental analysis gave 55% of the PVCmass as chlorine; this would produce a yield of HCl of 58% as shown inTable 1. Fig. 1a shows there was a 42% weight loss due to HCl loss in airand nitrogen. However, there is evidence of another volatile loss at400-500oC and assuming that this is also HCl the total weight loss due torelease of HCl is 52% close to the expected total yield of HCl. Bowes [8]commented that 70% of the expected HCl yield (41%) occurs rapidly at272oC and this was found, as shown in Fig. 1a.


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MASS LOSS ON THE CONE CALORIMETER

After 500oC there was little combustion ofhydrocarbons, as the pyrolysis weight losswas small. At around 700-750oC bothpyrolysis and combustion show a weightloss and this could be due to release ofhydrogen. After 800oC there was no furtherweight loss for heating in nitrogen or in airand this indicates that there is about 30% of

residual char that does not burn. This will beshown later to be a feature of the conecalorimeter results.


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MASS LOSS AND HEAT RELEASE RATE ON THECONE CALORIMETER

PVC weight Loss at different Heat Fluxes

0.7

0.75

0.8

0.85

0.9

0.95

1

0 500 1000 1500

Time (S)

Weight/initial

weight(g/g)

20 Kw/m2

30Kw/m2

40Kw/m2

Polypropylein Weight Loss at Different Heat

Fluxes

0.8

0.85

0.9

0.95

1

0 500 1000 1500

Time (s)

Weight/initialweight

(g/g)

20 KW/m2

30 Kw/m2

40 Kw/m2

Weight loss and HRR as a function of time in the cone calorimeter tests for three

incident radiant fluxes for PVC and PP .

Heat Release Rate at Varying Heat Fluxes for a

PVC Sample

-50

0

50

100

150

200

0 200 400 600 800 1000 1200

Time(s)

HRR(Kw/m2)

20Kw/m2

30Kw/m2

40 KW/m2

Heat Release Rate at Varying Heat Fluxes for a

Polypropylein Sample

-50

0

50

100

150

200250

300

0 200 400 600 800 1000 1200

Time (s)

HRR(Kw/m2)

20Kw/m2

30Kw/m2

40Kw/m2


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MASS LOSS AND HEAT RELEASE RATE ON THECONE CALORIMETER

Comparison between PVC and PP willbe made at 40 kw/m2 as PVC burnsfreely at this condition.

On all the graphs PVC is on the left andPP on the right.


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TOTAL HYDROCARBON EMISSIONS BY FTIR

Total HC

0

200

400

600

800

1000

1200

0 200 400 600 800 1000 1200

Time s

TotalFTIR

HC

ppm

Total HC

Concentration of THC Vs Time (s)

0

100

200

300

400

500

600

700

0 100 200 300 400 500

Time(s)

Concentration(ppm)

Total FTIR hydrocarbons for PVC and PP for 40 kw/m2

For the PVC fire the total HC results show a continuous

release of hydrocarbons throughout the later stages of the

fire and these do not get oxidised. For the PP cable fire thehydrocarbons are mainly release quickly along side the

benzene release that is shown later and then burn out and arevery low in the later stages of the cable fire.


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TOXIC YIELDS OF HCL g/gYields of HCl at Varying Heat Fluxes

-0.1

0

0.1

0.2

0.3

0.4

0.5

0.6

0 500 1000 1500 2000

Time (s)

Yield(g/g)

20 Kw/m2

30 Kw/m2

40 Kw/m2

Yield HCl

0

0.002

0.004

0.006

0.008

0.01

0.012

0.014

0.016

0.018

0.02

0 50 100 150 200 250 300 350 400 450 500

Time s

HClYieldg/g

Yield HCl

These yields are comparable with those reported by others forboth large and small scale equipment. They are higher thanmost previous work has found, especially in small scale testssuch as the Purser furnace test [10,11]. However, the

agreement was very good with the previous cone calorimeterwork of Babrauskas for PVC sheet samples.

Babrauskas, V., Harris, R.H., Braun, E., Levin, B., Paabo, M. and Gann, R.G.,

The Role of Bench Scale Test Data in Assessing Real-Scale Fire Toxicity,

NIST Technical Note 1284, 1991.


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TOXIC YIELDS OF HCL - 2

The HCl yields were significantly lower than the 100%conversion of chlorine to HCl yield of 0.58. This may bedue to incomplete conversion of the chlorine in the fire.The cone calorimeter debris was tested by TGA, asdiscussed above. There was a low temperature volatile gasloss, but at a lower temperature than was found for thefresh PVC. However, it was possible that the second stageHCl loss that occurred at higher temperatures in the TGAwas the source of the Chlorine that was not released in thecone calorimeter tests. This was about 30% of the totalChlorine and if this was the case then the measured yieldof HCl would be close to achieving a chlorine balance.

There was a low yield of HCl from polypropylene electriccables, which was at least a factor of 10 below the yield forPVC electric cables. The chlorine may have come from achlorine based fire retardant added to the cable sheathcomposition.


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Yields of Acrolein for PVC and Benzene for PP

Yield for Acrolein in 40 Kw/m2 fire

-0.01

0

0.01

0.02

0.03

0.04

0.05

0.06

0 200 400 600 800 1000 1200

Time(s)

Yield

Yield Vs. Time (s)

0

0.05

0.1

0.15

0.2

0.25

0.3

0 100 200 300 400 500

Time (s)

Yield

ACROLEIN PVC BENZENE PP

There was no significant acrolein for PP and no significantbenzene for PVC. The yield of benzene from PP was very

high and the yield of acrolein was very significant for PVC.

This was not expected as acrolein is generally formed fromHCO elemental fire loads such as wood and cotton.For example I reported yesterday acrolein yields of a peak of

0.016 for smoldering cotton towels and here it is 0.05 for PVC.


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YIELDS OF FORMALDEHYDE

Yield for formaldehyde in 40 Kw/m2 fire

-0.01

0

0.01

0.02

0.03

0.04

0 200 400 600 800 1000 1200

Time (s)

Yield

Yield Vs Time

-0.005

0

0.005

0.01

0.015

0.02

0.025

0.03

0.035

0 100 200 300 400 500

Time (s)

Yield

Yield

Yields of formaldehyde for 40 kw/m2 for PVC (left) and PP (rt)

Peak yields were similar but lasted longer with PVC.As for acrolein, the generation of formaldehyde from PVC

or PP was unexpected as they normally occur with HCOType fire loads. However, they are a low temperaturecombustion pollutant and it may be the entrained air thatis responsible for their generation.


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YIELD FOR 1.3 BUTADIENE FOR PVC (left) ANDMETHYL BENZENE FOR PP (Right)

Yield for 1,3 Butadiene Vs Time

-0.005

0

0.005

0.01

0.015

0.02

0.025

0.03

0.035

0 500 1000 1500 2000

Time (s)

Yield

Series1

Yield Vs. Time (s)

-0.001

0

0.001

0.002

0.003

0.004

0.005

0 100 200 300 400 500

Time (s)

Yield

Virtually all the toxicity for polypropylene fires was due tothe hydrocarbons benzene and small quantities oftri-methyl benzene.For the PVC fires 1,3 butadiene was the most significant

hydrocarbon in the later stages of the cable burning.


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RELATIVE TOXICITY

Comparing Total COSHH N for Varying Heat Flux

0

50

100

150

200

0 500 1000 1500 2000

Time (s)

COSHH(N)

20 Kw/m2

30 Kw/m2

40 Kw/m2

Comparing Total COSHH N for Varying HeatFluxes

0

20

40

60

80

0 200 400 600 800 1000 1200

Time (s)

COSHHN 20Kw/m2

30 KW/m2

40 KW/m2

The total COSHH 15 min. total relative toxicity N values are shown for PVCand PP. For the LC50 toxicity the N values are much lower but occur at thesame time and the trends with time are the same. The total toxicity wassensitive to the heat flux as this essentially changes the pyrolysis

temperature in the fire zone.The air dilution of the fire products in the freely ventilated test conditionsof the cone calorimeter encourages oxidation of products in the mixingarea. If the air entrainment is rapid then species are quenched and realisticyields can be measured, but for CO oxidation and the lower equilibriumValues that occur for leaner mixtures make the test unrealistic for CO toxici


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RELATIVE TOXICITY ON A COSHH 15 MIN.BASIS FOR HCL

Ratio of HCL concentration to COSSH Value

-10

0

10

20

30

40

50

60

0 200 400 600 800 1000 1200

Ratio

Tim

e

(s)

COSHH HCL

-1

-0.5

0

0.5

1

1.5

2

2.5

3

0 50 100 150 200 250 300 350 400 450 500

Time s

RELATIVECOSHHHCL

COSHH HCL

HCl for PP was significant, which was not expected and

indicates that the polypropylene may have had some PVCin its blend or had some chlorine based fire retardant added.The HCl levels are much lower than for PVC but have a simila

toxicity contribution to that of CO.The highest toxicity occurs in the time period of thehighest HRR.


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RELATIVE TOXICITY FOR COON A COSHH 15 MIN. BASIS

COSHH ratio for CO Vs Time(s)

-0.1

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0 200 400 600 800 1000 1200

Time (s)

COSHHratio

COSHH ration for CO

-0.5

0

0.5

1

1.5

2

2.5

3

3.5

0 100 200 300 400 500

Time (s)

CoshRatio

For PVC the overall toxicity was dominated by acrolein andHCl, CO were not a major contributor. This conclusion wouldnot change if toxicity was assessed based on LC50, as the

toxicity concentration ratio for HCl and acrolein is 12 on anLC50 basis and 16 on a COSHH 15 min. basis.For PP the toxicity was dominated by the production of

benzene and again CO was a low contributor.


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RELATIVE TOXICITY FOR ACROLEIN FOR PVCAND BENZENE FOR PP

COSSH ratio for acrolein

-20

0

20

40

60

80

100

120

0 200 400 600 800 1000 1200Time (s)

Ratio

COSHH ratio Vs Time (s)

0

5

10

15

20

25

30

35

0 100 200 300 400 500Time (s)

COSHHRatio

BENZENEACROLEIN

Peak relative toxicity for acrolein was 110 and for CO it was

0.7 and for HCL it was 50. Thus acrolein dominates the

toxicity. On an LC50 basis the conclusion in relation to COwould not change as the change in relative toxicity of CO

to acrolein for LC50 relative to COSHH 15 min. was 67 andthe acrolein to CO COSHH toxicity ratio is 157.


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CONCLUSIONS

1. The cone calorimeter with freely ventilated fires is a good method forthe evaluation of toxicity from materials in fires, but isunrepresentative of CO yields due to the high post flame oxidation ofCO.2. A heated sample system and FTIR is essential for work on toxicgases as the losses if water condenses in the sample system willremove many of the important toxic species.3. A comparison of toxic emissions for cable sheath materials of PVCand PP was carried out and this showed completely different toxic gasemissions but only slightly lower overall toxicity for PP, which was notexpected.4. For PVC the dominant toxic gas was acrolein and this was absent in

the PP tests. HCl was a significant toxic species but not as importantas acrolein and this has not previously been reported.

5. For PP the dominant toxic gas was benzene and this was notpresent in the PVC cable fires. Further work is required on thesesurprising results in relation to understanding the pyrolysis kineticsand to ensure that similar conclusions are valid for air starved fires.


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38.3 Electrical Cable Fire Toxicity