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HAZARDS OF COMBUSTION PRODUCTS

INDUSTRY SECTOR REPORT – ELECTRIC CABLES

By Terence L JourneauxPrysmian Cables & Systems, UK

Europacable Seminar “Safety during Fire”, Brussels 6th May 2009

This paper was given at Hazards of Combustion Products, 10-11 November 2008, The Royal Society, London




INTRODUCTION

The objective of this paper is to give an overview of the development, current status andpossible future direction of the way that cable manufacturers address the hazards of combustionproducts when electric cables are burnt. It is not intended to provide a detailed description of

the test methods used or a detailed discussion as to their validity other than by cross reference,as many other papers have addressed these issues.

The cable industry has had a long history in the development of test methods and productsdesigned to lessen the hazards resulting from burning cables, going back to the 1960’s. Theindustry was one of the first in the electrotechnical sector to develop tests for the assessment of the reaction to fire performance of its products and has continued to refine and improve thesemethods over the years. Standards covering flame spread, heat release, opacity, corrosivityand toxicity of fire effluent are today in use and the industry continues to sponsor research intothe improvement of the fire performance of its products and the definition of appropriate testmethods.

In order to provide an international perspective, reference is generally made in the paper to IEC(International Electrotechnical Commission) standards or regional standards e.g. EN (EuropeanStandard). Within this international framework there are considerable differences from region toregion and country to country in the way that the hazards of combustion of cables areapproached and the applications for which the established test methods are referenced.

The cable industry approach and current position has been largely customer driven and theindustry provides products with a wide range of reaction to fire performance to reflect thevarying customer requirements.

HISTORICAL DEVELOPMENT

Much of the early work relating to improvements to the fire performance of electric cables wascarried out in the 1970’s and 1980’s (1,2,3). A four stage approach was taken at the time:

- minimise the hazard by restriction of the amount of burning by reduction of thepropagation of fire along cable runs

- minimise the emission of smoke leading to obscuration of exits and prevention of escape- minimise the emission of acid gas leading to corrosion of equipment- minimise the emission of toxic fumes leading to incapacity and prevention of escape

It was recognised that these factors would assume varying levels of importance according to theparticular market sector and typical installation condition considered (4). The cable industry alsowas aware that it would need to develop its own tests to properly assess the performance of itsproducts. Although this early development led to a series of tests that could be said to lackintegration, the approach when taken as a whole still sits well with current thinking.

Another important consideration was that large scale tests should be developed wherever relevant so as to assess performance of the total product in something approaching an “asinstalled” condition. The International work concentrated on the development of test methods sothat these could be adopted as an “add on” to product standards such that the widest possiblelevels of fire performance could be achieved across the full range of product types.

Reduced flame propagation

Reduced flame propagation cables have been well established in the market since the 1970’s



Europacable Seminar “Safety during Fire”, Brussels 6th May 2009 59

for cabling in areas where, because of installation conditions e.g. vertically mounted bunchedwires or cables, the risk of propagation of fire is high.Work in this area began in 1967 when, following a serious fire at the La Spezia power station, itbecame apparent that it was necessary to devise a test to simulate the behavior of cables inlarge scale fire conditions. Until that time, it had been believed that cables insulated andsheathed with a polyvinyl chloride (PVC) compound were sufficiently safe with regard to firepropagation because of the high chlorine content of the polymer which made the cable self-extinguishing under small scale test conditions (similar to IEC332-1). Unfortunately, the incidentat La Spezia showed that the cables burnt completely.Collaborative research between ENEL, CESI and cable manufacturer Pirelli led to thedevelopment of a full scale test for non propagation of cables known as the “CESI” test.International interest resulted in the work being furthered by a specially formed working group of IEC TC20, This work resulted in the publication of IEC 332-3 “Tests on electric cables under fireconditions – Tests on bunched wires or cables” in 1982. The standard includes a performancerequirement. This large scale test was a simplified version of the “CESI” test but research at thetime showed that results on both rigs were similar. The importance of the actual installationconditions on flame propagation has always been recognized in the IEC work.

Figure 1 Schematic of IEC332-3 apparatus Figure 2 IEC332-3 test inprogress

Corrosive gas emission

As a result of the experience gained form the new propagation testing, users began to raise a




new concern relating to the large amounts of acid gas emissions from the burning reducedpropagation PVC cables. This corrosive and irritant gas had already been seen to havedevastating effects on electrical panels and instrumentation exposed to cable fire effluent (5,6) and also effects on metal structures.These ongoing requirements to limit acid gas evolution from cables affected by fire led to thedevelopment in IEC TC20 of a bench scale test that could be used to assess cable makingmaterials. This work resulted in the publication of IEC 754-1 “Test on gases evolved duringcombustion of materials from cables – Determination of the amount of halogen acid gas” in1982. The standard does not include any recommended performance requirements.

Compound type Acid gas emission (IEC754-1)

1960’s Reduced propagation PVC 30%

1970’s X-Flam 15 reduced propagation PVC 15%

Early 1980’s “Low acid” rubber sheath for marine use 5%

Late 1980’s “Halogen free” compounds Less than 0.5%

Figure 3 Stages in the development of cable making materials with reduced acid gas emission

The limitations of this test method, which was developed with materials commonly used at thetime in mind, were well known:

- inability to detect hydrofluoric acid- limit of detection at 5 mg/g (0.5%) halogen acid equivalent- restriction to halogen acid gas

Work continued in IEC TC20 to develop a better indirect corrosivity test method based upon themeasurement of the pH and conductivity of an aqueous solution of the fire effluent as this couldbe more accurately related to corrosive effects. The work resulted in the publication of IEC 754-2 “Test on gases evolved during combustion of electric cables – Determination of degree of acidity of gases evolved during the combustion of materials taken from electric cables by

measuring ph and conductivity” in 1991. The standard includes recommended values and ameans of estimating the pH and conductivity of the gases expected to be evolved by acombination of materials found in a specified cable by means of a weighted value. It isrecognized that, as the test is not carried out on a complete cable, for a hazard assessment theactual material volumes of the cable components should be taken into consideration.

1980's 1990's 2000's1970's

Reduced Flame Propagation

Smoke Emission

Corrosivity

Corrosive (Acid) Gas Emission




Figure 4 Historical overview of cable reaction to fire performance development

Smoke emission

Low smoke emission cables have been manufactured since the 1970’s but it was not until

advances were made in cable making material technology in the 1980’s that cost effectivedesigns become widely available on the market.As cables containing conventional sheathing materials such as PVC were seen to give off largeamounts of dense smoke when affected by fire, an interest in the development of cablesemitting lower levels of smoke came from a number of users. Metro operators were particularlyconcerned about smoke preventing escape from underground tunnels. “Low smoke” cablesmanufactured by Pirelli were first subjected to full scale tests in Milan and London undergroundtunnels in 1975. Although bench scale smoke tests existed, their deficiencies required that alarge scale test capable of product testing in a simulated installation and end use condition bedeveloped. The LTE (London Transport Executive) smoke chamber was thus conceived as alarge scale laboratory alterative to the testing of cables in underground railway tunnels. The firesource was chosen as a reasonable simulation of a typical small fire to be anticipated in an

underground railway station and the smoke is collected in a 3m x 3m x 3m cube.International interest, particularly in the wake of the London Kings Cross metro fire in 1987, ledto IEC TC20 further developing the 3m cube test. The work resulted in the publication of IEC61034 “Measurement of smoke density of cables burning under defined conditions” in 1990.The standard includes recommended performance requirements which were derived fromcontemporary guidance on minimum visibility for escape.

Figure 5 IEC 61034 typical output curves Figure 6 IEC 61034 3m cube smokeapparatus

Toxic fume emission

The cable industry has been aware of the ongoing discussion of toxic emissions and theconflicting views generated. The high sensitivity to combustion protocol has been recognized asa major issue and a pragmatic view that a concentration on the securing of escape routesthrough control of smoke emission should enable personnel to escape and thus prevent

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prolonged exposure to toxic emissions was taken. The industry has not progressed anyinternational “toxicity” standards of its own, preferring to follow guidance from other expertgroups. It has however responded with suitable products to those users having their own “toxicemission” standards. These range from a simple restriction on certain elemental groups toindices derived from an analysis of the gases evolved and weighted according to the knownmammalian toxicity factors of the gases present such as NES 713 (UK), NFX 70-100 (France)and CEI 20-37/7 (Italy).

Historically the industry sponsored research work at FRS, the continuing development of whichhas led to the “Purser furnace” test method. However, at an IEC level the industry recognizedthat it did not have the expertise or resource to study “toxicity” and that the issue was muchwider than cables. Recognizing the close liaison between IEC TC20 and TC89, the work waspassed to the later group. Their work resulted in the publication of IEC60695-7-50 “Fire hazardtesting: Toxicity of fire effluent – Estimation of toxic potency: Apparatus and test method” (7) The cable industry view on the need, or otherwise, for a “toxicity” test was further influenced bywork carried out by the UK CEGB which concluded that “From this brief excursion intoconsequences it appears that the acidic gas concentrations likely to cause harm to people and

plant are similar”

(8,9)

.

Compound type pH Conductivity( µS cm-1 )

Chloride yield(mg/g)

Toxicity index(NES713)

EPR insulation 4.36 Less than 20 Less than 0.015 2.5

XLPE insulation 4.07 22.0 Less than 0.015 2.6

Elastomeric sheath 4.25 Less than 20 Less than 0.015 3.6

Thermoplastic sheath 4.15 28.5 Less than 0.3 3.0

Cable requirement 3.8 to 10 less than 80 Less than 0.5 Less than 5.0

Figure 7 Properties of materials used in power station cables circa 1990

CURRENT SITUATION

The IEC test standards have remained largely unchanged over recent years and have beensubject to ongoing refinement rather than major change (10,11,12,13). For example, IEC 332-3 hasdeveloped into a multipart standard with each part covering a different installation condition or time of exposure to the fire source.The principles established in the early development of these standards still serve cablemanufacturers and users well and the IEC standards are adopted in many countries. Some

regional and national variations do occur and these are discussed together with the relevance of the various methods in a series of review standards produced by IEC TC89 (14,15,16,17).Based on the use of these tests, one can find in the market;

Reduced flame propagation (RP) cables which when installed in vertical bunches inaccordance with the recommended procedures do not propagate fire more than a limiteddistance from the source. They are tested to the various parts of IEC60332-3.Low smoke cables which have limits on smoke evolution when assessed in the 3m cubesmoke chamber with performance limits chosen to give visibility over 10 m distance. They aretested to IEC61034-2.Generally cables of this type also combine the properties of low corrosive gas emission and aremanufactured using “halogen free” materials.




Low corrosive gas emission cables which have limits on acid and corrosive gas emissionwhen assessed by burning samples of materials in a bench tube furnace. The acid gasemission test of IEC60754-1 and / or the indirect corrosivity test of IEC60754-2 may be used.Products meeting the requirement of less than 0.5% acid gas emission when tested toIEC60754-1 are often referred to as “halogen free”. In some standards an additional test for fluorine content (IEC60684-2) is required.Cables having low emission of toxic gases are generally restricted to specific applications wherethe users have imposed such a requirement. Such cables are particularly found in the railsector.

Although, as previously stated, the IEC fire tests are normally used in conjunction with Nationalor Regional cable product standards, the second edition of IEC60502-1 published in 2004 (18) includes cables “which exhibit properties of reduced flame spread, low levels of smoke emissionand halogen free gas emission when exposed to fire”.

Although the existing suite of IEC TC20 standards does allow an overall approach to fire safety,as illustrated by IEC60502-1, by combining the various elements (it is common for product

standards to call up IEC60332-3, IEC61034, IEC60754-1 and/or IEC60754-2), recentdevelopments have been towards a more integrated approach. Examples of National andregional standards that incorporate the measurement of heat release, smoke obscuration andcombustion gas release in a vertical flame propagation test already exist in North America andEurope. Of particular importance to the European market is the development of EN50399 (19,20) which is a test standard based upon the apparatus of IEC60332-3-10 with the addition of anexhaust duct equipped to measure heat release rate and smoke production rate.

Figure 8 Schematic of prEN50399 test apparatus

TEST CHAMBER

IEC 60332-3-10

TEST CHAMBER

IEC 60332-3-10

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Figure 9 prEN50399 apparatus Figure 10 Typical SPR and HRR curvesEN50399 has been developed to support the classification “Classes of reaction-to-fire for electric cables” given in Commission Decision of 27 October 2006 amending Decision2000/147/EC implementing Council Directive 89/106/EC as regards the classification of thereaction-to-fire performance of construction products. The essential requirements of theCommission Decision are given in the following table.

(1) For the product as a whole, excluding metallic materials, and for any external component (i.e. sheath) of theproduct.(2) s1 = TSP1200 ≤ 50 m

2and Peak SPR ≤ 0.25 m

2/s

s1a = s1 and transmittance in accordance with EN 61034-2 ≥ 80%s1b = s1 and transmittance in accordance with EN 61034-2 ≥ 60% < 80%s2 = TSP1200 ≤ 400 m

2and Peak SPR ≤ 1.5 m

2/s

s3 = not s1 or s2(3) For FIPEC20 Scenarios 1 and 2: d0 = No flaming droplets/particles within 1200 s; d1 = No flaming droplets/

particles persisting longer than 10 s within 1200 s; d2 = not d0 or d1.(4) EN 50267-2-3: a1 = conductivity < 2.5 μS/mm and pH > 4.3; a2 = conductivity < 10 μS/mm and pH>4.3; a3 = nota1 or a2. No declaration = No Performance Determined.(5) Air flow into chamber shall be set to 8000 ± 800 l/min.FIPEC20 Scenario 1 = prEN 50399-2-1 with mounting and fixing as belowFIPEC20 Scenario 2 = prEN 50399-2-2 with mounting and fixing as below(6) The smoke class declared for class B1ca cables must originate from the FIPEC20 Scen 2 test.(7) The smoke class declared for class B2ca, Cca, Dca cables must originate from the FIPEC20 Scen 1 test.(8) Measuring the hazardous properties of gases developed in the event of fire, which compromise the ability of thepersons exposed to them to take effective action to accomplish escape, and not describing the toxicity of these gases.

Figure 11 Classes or reaction-to-fire performance of electric cables

Smoke production (2, 7) and

Flaming droplets/particles (3) and

Acidity (4, 8)

FS ≤ 1.5 m; and

THR 1200s ≤ 15 MJ; and

Peak HRR ≤ 30 kW; and FIGRA ≤ 150 Ws-1

FIPEC20 Scen 1 (5)

and

B2ca

No performance determinedFca

H ≤ 425 mmEN 60332-1-2Eca

H ≤ 425 mmEN 60332-1-2



Acidity (4, 8)


Peak HRR ≤ 400 kW; and

FIGRA ≤ 1300 Ws-1

FIPEC20 Scen 1 (5)

and

Dca

H ≤ 425 mmEN 60332-1-2



Acidity (4, 8)

FS ≤ 2.0 m; and


Peak HRR ≤ 60 kW; and

FIGRA ≤ 300 Ws-1

FIPEC20 Scen 1 (5)

and

Cca

H ≤ 425 mmEN 60332-1-2

H ≤ 425 mmEN 60332-1-2



Acidity (4, 8)

FS ≤ 1.75 m and

THR 1200s ≤ 10 MJ and

Peak HRR ≤ 20 kW and

FIGRA ≤ 120 Ws-1

FIPEC20 Scen 2 (5)

and

B1ca

PCS ≤ 2,0 MJ/kg (1)EN ISO 1716Aca

Additional classificationClassification criteriaTest method(s)Class




Although the EN50399 test equipment is based upon the pre-existing IEC60332-3 series, resultsfrom the two procedures are not comparable because the EN50399 procedure is based upon astandardised “worse case” cable mounting as adopted in the Decision, whereas the IECprocedure is based upon an “as installed” cable mounting. Smoke production is also measuredin the dynamic EN50399 test but the resolution is such that the test is not capable of measuringthe low levels of smoke associated with state of the art low smoke cables for metro applicationsand the like. The IEC(EN)61034-2 method is therefore included to assess the highest class.Acidity is assessed using the existing EN50267-2-3 method which is technically equivalent toIEC60754-2.Whilst the inclusion of an “acidity” requirement in the decision has caused much debate, currentwork (21) is continuing to demonstrate the large contribution of HCl to the fire hazard.Interesting new work studying the effect of important irritant gases on animal lungs (22) has alsofound that HCl and PVC smoke inhalation caused an acute effect with a rapid decline in the lungphysiology parameters. This work would appear to support a cable industry position to offer for particular applications products not releasing important irritant gases (i.e. HCl from PVCcompounds) due to the ability of such gases to hinder escape and damage lungs and reinforce

the position of “acidity” as an “indicator” for important irritant toxic effects.

POSSIBLE FUTURE DEVELOPMENTS

It is clear that the cable industry, particularly in Europe, will be heavily involved with thenew integrated reaction to fire test approach of EN50399 and this, together with the other testrequirements of the European classification, will dictate its position towards the hazards of combustion products for the immediate future. It is likely that EN50399 will be considered byIEC at the next major review of spread of flame test methods. This is due to be conducted 2012 – 2015. It is also intended at this review to seek a greater alignment between IEC and NorthAmerican test methods through the established IEC / IEEE liaison.

The future direction of any work on toxicity is less clear. In the absence of any strong regulatoryof user driven impetus, it is likely that the industry will continue to monitor with interest thedebate in expert areas such as ISO TC92. Some consideration as to suitable product tests thatcould provide data to be used in fire safety engineering studies such as escape modelling will benecessary to put the industry in a position to respond to any external drivers. In line with theindustry’s known preference for larger scale product tests and with the development of real timeFTIR techniques for measuring effluent in ISO, some preliminary work has already been carriedout in Europe to determine if it is possible to measure effluent quality during the EN50399 test(23).




Figure 12 Typical output curve of toxic gas productionFor the commonly used cables investigated in this work, a good corroboration in ranking wasdemonstrated when using:

- FEC index derived from modified EN50399 test- ITC index from bench scale toxicity test (NFX70-100)- Acidity classification from IEC60754-2

and it could be concluded that acidity was shown to be a good predictor of irritant effectdetermined as an FEC index according to ISO TS13571. However, it is clear that much further work would be necessary before any standardisation could take place.Other workers are known to be working on the modelling of the large scale test by a small scaletest such as the Purser furnace.

CONCLUSIONS

In reacting to the requirements of its customers with regards to the reaction to fire performanceof its products, the cable industry has historically taken a somewhat pragmatic approach rather than seeking perfection. This approach has concentrated on the macro rather than microeffects relating to burning cables. The industry has been at the forefront when considering thefire performance of its products and the current activities suggest that this will continue. An

ability to react to user needs and develop new tests and requirements and embrace consequentchanges to product offering has been clearly demonstrated. The industry continues to offer arange of products with different reaction to fire performance consistent with users needs for particular applications and installation conditions.The principles established some 20 years ago:

- control the burning- control the smoke emission- control the emission of the most important corrosive and irritant gases- hold a watching brief on the development of toxicity guidance and test methods

still remain valid today. Refinement and improvement have been achieved in the interveningyears and the advent of new requirements based on integrated tests involving heat release has




been, and will continue to be a major challenge particularly in Europe. The effects of the newEuropean regulatory framework for the classification of a cable’s reaction to fire performance willbe a major change from the existing voluntary position.Through its ongoing and demonstrated ability to invest in the study of the fire performance of itsproducts, the cable industry will no doubt meet these challenges and any that may result fromregulatory or end user demand to introduce any “toxicity” requirement.




REFERENCES

1 Zanelli, C, Philbrick, S, Beretta, G, “Cavi e pericolo di incendio”Cired London 19732 Philbrick, S, McConnell,J, Cables having improved fire performance” Jicable 84,

Versailles 19843 Journeaux, T, Beratta, G, Gautier, P, “Development of cables with improved fireperformance characteristics” Jicable 87, Versailles 1987

4 Journeaux, T, « The development of new standards for offshore cabling » PRIConference Polymers for Offshore Cabling Proceedings, London 1987

5 Gibbons, J, Stevens, G, “Limiting the corrosion hazard from electrical cables involvedin fires” Fire Safety Journal 15 p183-190, 1989

6Telecom Australia, Design standards Branch, HQ Fire loss report, 19877 IEC Technical Specification IEC60695-7-50, TS Fire hazard testing – Part 7-50:

Toxicity of fire effluent – Estimation of toxic potency: Apparatus and test method8 Stevens, G, “The appraisal and significance of acidic gas emissions from burning

electric cable materials”, 5th BEAMA International Electrical Insulation Conference,

Brighton 19869 Journeaux, T, “The development and manufacture of Sizewell B cables”, Proceedings

IEE International Conference on control aspects of the Sizewell B PWR, p74-79London 1992

10 IEC Standard IEC60332-3-10 Tests on electric cables under fire conditions – Part 3-10: Test for vertical flame spread of vertically mounted bunched wires or cables –Apparatus

11 IEC Standard IEC 60754-1:1994 Test on gases evolved during combustion of




materials from cables – Part 1: determination of the amount of halogen acid gas12 IEC Standard IEC 60754-2:1997 Test on gases evolved during combustion of electric

cables – Part 2:Determination of degree of acidity of gases evolved during thecombustion of materials taken from electric cables by measuring pH and conductivity

13 IEC Standard IEC 61034-2:2005 Measurement of smoke density of cables burningunder defined conditions – Part 2: Test procedure and requirements

14 IEC Technical Specification IEC TS 60695-9-2:2005 Fire hazard testing – Part 9-2:surface spread of flame – Summary and relevance of test methods

15 IEC Technical Specification IEC TS 60695-5-2:2002 Fire hazard testing – Part 5-2:Corrosion damage effects of fire effluent – summary and relevance of test methods

16 IEC Technical Specification IEC TS 60695-6-2:2005Fire hazard testing – Part 6-2:Smoke obscuration – Summary and relevance of test methods

17 IEC Technical Specification IEC TS 60695-7-2:2002 Fire hazard testing – Part 7-2:Toxicity of fire effluent – summary and relevance of test methods

18 IEC Standard IEC 60502-1:2004 Power cables with extruded insulation and their accessories for rated voltages from 1 kV up to 30 kV – Part 1: Cables for ratedvoltages of 1 kV and 3 kV

19

Journeaux, T, “Development in regulatory classification methods that will affect theEuropean cable industry and its suppliers”, Flame Retardants 2008, InterscienceCommunications Ltd, London

20 Draft prEN 50399 Common test methods for cables under fire conditions – Heatrelease and smoke production measurement on cables during flame spread test –Test apparatus, procedures, results, CLC TC20/Sec1577/INF June 2008

21 Hull, T, Stec, A, Paul, K, “hydrogen Chloride in Fires”, IAFSS 9th Internationalsymposium, Karlsruhe 2008

22 Hertzberg, T, Blomqvist, P, Lastbom, L, “Influence of HCl and PVC-smoke on isolated




and perfused guinea pig lungs, SP Fire Technology Report 57, 200623 Journeaux, T “Investigation of smoke toxicity of burning cables”, SCI Fire Chemistry

discussion group, London 2005

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