Properties of PVC compounds with improved fire performance for electrical cables

Fire Safety Journal 19 (1992) 55-72

~

Properties of PVC Compounds With Improved Fire Performance for Electrical Cables Alister. F. Matheson, Robin Charge Hydro Polymers, Newton Aycliffe, Co. Durham, DL5 6EA, UK

& Tor Corneliussen Norsk Hydro a.s., Petrochemical Division, Porsgrunn Fabrikker, N 3901 Porsgrunn, Norway

ABSTRACT This paper discusses the features of PVC-based compositions designed for use as insulation and sheathing on electrical cables which offer improved resistance to ignition and fire spread compared to conventional PVC cable compounds. Data is provided which demonstrates reduced emission of hydrogen chloride following combustion for these fire retardant low acid (FRLA) materials. A comprehensive range of standard tests for fire behaviour and also for other important cable parameters such as mechanical strength, ageing behaviour and electrical performance are reported for PVC, F R L A - P V C and for non-halogenated low smoke and fume (LSF) compositions. A summary chart lists the strengths and weaknesses of the different materials. It is concluded that all of these cable materials have a role to play depending on the applicational requirements.

INTRODUCTION PVC in its plasticised form provides a highly versatile, flexible material which combines the key benefits of ease and speed of processing, numerous formulation options ranging from semi-rigid to very soft and an outstanding cost/performance relationship. In the cable industry it has a proven track record in a wide range of service conditions and is 55 Fire Safety Journal 0379-7112/92/$05.00 © 1992 Elsevier Science Publishers Ltd, England. Printed in Northern Ireland

56

Alister F. Matheson, Robin Charge, Tor Corneliussen TABLE 1

UK Breakdown of Materials for Cables Material type

Proportion

(~) PVC compositions Polyolefins Elastomers Specialities

58 26 16 <1

used in applications ranging from thin wall insulation, for example on telephone singles and switchboard j u m p e r wire, to sheathing for large power cables. PVC-based materials are not hygroscopic and they provide cables which can be easily installed and jointed. A typical UK breakdown of polymer based material usage in cables is tabulated in Table 1. In western Europe, PVC c o m p o u n d offtake for cables has increased steadily with a growth trend of +3-7% per year over the 5-year period, 1984-1988 as shown in Table 2. The subject of fire performance of cables first came to the fore after a major conflagration which burnt down the La Spezia power station in Italy in 1967. One lesson that was learnt from this event was that high cable densities with their larger fuel load of polymeric insulation and sheathing required much more stringent and relevant fire test evaluation of the complete cable assembly than had been the case hitherto. PVC compounders responded by formulating compositions with enhanced resistance to fire propagation to meet the new requirements, and the search for ever better fire performance materials has continued since. Fire is an unexpected event which is often frightening as well as dramatic. It can lead to loss of life and to expensive damage to property and equipment. Very large fires, especially in public areas, are TABLE 2

PVC Cable Compound Usage, W. Europe Year

PVC compound (KT)

1984 1985 1986 1987 1988

640 670 700 720 740

Features of fire performance PVC for electric cables

57

highlighted in the media with severe resultant pressure of public opinion on the authorities and specifiers involved to be seen to take quick and decisive corrective action. In this environment responses can, understandably, often be emotive, hasty and ill-considered. A good example of misleading statements after a fire episode can be quoted from the South Atlantic in 1982. During the conflict the British warship HMS Sheffield was hit by a missile, took fire and had to be abandoned. Pundits, having observed the spread of fire accompanied by a pall of dense black smoke, attributed the cause to PVC cabling. However, there were virtually no PVC cables employed on this ship (PVC represented <4% of cabling material) and the subsequent official report by the House of Commons Defence Committee exonerated the cables and pointed to inadequate fire zoning as the cause of fire and smoke spread.~ In this paper, fire performance parameters of PVC, FRLA and LSF, (non-halogenated) will be discussed, making use of six aspects of fire behaviour tabulated below: Primary features

Ignitability Fire propagation Heat release

Secondary features

Corrosive emission Smoke Toxic fumes

In addition, other performance parameters relevant to cable design will be examined including mechanical and electrical behaviour under various test regimes.

FRLA-PVC WITH IMPROVED FIRE PERFORMANCE CHARACTERISTICS PVC when exposed to heat emits hydrogen chloride into the gaseous phase and forms a cross-linked char in the solid phase. It interacts with the developing fire in both the solid and gaseous phases. Thus the char acts to insulate parts of the solid phase more remote from the fire source from heatspread as well as reducing oxygen penetration, while the HCI reacts with the energy rich, oxygen-based, chain branching free radicals in the gaseous phase which propagate the flame, and thus acts as a chain terminator. The subject of PVC in fire has been discussed by Briggs. 2

581

Alister F. Matheson, Robin Charge, Tor Corneliussen

By formulation the compounder seeks to optimise fire retardant characteristics whilst retaining the well known beneficial mechanical and electrical features of conventional PVC compositions. The first advance achieved was the development of highly fire retardant compounds which have proved successful in passing stringent cable fire propagation tests such as IEC 332-3 where a number of cables are tested in the vertical mode. Outside the cable industry another proven example is successful use of PVC conveyor belting in deep mines by British Coal following a serious fire at Cresswell in the English Midlands some 30 years ago. By the 1970s the risk of corrosive damage by hydrogen chloride was causing concern, though this has sometimes proved exaggerated. Thus work on the effect of fire on reinforced concrete by the Fire Research Station in the UK demonstrated that 'long term effects associated with exposure to PVC fires was unlikely to be a special problem'. The main cause of damage to the structure was identified as thermal effects of the fire itself. 3 Another factor of importance is that HCI decays by depositing from the fire gases and thus is only transported a limited distance from the seat of the fire. 4 As a result of these concerns however, a new generation of fire performance products was introduced which combined good fire retardant properties with reduced acid emission. These compounds are defined in this paper as F R L A PVC. Tables 3 and 4 list some basic mechanical and fire parameters of the conventional cable materials, L D P E and PVC, and of the improved fire performance products, FRPVC (fire retarded PVC) and F R L A - P V C . The tensile parameters are maintained well by the improved PVC compounds (Table 3) and it can be seen in Table 4 that FRPVC exhibits the highest resistance to combustion as shown by oxygen index (OI) and flammability temperature (FT) but also generates more hydrogen chloride. F R L A - P V C provides a better compromise in that it retains good OI and FT values but reduces HCI emission. L D P E has, of course, a low oxygen index and burns readily. TABLE 3 Mechanical Properties of Cable Materials

Shore A

Tensile strength

Elongation at break (%)

(l~Pa) LDPE PVC FRPVC FRLA-PVC

97 91 94 94

14-8 18"4 18"8 19-4

600 230 250 240

Features of fire performance PVC for electric cables

59

TABLE 4 Fire Parameters of Cable Materials

LDPE PVC FRPVC FRLA-PVC

Oxygen index (%)~

Flammability temperature(°C)~

% By weight of HCI emitted (%)c

17 25 36 32

n/a 180 370 350

0 23 27 15

~ ISO 4589. ~ BS 2782 Pt 1: 143A. c After 60 min in muffle furnace at 600 °C.

By use of fillers such as calcium carbonate which convert hydrogen chloride into the chloride salt, FRLA-PVC compositions are designed to reduce the escape of HCI into the atmosphere by binding the acid into the residual char. Some data comparing FRLA-PVC to PVC is given in Table 5. Both materials start with the same chlorine content but the reduced emission of HCI is clearly apparent for FRLA-PVC from this table. There is an increasing interest in examining the corrosive effect of fire gases rather than simply carrying out chemical analyses of the type listed above. A test developed by the Centre National d'Etudes des T616communications (CNET), the research laboratory for France's telecommunication authority, PTT, is discussed in the paper by Rio & O'Neill. 5 The change in resistance of a standard printed circuit board on which combustion fumes have been condensed is recorded as a percentage change. The CNET test for suitability of materials includes consideration of ignitability, propagation (expressed as oxygen index, OI) and change of resistance (COR). Data for PVC and FRLA-PVC are outlined in Table 6. TABLE 5 Chloride Binding Data

Initial chlorine ( ~ )

After combustion: pH ~

PVC FRLA-PVC

29 29

a CEGB E/TSS/EX5/8056 (1984). b IEC 754-1.

2.08 2.51

CI- (mg/g) a

89 35

HCI in ash (%)

HCI emission

19.4 44.5

24 14

(~)~

Alister F. Matheson, Robin Charge, Tor Corneliussen TABLE 6

CNET Corrosivity Assessment Change of resistance

Oxygen index (%)

Self ignition temperature

COR (%) PVC FRLA-PVC

CNET assessment

(°C) o

14 9

25 32

385 410

Fail Pass

a ASTM D1929.

This table demonstrates that the level of corrosion has been reduced for F R L A - P V C while ignitability has also been lowered. This material passes the criteria laid down while PVC is not acceptable. F R L A - P V C was introduced at the start of the 1980s and has been widely specified by end-users in critical service conditions ranging from sophisticated multicore shielded data transmission cables to insulation and sheathing of power cables for use an offshore oil rigs in the harsh conditions of the North Sea. 6"7 FIRE PARAMETERS HALOGENATED

FOR FRLA-PVC AND NONLSF CABLE MATERIALS

A number of commercially available non-halogenated cable compounds d e s c r i b e d in this p a p e r as L S F w e r e c o m p a r e d to F R L A - P V C in a r a n g e of fire tests. T h e L S F m a t e r i a l s w e r e highly filled c o m p o s i t i o n s b a s e d o n fillers s u c h as a l u m i n a t r i h y d r a t e ( A T H ) w h i c h d e c o m p o s e s e n d o t h e r mically at e l e v a t e d t e m p e r a t u r e t o r e l e a s e w a t e r v a p o u r w h i c h dilutes t h e final v a p o u r . T h e m a t e r i a l s i n c l u d e d in t h e p r o g r a m m e a r e d e s c r i b e d in T a b l e 7. TABLE 7

Cable Materials Evaluated in Test Programme Fire performance material

Polymer structure

Shore A hardness

SG

Main polymeric constituent

Filler content (phr)

FRLA-PVC LSF I LSF II LSF III LSF IV

TP~ TP ~ TP ~ T1~ XL b

94 96 92 95 94

1-6 1.5 1-5 1.6 1.4

PVC EVA EPR EVA EVA

60 140 160 230 120

a TP is thermoplastic. b XL is silane cross-linked.

Features of fire performance PVC for electric cables

61

TABLE $ Fire Temperature Parameters

Ignitubility Flash ignition temperature PVC FRLA-PVC LSF ~

Self ignition temperature

(~c)

(oc)

330 400 390

385 410 420

Propagation flammability temperature

(°c) 180 350 305

a Mean of four LSF materials.

Ignitability and propagation were examined by measuring the flash and self-ignition temperatures (Setchkin furnace method, ASTM D1929) and the flammability temperature. This latter value represents the temperature at which the material will just burn in 21% oxygen and 79% nitrogen, i.e. the atmospheric ratio. The results are given in Table 8. Two important findings can be observed: (1) PVC is more ignitable than either F R L A or LSF. (2) F R L A - P V C is the most effective material at resisting fire propagation. The relationship between the oxygen index (OI) and temperature gives a better insight into the performance of materials in resisting fire propagation at different temperatures. Figure 1 compares two cable PVCs with two cable LSFs and demonstrates a less rapid decline in OI with temperature for the halogenated materials. Thus the nonhalogenated compounds require higher oxygen indices to maintain a high flammability temperature and many fire scientists believe that this latter parameter is a more meaningful assessment of resistance to fire spread under actual fire conditions than OI. More detailed studies of ignitability and propagation behaviour were carried out to follow up on the findings discussed above and these are summarised in Table 9. Four tests were employed to investigate fire spread. The glow wire test (IEC 695-2-1) makes use of a glow wire source, UL94 a bunsen burner with the test being carried out in the vertical mode. The needle flame (IEC 695-2-2) is the smallest standard-sized diffusion flame and is produced from a ground-off hypodermic syringe. In this case, the sample was hung vertically, the flame applied for 120 s and then the distance of travel of the flame before it extinguished was measured.

Alister F. Matheson, Robin Charge, Tor Corneliussen

62

OXYGEN INDEX

~,0%

PR-PVC

~0%

<..

H

N?N

20%

._FLAMM~UTY ~ -TEM~RATUI~ELINE

~

~

TEHPERATURE('El "~'-~" i

Fig. 1.

O I vs

t

,.oo

temperaturfor e

cable materials.

IEC 332-1 is the cable industry's test for a single wire located vertically. A burning/charring limit is required by the specification after exposure to a pre-mixed flame. The results from the four test methods correlate very well and confirm FRLA-PVC's superior performance. Of the LSF materials,

9

TABLE Ignitability a n d P r o p a g a t i o n Tests I

Test

UL94 (vertical)

Plaque Value

Glow wire* 1 mm °C

FRLA-PVC LSF I LSF II LSF III LSF IV

960 850 850 960 750

Test

Material

a IEC 695-2-1. b Unclassifiable. c IEC 695-2-2.

I

1 mm Rating

Needle flamec (120 s) 3 mm

IEC/332-1 Single wire Pass/fail

94 V-0 UC b UC b 94 V-0 UC b

30 80 65 29 95

Pass Fail n/a Pass Fail

Features of fire performance PVC for electric cables

63

TABLE 10 Cone Calorimeter Heat Release Data for Irradiance of 30 kW/m 2

Heat release

FRLA-PVC LSI~

Total (MJ /m 2)

Max. rate (kW/m 2)

53 92

182 286

a Mean of four LSF materials.

compound III is comparable to F R L A - P V C , while the others are inferior. It is apparent that the higher filler loading in LSF III is beneficial in terms of resistance to fire propagation. A very important parameter in assessing the likely contribution of a material to fire growth is heat release and particularly the peak rate of heat release. The higher this value the more likely is the fire to grow quickly and spread throughout a compartment. Listed in Table 10 are heat release data for F R L A - P V C and LSF determined by the cone calorimeter. The halogenated F R L A - P V C has lower heat release values and thus would be predicted to contribute less to the growth and size of a conflagration than the LSF materials. The aspect of toxic emissions is one which is recognised as highly complex. A good review of the current position on fire toxicity, particularly as it applies to cables, has been given by Ness and Scott. 8 A programme reported by the National Bureau of Standards draws the conclusion that fire-retarded products reduce production of heat and toxic species. 1° Hydrogen chloride has been attributed in some standards 11 with unduly high levels of toxicity which appear to date back to values quoted in 194312 but fire scientists now rate it similarly to carbon monoxide, the main toxic emission in fires. 12.13A summary table of toxicity aspects of hydrogen chloride, the initial major emission from burning PVC, and carbon monoxide is given in Table 11. TABLE 11 Toxicity Aspects of HC1a and CO

UN toxicity rating Concentration for death after 30 min (ppm) Toxic potency (rag per 1. min) Odour ~ Vapour.

HCI

CO

Harmful

Harmful

2 000-,l 000 155-190~ Sharp

2 500-4 000 210 None

Alister F. Matheson, Robin Charge, Tot Corneliussen

The UN rating system is three level with 'harmful' being the lowest level. Hydrogen chloride is an irritant as opposed to a narcotic like carbon monoxide, so its presence in a developing fire situation can serve as a valuable warning at levels well below those of dangerous toxicity. S M O K E EMISSION Two procedures have been used in this work to evaluate smoke behaviour. The first makes use of the 3m cube originally developed by London Underground's fire research laboratory to provide a m e t h o d for testing complete cables. The m e t h o d is well known in the European Cable industry and is, for instance, called up in British Standard Specifications, e.g. BS 6724:1986 Appendix F, BS 6853:1987 Appendix B. This smoke test is a cumulative one. IEC's TC20 is currently preparing an IEC specification. The test apparatus is a 3 m cube containing a fan located on the floor to mix the smoke generated, a fire source consisting of a standard container with 1 litre of alcohol which is ignited and above which the test-pieces are placed, a screen to protect the fire from turbulence and a light beam which measures the degree of obscuration as absorbance. The absorbance was calculated in this work by the following equations: Io

for wire; Ao = loglo ~ × F I0

for plaque; A0 per ml = log10 ~- × F ×

SG

sample weight

where Ao is standard absorbance I0 is initial light transmittance It is final light transmittance F is factor related to the cube's geometry and number of samples ( = cube volume/length of light beam × no. of cable samples) The second m e t h o d is the cone calorimeter developed by the National Bureau of Standards in the USA, which contrasts with the 3 m cube in that it represents a dynamic smoke test. The smoke parameter was first proposed by Babrauskas 9 and is calculated thus: smoke parameter = average rate of heat release × specific extinction area

Features o f lire performance P V C for electric cables

65

T A B L E 12 S m o k e D a t a for H a l o g e n a t e d and N o n - h a l o g e n a t e d Materials

Chemical description

Material c

3 m cube

Cone calorimeter

Plaque a

Wire b

Ao max x 10 z (m 2/cm 3)

Ao max (ON) ~ 10 (m 2)

Plaque a smoke parameter × I0-'~ (kW2/kg)

Halogenated

Non-

halogenated

PVC FRLA-PVC CPVC FEP PS/PPO LDPE EVA/ATH

10.1 7.9

1-7 3.2 31.5

1-5 2.1

14.0 14.4

10.3 5-1

na a

na a

2-0 19.4 3.3 0-7

21.4 3.9 1.4

nad

a 3 mm Thick. b 1.5 mm 2 Copper conductor, 2.8 mm OD.

c Commercially available materials. a Not available.

A heat flux of 30 kW/m 2, representing a developing fire, was selected and the smoke parameter was calculated after the specimen had been burning for 5 rain. The data for a number of halogenated and non-halogenated materials are tabulated in Table 12. The smoke data shown in the table confirms that halogen content has no correlation with smoke emission. Since smoke level is associated with particulates, this is as expected but it is useful to make the point to prevent any misconceptions arising concerning the effect of halogen on smoke generation. Thus chlorinated PVC (CPVC) and fluorinated ethylene propylene (FEP) have similar low smoke emission levels to LDPE and ethylene vinyl acetate compounded with alumina trihydrate ( E V A / A T H ) while PVC and F R L A - P V C provide less obscuration than a polystyrene modified polyphenylene oxide cable grade. It should be noted that the PVC compounds in this table were commercially available products which had not been formulated with smoke suppressant additives. Figure 2 graphs the values obtained on 3-mm-thick plaques by the two methods. It can be seen that there is an encouraging degree of correlation for the limited data available and it has been possible to plot a straight line. This suggests that the smoke parameter may serve as a helpful predictor of smoke behaviour for materials.

66

Alister F. Matheson, Robin Charge, Tor Corneliussen

SMOKEP~RAMETER x10-~' 25 ~

®PSIPPO

15 P 5

~LA ,~EVAIATH '

~IoM~ ~

I;

'

2;

'

3~

'

Cone calofimete[ smoke pa[amete[ (@ 30 kW/m ~) ~s 3 m cube absorbance.

The table also shows that the 3 m plaque results successfully provide a similar ranking to the 30 bundle wire test which thus takes the work forward from simple materials to a composite product where copper metal accounts for approximately 30% of the volume of material and the test-piece consists of a complex bundled format. The E V A / A T H composition which typifies the LSF products examined in this paper was confirmed as a low smoke emitter in both plaque and wire tests in the 3 m cube as well as possessing a low cone calorimeter smoke parameter.

M E C H A N I C A L P A R A M E T E R S F O R F R L A A N D LSF MATERIALS Figure 3 depicts a plot of tensile strength against elongation at break for PVC and LSF materials. The highly filled LSF compositions tend to have both lower tensile strength and elongation than the PVC-based materials (both PVC and F R L A - P V C are plotted). In a further test, material extruded onto wire was aged in air (7 days @ 100 °C) and water (7 days @ 23 °C), and the retention of the initial elongation at break for each test material has been depicted in the histogram in Fig. 4. A decline in elongation is evident for the LSF compounds after heat ageing and, in the case of silane cross-linked product, water immersion

Features of fire performance PVC for electric cables l

20

67

~._~

/ O O x~

TENSILE

I

STRENGTH

~

0 "-

(1"1Pa)

o ~ ,

~"

® PVE & FRLA ,"C LSF

/ i

I ~ I I

I I ~ i

\\>( \\

10

~,

\\ \ \

\ \

\\\\

\\\\ x. "~

\ ~

~

~

%

~

~ ~ .~.g~

~ ELONaATIONN ~ K ( ~ ) ~

l~ig. 3.

Tensile strength vs elongation at break.

%RETENllON ~NDEX -110

- 100 B

C

A

-9O

B

cI

~-60

PVE

TP LSF

XL LSF

R e t e n t i o n o f e l o n g a t i o n at break on stripped w i r e insulation. A , initial; B, 7 days at 100°C, air; C, 7 days at 23 °C, water.

Alister F. Matheson, Robin Charge, Tor Corneliussen TABLE 13

Tear Strength (N/mm) Data Material

Ageing conditions Initial

7 days/air/lO0 °C

7 days~H20~23 °C

1 day/oil~/lO0 °C

49 25 42 10

47 32 25 l0

28 26 23 12

42 Nil Nil Nil

FRLA-PVC LSF I LSF III LSF IV a ASTM NO. 2 oil.

also leads to a drop in elongation, presumably due to further curing taking place. In order to evaluate toughness, a tear test on 3-mm-thick plaque, the so-called 'trouser' tear test, and an abrasion test on wire were carried out before and after various ageing regimes. The wire e m p l o y e d throughout the w o r k reported in this p a p e r consisted of 1.5 mm 2 solid copper conductor with a radial thickness of insulation of 0-7 mm and a total overall diameter of 2.8 mm. The next two tables, Tables 13 and 14, summarise the data obtained. In the tear test F R L A - P V C gives the best resistance to tear propagation and is the only material which withstood immersion in hot oil. The silane cross-linked L S F IV displayed the p o o r e s t figures. The abrasion test jig consisted of a chisel-edged jack which scraped a length of 10 mm at a rate of 60 cycles per min. The effect of abrasion is complex, involving not only toughness but surface finish and hardness. F R L A - P V C ' s initial value fell to a constant value of approximately 200 after different ageing conditions. In the case of oil treatment the high n u m b e r of cycles recorded suggests a surface lubrication effect, but the important point was that the i n s u l a t i o n r e m a i n e d in place so that the TABLE 14

Abrasion Testing on Wine (BS G 212) 1d/oil a

Shore A

Initial

FRLA-PVC LSF I LSF III LSF IV

94 96 95 94

520 560 50 460

a ASTM, No. 2 oil.

Cycles to failure 7d/H20/ ld/H20/ 23°C 70°C

180 440 40 380

180 370 25 250

7d/air/ 100oc

ld/oil 90oC

220 990 70 530

> 1000 Nil Nil Nil

69

Features of fire performance PVC for electric cables TABLE 15 Water Absorption (% by Weight) (BS 2782 Method 430D)

Material immersed in water for:

FRLA-PVC LSF I LSF II LSF III LSF IV

phr filler

1 day @ 23 °C

7 days @ 20 ~C

I day @ 70 °17

60 140 160 230 120

0.04 0-32 0.16 0.58 0.12

0.19 0-86 0.29 1.58 0.23

0-26 1.85 0.72 3-07 0.37

test could be carried out. LSFs tended to harden on exposure to heat ageing. In water there was evidence of a progressive decline with more severe immersion conditions which could raise questions about longterm performance in applications where water is expected to be present. In oil the thermoplastic LSFs tended to partially dissolve while the cross-linked material, LSF IV, swelled massively both radially and longitudinally with accompanying loss of strength. An interesting point was the low values recorded for LSF III which was the most highly filled compound with a similar high level of resistance to fire propagation to F R L A - P V C . It is apparent that there is a trade-off for the LSFs examined in this programme between fire performance on the one hand and mechanical and electrical parameters on the other. Another property which can be important for some applications is the ability to limit water and moisture uptake. Results expressed as percentage increase in weight for a 3-mm-thick test-piece, are enumerated in Table 15. F R L A - P V C has the lowest uptake followed closely by LSF IV. Some of the LSF materials exhibited high uptake, notably LSF I and LSF III, and these would probably be unsuitable, for example, for use in sheathing underground cables without design precautions to protect the interior of the cable structure.

E L E C T R I C A L P A R A M E T E R S F O R F R L A - P V C A N D LSF MATERIALS In addition to water absorption, the electrical performance of insulated wire, while immersed in water, was also evaluated as tabulated in Table 16. F R L A - P V C has the best electrical performance and the highest safety margin in terms of voltage breakdown while LSF IV is also

Alister F. Matheson, Robin Charge, Tor Corneliussen

70

T A B L E 16 Electrical Data on Wire

Insulation resistance (M ~ /km ) Initial

FRLA-PVC LSF I LSF III LSF IV

425 36 5 55

at 20 °C 1 day

14 days

225 0.3 -55

225 0 0.0001 56

Voltage breakdown At 70 °C

(kV)

0.4 <0-0005 <0-0005 0-5

>30 5 5 15

satisfactory for low voltage insulation. Neither LSF I or III would be acceptable for insulation purposes.

CONCLUSIONS The programme reported here makes use of standard tests in order to provide guidance on behaviour during combustion. However, it is necessary to point out that full-scale fire safety assessments of cable configurations have not been carried out in this work. The results discussed confirm the value of halogens in polymeric compositions. They help to reduce heat generation in a fire, they contribute towards mechanical and toughness durability and they improve resistance to oil and other solvents. It has also been clearly demonstrated, as expected, that halogen content, whether chlorine or fluorine, has no correlation with smoke emission levels of the final compounded material. F R L A - P V C compounds offer a balanced blend of performance parameters which can be summarised thus: 1. 2. 3. 4. 5.

Enhanced resistance to fire propagation combined with reduced acid emission. Good mechanical and electrical properties. High speed processability on conventional PVC cable extruders. Proven in demanding internal and external service environments. Cost-effective.

The test programme also confirms that the LSF compounds evaluated herein exhibit low smoke and halogen acid emission. Mechanical and electrical performance tended to be lower than F R L A - P V C and there

Features of fire performance PVC for electric cables

71

is some evidence of a trade-off between fire propagation and mechanical behaviour. The final table, Table 17, summarises the features of the different fire retardant compounds designed for cables which have been discussed in this paper. In addition, cost indices have been listed in which the materials are compared to the reference price of conventional 70 °C rated PVC cable compound (BS 6746 Types TI1, TM1). These figures were representative of UK prices pertaining at the time this paper was prepared. In selecting materials for cable applications the end-use is taken into account, together with the environmental extremes the cable is expected to encounter in service and the fire safety requirements. In the latter case primary and secondary fire features must both be considered and weighted. Fire hazard which can be defined as the physical situation and its potential for causing fire damage, and fire risk which is the probability that fire will occur are important considerations.

TABLE 17 S u m m a r y of P r o p e r t i e s for P V C , F R L A - P V C a n d L S F M a t e r i a l s

Primary fire: Ignition Propagation Heat release Secondary fire: Corrosion Smoke Toxicity Toughness: Initial Heat aged Water immersion Oil ageing Durability: Ext. weathering Water absorption Electrical: Insulation Resistance Breakdown voltage Cost index: P, poor; F, fair; G, good.

PVC

FRLA -PVC

EVA/A TH

EVA/A TH (High filler)

XL EVA/A TH

F F G

G G G

G F F

G G F

G F F

P P F

F P F

G G F

G G F

G G F

G G G F

G G G F

G G G P

F G F P

F G G P

G G

G G

? P

? P

? P

G G 1.0

G G 1.3

P P 2-5

P P 2.5

F F 3-0

72

Alister F. Matheson, Robin Charge, Tor Corneliussen

The objective is to select the right materials for the j o b and at the right price. PVC, F R L A - P V C and L S F all have a role to play in the design of cables which are 'fit for purpose'.

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