Development of electric cables for fire situations During the past 25 years, many authorities have become concerned about the dangers of fire: for the safety of the people who might be trapped, the continued working of the electrical circuits associated with safety, the long- term effects of smoke and fume damage on sophisticated electronic equipment and even the effects on the buildings themselves. Although electric cables very rarely cause a fire, they are often engulfed in fires started elsewhere and consequently their constituent parts should not contribute to the fire, help spread it, nor emit gases during combustion that could harm people or damage equipment. This article highlights work carried out by the electric cable industry, often encouraged by customers, to develop new and better materials that will be safer to use should a fire occur. It also gives details of the current standards, both British and the IEC equivalents, which relate to the performance of cables in fires and measure such characteristics as fire survival, fire resistance, fire propagation, smoke emission and toxicity.

by D. W. Brown

Historical Polyvinyl chloride (PVC) has been a popular cable-making material since the 1950s because it has good mechanical and electrical properties, it is easy to extrude onto cables and is relatively cheap to produce. However, although PVC is slow to ignite, when it is involved in a fire it emits dense smoke, which rapidly obscures visibility in a confined space and, in addition, emits hydrogen chloride gas that can maim or kill.

In the late 1970s, London Underground Ltd. (LUL) became concerned a t the incidence of fires that occurred in their railway tunnels due to flammable rubbish collectinq in

BlCC Cables’ response was the introduction of the low smoke and fume (LSF) range of cables for all circuits. These cables do not contain PVC, nor any other materials which would generate dense smoke in the event of a fire and nor do they contain halogens. In the past, some specialist cables with oil or other fluid resisting properties have contained fluorine, but it is now recognised that all halogens, i.e. chlorine, bromine and fluorine are hazardous to health, because they form the acid gases HCI, HBr and HFI when subject to fire and London Underground, therefore should be eliminated from all 22 kV cables and joint systems. Other manufacturers introduced bay

certain areas (caused by the eddiesof passing trains) and then being ignited by discarded cigarette ends (smoking was still allowed on the trains a t that time) or sparks from the shoehail connection. These small rubbish fires often occurred under rows of electric cables for the power, control, signalling and communication circuits, that are installed on hangers fixed to the sides of the tunnels. The fires ignited the PVC oversheaths of the cables and the resultant smoke filled the tunnels and stopped the trains.

British cable makers were each asked to try to find a solution to the problem and the first attempts incorporated flame-retardant additives into the PVC, which limited the fire propagation but did nothing for the emission of smoke and acid gas. The hydrogen chloride gas comes from the polymer and the smoke from the polymer and plasticisers that are incorporated into the PVC to make it more pliable. To make a breakthrough, the cable industry had to move awayfrom PVC.

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Three-core 120 mm2 21 kV cable as supplied for the Channel Tunnel

their own range of low smoke and fume cables, which they designated LSOH, LSZH, LSHF etc.

Three-core 240 mm2 22 kV cable as supplied to London Underground

Materials development The fire retardancy of LSF cables is

achieved through the incorporation of about 60% by weight of aluminium trihydrate (ATH) in the formulation. ATH confers retardancy through the combined effects of diluting the quantity of combustible material available to burn and an endothermic decomposition that liberates water, i.e.:

A1,0,.3H20 -+A120, + 3H,O a t 170°C

The liberation of water, plus the different burning mechanism of LSF type compounds, results in the low smoke characteristic and the problem of corrosive acid gas is avoided by eliminating all halogens from the polymers in the formulation.

Early LSF compounds were based on highly amorphous materials such as ethylene propylene rubber (EPR), but above their glass transition temperature of about 0°C these compounds are intrinsically weak, giving rise to poor abrasion resistance, poor tensile strength and, more importantly, poor tear resistance. Arguably the most important mechanical property of a cable sheathing material is i ts resistance to tearing because cables are often scuffed or cut during installation and this can lead to the sheath

Table 1 Characteristics of LSF and PVC c bles

splitting, particularly a t elevated temperatures, when the underlying construction imposes a hoop stress on the oversheath.

BlCC Cables set itself the target of producing an LSF compound with a tear resistance a t least equal to that of PVC a t both 20°C and 70°C. The latter temperature was chosen as being the sheath temperature of a cross-linked polyethylene (XLPE) insulated cable when fully loaded. The constraints of a high filler content meant that it was necessary to select an ethylene-based copolymer with a melting point in the range 95-1 15°C and, using such a polymer, it was found possible to develop a formulation which easily met the target performance.

The early LSF compounds had another major defect, which was their poor performance in water when compared to PVC. Water vapour permeation rates typically of the order of 25g/m/24h, compared to 3.2g/m/24h for PVC, were the norm. By washing the ATH to remove the bulk of the sodium hydroxide impurity and rendering the balance inactive by the use of organosilane, the surface of the ATH becomes water repellent and the result is an LSF compound which has moisture properties better than those of PVC.

A l ist of the more important characteristics that ought to be considered when customers are specifying the LSF cables required is given in Table 1; the values of PVC are included for comparison.

LSF PVC units ._

tensile strength at break elongation a t break aged 7 days a t 1 OOT, tensile strength at break aged 7 days a t 1 OO'C, elongation a t break hot pressure at 80°C cold elongation a t -1 5°C tear resistance at 20°C tear resistance at 70°C oxygen index temperature index water vapour permeation Kvaluefollowing 7 days in water at 70°C retention of tensile strength following 7 days in water a t 70°C

1 3 160 13

145 5

> 50 9 5

38 290 0.6 130 95

>12.5

> 12.5 >I50

>I50 < 50 >20 8 5 35 NIA 3 .2 30 95

Nlm m2 %

N/mm2 % O h

% N/m m N/mm %02 "C

g/m/24 h MQ km

YO

102

~ ~ ~~

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Fire safety engineering

Engineering, was created. Previously, groups in the International Standards Organisation had considered fire safety overwhelmingly in terms of reaction to fire of the materials and products, i.e. ignition, propagation and spread of fire, smoke density, heat release and corrosive/noxious gas formation. The majority of existing reaction to fire tests look a t only one of these responses to the model fire, and product specifications give individual values for each of the listed response factors. Test requirements often seem arbitraryand based on custom and practice, rather than consideration of actual risk or hazard.

IS0 TC92 SC4 has five working groups developing fire strategy engineering standards:

In 1992, IS0 TC92 SC4 - Fire Safety

WGI Application of fire safety performance concepts to design objectives WG2 Fire development and smoke movement WG3 Fire spread beyond the compartment of origin WG4 Detection, activation and suppression WG5 Evacuation and rescue.

The British Standards Institution has published a draft for public comment entitled ’Draft Code of Practice for the Application of fire safety engineering principles to fire safety in buildings’, a 192 page draft publication issued in 1995. It is designed for use by fire safety engineers as ‘a structured approach to assessing the effectiveness of the total fire safety system in achieving desig n objectives’ .

In the electrotechnical area, International Electrotechnical Commission, Technical Committee 89 has responsibility for fire hazard and i ts WG2 has issued a revised IEC 695-1 -1, ’Guidance for assessing fire hazard of electrotechnical products’, which is in draft form for voting.

Both these draft documents highlight the fire safety engineering approach to fire hazards and they both indicate that ‘the principles, methodology and calculation tools can be applied to the fire safe design of buildings, tunnels, petrochemical plants, offshore oil/gas accommodation modules and transportation’.

Practical experience Cables installed in buildings, tunnels or

other enclosed premises, especially those likely to be used by people, or which contain sensitive electronic equipment, should now fall into three categories:

(b) Those cables that will continue to work for a limited period during the fire, to allow the orderly and controlled shutdown of plant.

(c) Those cables that can be disconnected a t the outbreak of the fire and do not need to survive i t

The cables in all three categories must not contribute to the fire, must not allow flames to propagate along them (either in horizontal or vertical mode), must not generate dense smoke nor emit toxic gases.

Unfortunately it is the well-publicised disasters, like Summerland in the Isle of Man, HMSSheffield in the Falklands, Kings Cross in London, Piper Alpha in the North Sea, and Dusseldorf Airport in Germany which highlight the fact that if PVC cables are involved in a fire then the emission of dense smoke and toxic gases will add to the hazard and may well increase the fatalities and damage. The devastating fire a t Kings Cross underground station on the 18th November 1987 has completely changed the way that LUL now approaches fire safety engineering and the products it permits to be installed in the underground railway network. Similarly, the fire in HMSSheffield altered for ever the specifications of the materials to be used in fighting ships and no doubt the fire at Dusseldorf Airport on the 1 1 th April 1996 will change the attitude to what is allowed to be installed at airports in future.

A customer-led demand criterion often becomes well advanced of the national and international standards appertaining a t the time and the result is that clients with large orders to place overcome their frustration at the lack of suitable specifications by writing their own. A good example of this is LUL,

Channel Tunnel British CrOSSover cavern under construction

(a) Those cables that will survive the fire and continue to operate satisfactorily, even when sprayed with water and hit by falling masonry or other debris.

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Fixing a trough cleat to the21 kvcable in the Channel Tunnel

which realised that a true fire survival cable was required after Kings Cross, i.e. one that would survive an actual fire and the conditions appertaining during the conflagration. In practice this means a cable that will continue to operate with prolonged exposure to heat and flames, while a t the same time being directly impacted by falling objects and receiving a pounding and thorough soaking from a fireman's hose- all a t the same time and on the same piece of cable.

The 1994 LUL specification requires a sample of cable to be subjected to a three hour flame exposure while being directly impacted a t the centre of the heated section throughout the test. The same sample, still energised a t rated voltage, is then sprayed with water for 1 5 minutes, bent through 180" (into a U shape) and given another impact. Finally the same sample must be immersed in water for an hour and then re- energised for a final integrity test. London Underground believes that such a test is representative of the conditions that could so easily occur in a railway tunnel or station fire. The Channel Tunnel Builders believed that in a major conflagration all the cable support systems could collapse, leaving the copper weight of all cables suspended to the fire survival cable. They therefore insisted that the equivalent of 25 kg/m be suspended from the cable, in the centre of the heated section, during the three hour fire test a t 950°C and that circuit integrity a t rated voltage be maintained throughout. Only mineral- insulated copper-clad cables will survive the LUL and Channel Tunnel tests.

On Monday, 18th November 1996, a lorry on a freight wagon caught fire and this led to

.

a major incident in the Channel Tunnel, when the fire spread to adjacent wagons. At the centre of the fire, the heat exceeded 1 OOO"C, sufficient to destroy all the cables a t this position and, in addition, spall the concrete linings such that the reinforcing rods were exposed. However, only a short distance away the mineral-insulated cable survived and continued to power the emergency lighting, albeit without i ts LSF oversheath. In no cases did any of the LSF cables propagate the fire, or add to it in any way, nor did they emit dense smoke or acrid gases. At a position 350 m from the centre of the fire the power cables were examined in detail and although the LSF oversheaths were blistered (due to the heat I i berati ng the water vapour), the insulated cores were untouched and could have permitted the HV power to be reconnected.

Cable standards and their fire-related properties BS 6724: 1990- 600/1000 Vand 1900/3300 VXLPE insulated, armoured cables

British Specification BS 6724, 'Armoured cables for electricity supply having thermosetting insulation with low emissions of smoke and corrosive gases when affected by fire', was first published in 1986. This standard covers the range of single core and multicore armoured cables and requires compliance with the vertical fire test of BS 4066 Part 3, Category NMV 1.5 (IEC 332 Part 3, Category C). Due to advances in material technology many cables are now able to pass the more onerous requirements of Category NMV7 (IEC Category A).

The 1996 version of this specification will require all cables to have minimum light transmittance values of 70% when tested in accordance with BS 7622 (IEC 1034). The light transmittance value is the percentage of light reaching the detector during the 3 m cube smoke test.

As with all cables described as low smoke and fume (LSF), the materials used in the construction comply with the zero halogen requirements of BS 6425 (IEC 754). Unfortunately the test method used is only sensitive down to a limit of 0.5% and this is why this value is stated as the maximum acid gas content for zero halogen cables.

BS 7835: 1996 - 6 kVto 30 kVXLPE insulated, armoured cables

cables in thevoltage range 6-30 kV have been manufactured to BS 6622 (IEC 502). These incorporated PVC as the bedding and sheathing material and, on occasions, special reduced fire propagating PVC was included for use in power stations. The new standard includes many of the established features of BS 6622, but eliminates all the PVC components. Cables to BS 7835 will be 'zero halogen' and will comply with the reduced fire propagation requirements of BS 4066 Part 3, Category NMV 1.5 (IEC 332 Part 3,

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For many years XLPE insulated, armoured

104

Category C). Cables manufactured by some companies will meet the more stringent vertical fire test requirements of Category A.

As the medium-voltage LSF cables are considerably larger in diameter than the 600/1000 V equivalents, it is expected that slightly larger volumes of smoke will be produced in a fire. The maximum value of smoke evolution for these products is sti l l in debate, but for cables up to 70 mm diameter, a minimum light transmittance value of 60% has been agreed. It is probable that the very large cables, with a diameter of 70 < 120 mm, will require a lower test limit of perhaps 50%. When considering the specification values for minimum light transmittance over the 30-40 minute test period, it is worth remembering that similar tests on PVC sheathed cables result in zero light transmittance in less than five minutes.

BS 6387 Fire survival testing BS 6387 'Specification for performance

requirements for cables required to maintain circuit integrity under fire conditions' was introduced in 1983 to try to help installation designers assess the comparative claims of different products. This standard was issued to cover those small wiring cables used for fire alarm and emergency lighting circuits a t 450/750 V and for mineral cables complying with BS 6207, but i ts scope appears to have grown. It was revised in 1994 and attempts to embrace the three physical criteria which cable systems encounter in a fire, i.e. heat, water (from sprinklers) and mechanical shock (from falling debris). The tests are as follows:

fire alone: The 1500 mm sample of cable carries i ts test current a t rated voltage while being heated by a line of gas burners. The cable must maintain circuit integrity for three hours a t 650"C, 750°C or 950°C to obtain an 'A', 'B' or 'C' category rating, 'C' being the highest/ most difficult to achieve. Waterspray: Afresh sampleof cable, with a test current a t rated voltage, is heated to 650°C and this temperature is held for 15 minutes. With the flames still burning, a water sprinkler is applied to that part of the cable in the flames for a further 15 minutes and if circuit integrity is maintained, the cable receives a category 'W' pass. Mechanicalshock: Another fresh sample of cable is fixed to a board of specified dimensions with cable clips. The cable has two right angle bends as specified and the board is hit for 15 minutes while the cable is exposed to the gas flames. Circuit integrity must be maintained a t 650"C, 750°C or 950°C to obtain an 'X', 'Y' or 'Z' category rating, 'Z' being the highest.

Thus the highest category rating of all for BS 6387 is 'CWZ', but because the standard allows the use of three separate samples,

each only has to pass 'C', 'W' or 'Z', not that one sample passes all three.

BS 6207 Mineral insulated copper covered cables

BS 6207 'Specification for mineral insulated copper sheathed cables with copper conductors' covers both the light- duty and heavy-duty product range with voltages of 500 and 750 V. The cable can be supplied with a corrosion-resistant outer covering over the copper sheath and, when required, this outer sheath will have the properties and pass the test for LSF cables.

MlCC cables are insulated with the inorganic mag nesi u m oxide, which gives them a fire survival property and, although the most popular use is for small wiring, the range is quite comprehensive, as follows:

e Light duty 500 V

4 mm2

2.5"'. e Heavy duty 750 V:

400 mm2

25 mm2

-single and twin conductor cables up to

- 3,4 and 7 conductor cables up to

-single conductor cables up to

- 2,3 and 4 conductor cables up to

- 7 conductor cables up to 4 mm2 - 12 conductor cables up to 2.5 mm: - 19 conductor cables up to 1.5 mm

MlCC cables fully comply with BS 6387 and one sample of cable can be used for the three tests to prove the categories CWZ. They also comply with the vertical fire test of BS 4066 Part 3, Category A and the smoke test of BS 7622.

BS 72 7 I Non-armoured thermosetting cables

requirements for non-armoured cables with thermosetting insulation, with rated voltages of 450/750 V and which produce lower levels

This British Standard specifies the M,CC cable and fire survival junction box in the Channel Tunnel

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Channel Tunnel main substation and terminal at Folkestone

of smoke and corrosive gases when subjected to fire than PVC cables manufactured to BS 6004. The single core cables must comply with the relatively simple, short duration flame tests of BS 4066 Parts 1 and 2, but the multicore sheathed cables are also required to pass the more onerous BS 4066 Part 3, Category NMV 1.5 test.

BS 7629 Cables with a limited circuit integrity Cables manufactured to this specification

are primarily intended for use in fire alarm and emergency lighting circuits a t 300/500 V. The cables have annealed copper conductors to BS 6360, type E l 2 silicon rubber insulation to BS 7655, a trilayer flame and electrostatic screen barrier and an LSF oversheath which is also abrasion resistant and complies with the requirements of BS 7629 Type B.

This range of fire-resistant cables has been designed for use where a limited measure of circuit integrity is needed for the safe evacuation of personnel. The cables comply with BS 6387 Category BWX (when the three properties are tested separately) and some manufacturers can achieve Category CWZ. The cables also comply with the fire resistance test of IEC 331 (750°C for three hou rs) .

Emergency Lighting Part 1 Code of Practice, and BS 5389, the Fire Detection and Alarm System Part 1 Code of Practice. Because of the limited applications for this range, the sizes available are also limited to 2, 3 and 4 conductors in 1-0 mm2, 1.5 mm2, 2.5 mm2 and 4 mm’.

These cables are referred to in BS 5266, the

BS 7846 Armoured cables with limited circuit integrity

This standard was issued in September 1996; it covers the same range of 600/1000 V cables as BS 6724, but these cables are of an enhanced design such that, in addition to complying with the fire performance requirements of BS 6724, they will also operate for three hours a t 950°C when tested to BS 6387 Category C. Cables which meet this standard have been available from some

manufacturers for a period of time.

Summary This article has given an indication of the

large amount of research and development work that has taken place during the past 25 years to produce a range of cables that will meet today’s fire performance criteria. The work continues to try to make further improvements because customers are always seeking better characteristics a t lower prices. Unfortunately it has often been disasters that have led to quantum leaps in technology in the past and it is to be hoped that this will not be necessary in the future.

effort is being applied by experts on fire safety engineering to try to ensure that all buildings, tunnels and other enclosed spaces are properly designed with safety in mind, and also that the cables and accessories perform as well as, if not better than, predicted when involved in an actual fire.

The low smoke and fume cable range is now well established and is available for all products, from large power cables to optical fibre cables, where the improvement in the latter is such that even a 288 fibre cable will now pass the flame propagation test of BS 4066 Part 3, the smoke emission test of BS 7622 and the acid gas emission test of BS 6425. There is also a full range of accessories to match the cables, to ensure that the whole system is low smoke and halogen free, i.e. safe for people to use.

your requirements, but for your contract please specify exactly what you require, because the price quoted will reflect your choice.

It is pleasing to note that much time and

The cable industry has a product to meet

0 standard low smoke zero halogen (LSF) flame-retardant cable

0 fire resisting with a limited circuit integrity 0 fire survival

0 IEE: 1997

David Brown is Business Development Manager, BlCC Cables Ltd., Wrexham, Clwyd LL13 9PQ, UK.

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