The cost effective replacement of halogenated materials for the wiring of buildings

Transcription

1 The cost effective replacement of halogenated materials for the wiring of buildings James E. ROBINSON Over the passed two years the volume sales of new technology non halogen flame retardant materials for the wiring of buildings has seen rapid growth. It is apparent that cost effective replacement of PVC for data distribution and mains wiring has become a reality. The object of this paper is to take stock of the current situatuation. To describe both successes and limitations of the new technology and to present current and future possibilities for cable design. 1. Introduction The wiring of buildings is an emotive subject. Hitherto the unchallenge domain of PVC, the status quo is under threat from many directions. Change is in the air and from all points of view the questions are daunting. PVC is by far the largest wire and cable segment (in Europe 7 kta of compound) and building wire is approximately 5% of this volume. Industrial power cables (e.g. 1 kv) is the second largest category. Low smoke zero halogen materials, which in Europe are the only commercial alternative to PVC, have made significant inroads into the power cable market but with exceptions rather more limited progress in building wire applications. Building wires fall into three categories: - mains (22/24 Volt) wiring - telephone cables - data cables. In most European countries the mains wiring has PVC insulation and jacket, the telephone cables PVC insulation and jacket and the data cable PVC jacket and polyolefin insulation (mostly MD or HDPE). Although the above is well established a few exceptions have become apparent - notably in the UK where, following a fire disaster at the Kings Cross underground station, the use of LSZH cables in public premises is mandatory. As a result a series of specifications have become established:

2 Table 1. UK - LSZH specifications for Public Building Application PVC LSZH Mains wiring BS 64 BS 7211 Telephone CW 138 CW 16 Data Performance + Composition Performance + Composition Elsewhere in Europe change is rather more piecemeal. Based on power cable technology, mains wiring can generally be specified as LSZH but Telephone and Data are more generally described in terms of performance and/or composition. Although building wire falls into 3 categories much of the ongoing debate concerns the explosive growth of Local Area Network (LAN) computers and their associated data cable which link them all together. It is generally estimated that on average each computer has approximately 1 m of LAN cable. Data cable has electrical performance far exceeding that of telephone cable and so a futher tendency is to also use LAN cable for the wiring of the internal telephone systems. These cables are invariably installed in hidden voids above false (plenum) ceilings, behind walls or under suspended floors (1). A further problem is that as computers become more powerful so to must the LAN cables be upgraded. Invariably redundant cables are not removed but the new replacements just installed on top. The concern now is the fire loading that such installations imply. A further area of debate concerns the possible use of US style plenum cables (2,3). The proponents of these systems have produced impressive data demonstrating their improved fire performance compared to LSZH types. This conclusion is of doubtful value as the overwhelming majority (>> 8%) of existing installed data cable is jacketed with PVC. It is unclear if they are proposing plenum cables as suitable for installation on top of existing PVC cables or merely as options in start again scenarios. The latest twist in this scenario is the impending Euroclassification of cables (4). Although initially intended as a trade liberalisation measure, the estabishment of Euroclassifications will give the civil engineer the possibility to select cables on the basis of fire performance categories and hazard assessment. Additional guidance in the form of building directives will also move us to a more rational basis for the specification of building wires. 2. Economics of Cable Manufacture Essentially 3 types of cable have to be considered - the established PVC standard, LSZH types and US style plenum cable. Unfortunately they are not equally priced. As we all know in the short term cost and price are not always related. To address this problem we have circulated draft versions of the costings shown Table 2 to a number of cable makers for comment. In passing it is perhaps fair to note that the biggest debate concerned the correct allocation of overheads!! In the light of these comments the numbers have been adjusted and we believe reflect the long term cost implication of adopting the cable types described. Ultimately the rules of supply and demand will ensure prices match these ratios.

3 Table 2. Cable Cost Comparison Cost PVC LSZH (Low Cost) LSZH US style Plenum Copper Polymer Manufacture Overheads Total The reference PVC refers to cable jacketed with a low cost plasticised PVC of a type typically used for building applications such as data cable. The cable had a polyolefin insulation. LSZH (low cost) refers to a similar cable with a NHFR jacket based on CASICO technology. CASICO is an acronym for a flame retardant technology (5, 6, 7) based on the composition - Calcium Carbonate, Silicon Elastomer, Ethylene Copolymer. The reference LSZH refers to a similar cable this time jacketed with metal hydrate filled EVA. US style "Plenum" refers to cable with a very flame retardant PVC or FEP jacket and FEP insulation. In this paper the LSZH reference refers to the low cost cables based on CASICO technology. 3. Technical Performance 3.1. Heat Release According to some... all will be well if the heat release is controlled. Whilst not fully subscribing to that argument it is clear that heat release is a very important parameter in a given fire scenario. In addition Heat Release will be an important criteria for Euroclassification. Data for a number of data cables is shown Table 3/Fig 1. Fig 1. Cone Evaluation of Data Cables (Taken from Heinonen et al (8)) Rate of heat release Rate of smoke release Heat [kw/m] 4,5 4 3,5 3 2,5 2 1,5 1,5 Smoke [m²/sm],1,8,6,4,2 FTP-PVC FTP-FR487 UTP-PVC UTP-FR Time [s] Time [s]

4 Table 3. Summary of cone data Property FTP-PVC UTP-PVC FTP-FR 487 UTP- FR 487 Heat of Combustion kj/m Heat Release (average) Heat Release (peak) Ignition Time SEA CO CO CO/CO Heat release can ce controlled either by using material with inherently lower heat of combustion or by using less of a given material. Recently published results (8) have demonstrated how the use of improved dielectric materials allows cable downsizing with consequential improved economics and fire performance. In terms of data cable the use of foamed dielectrics + reduced jacket thickness seem obvious avenues to explore. Cable design on this basis is not difficult as heat release can be readily established by summing the component parts of a given cable construction Fig 2. Fig 2. Summation of Component Parts to predict overall Heat of Combustion Insulation Bedding Jacket Cable Data PVC/PVC 481(.7mm)/ (.5mm)/484 BC11W(.5mm)/ Smoke Control the heat release and you control the smoke... Recent data established in large scale fire experiments indicates this is not always true. This was hinted at in the building wire work where poor correlation between smale scale (cone calorimeter) and medium scale (IEC metre cube) was noted (Fig 3). Recent studies (9) involving the same cables in a large scale plenum fire test (Fig 4) completely vindicate the IEC 134 results.

5 Fig 3. Smoke density - IEC 134 (from Heinonen et al (8)) Transmittance [%] NHMH(1) -.5mm PVC -.7mm NHMH(2) -.5 mm Time [h] Studies on the large scale plenum rig give rapid fire spread and dense smoke for the conventional PVC cables. The LSZH cables gave significantly superior smoke performance in all stages of the fire - although exhibiting high heat release between 5-1 minutes after the start of the fire. On this basis the concern for the safety in building having overlaid PVC cable network appears justified. Fig 4. Large scale building wire fire test in BRE plenum rig (from Robinson (9)) (In Preparation) The dissapointing correlation with small scale cone calorimeter data is also of concern as we lack a route to predict cable performance in this area. Fig 5. Smoke Density - Cone Calorimeter (from Heinonen et al (8)) Specific Extinction Area - m2/m 8, 6, 4, 2, Insulation Bedding Jacket Cable Data 8,, PVC/PVC 481(.7mm)/ (.5mm)/484 BC11W(.5mm)/484,

6 3.3. Toxicity and corrosivity. Toxicity is more related to heat release than compostion... This concept is difficult to accept. The idea that we could knowingly fill buildings with materials giving highly toxic products of combustion but low heat release seems unacceptable. Further understanding is needed. Chapin has recently proposed a mechanism showing how hydrochloric acid produced during the combustion of PVC reacts with calcium carbonate to produce calcium chloride. This mechanism is not fully robust as studies comparing combustion fumes from PVC and LSZH cables resulted in the PVC cables generating copious quantities of hydrochloic acid. Presumably the scavenging of the combustion fumes is also linked to combustion conditions. Table 4. Analysis of Internal Telephone cable fire gases - MSC.41(64) (from McKeirnan et al (1)) Compound 1 pair PVC 1 pair NHFR 1 pair NHFR CO 55ppm 141ppm 89ppm CO HCl 181 <35 <35 HBr <5 <5 <5 HF <5 <5 <5 HCN <1 <17 <17 NO X <1 <38 <38 SO2 <5 <15 <15 Acrolein 74 <5 <5 Formaldehyde 87 <5 <5 4. Discussion 4.1. Safety Safety like quality is not an abstract concept. Aircrafts are designed to provide safe and reliable transport from A to B and the basis of this is reliability analysis. Two concepts are important - fail safe and safe life. Fail safe implies redundancy - if an item fails the system functionality will be maintained by a duplicate item. An example of this is split braking systems in cars. Another example could be the use of plenum rated cables which in the event of fire will not burn in whatever circumstances. Safe life involves the calculation of cumulative probabilities that an event will occur. The starting point is the acceptable frequency for failure. Invariably a catastrophy is the result of a series of events and overall frequency the cumulative probability of the series of events occuring in the required sequence. Both concepts are equally valid but generally safe life is more cost effective. In cases where redundancy is not possible safe life is by definition the design standard. In the case of hidden cables it may be equally appropriate. For example, if the risk assessment demonstrated that the consequences of a fire involving an existing cable ladder in a hidden void were unnacceptable, the probability of the cable catching fire could be reduced by retrospectively replacing the cable ladder by a metal tray, fixing permanently the ceiling

7 tiles beneath the existing tray or perhaps modifying the ventilation sytem if the void is also used for space ventilation. The key to this is the definition of building design standards coupled with a good understanding of product performance. The risk assessment will then be based on the acceptable frequency of events occuring: Major Fatallity 1 in 1 years Extensive Building Structural Damage 1 in 2 years Extensive Equipment Damage 1 in 3 years Minor Short Business Interruption 1 in 5 years 4.2. Installed PVC cable Although there are mitigating factors, the shear volume of installed data cable is of concern. It is clear that the explosive growth has not allowed the appropriate building standards to become established and the safety of the installed cable must be cause for concern. The first step must be to assess the hazard. Plasticised PVC burns with rapid flame propagation and with heavy smoke. Routes to improve fire safety could include replacing ladders with trays - preferably covered. The use of sprinklers. Segregation of mains power cables - a potential source of ignition. Assessment of ventilation strategy to allow fire smoke extraction. Perhaps a greater problem is upgrading an installed network. Laying new cables of no matter what type on top of existing PVC cable does not seem sensible. Removal of the old cables is certainly the best option. If this is impossible, then segregation of the new cables (using adequate installation practice) may permit future removal of the old during some future updating of the system The use of LSZH cables The use of these cables is well established in LV applications. The evidence presented shows that these cables exhibit low smoke and reduced toxicity/corrosivity (CO and HCl) compared with standard PVC cables. Recently introduced polyolefin based jacketing products give the possibility to achieve this improvement with only a modest cost increase. These cables are not inherently safe. They are not fail-safe and as a result must be used in a properly considered installation strategy (11). Higher performance cables using materials such as traditional hydrate fillers or inherently flame retardant polymers such as FEP are much more costly and their relative safety is still a matter of debate. Again their use seems satisfactory as part of a proper cable strategy. The doubt is whether their much higher cost could be compensated with reduced installation costs.

8 5. Conclusion Change in itself is not a bad thing and out of change new, safer and more cost effective buildings will result. Piecemeal upgrading of technology is not a good option. The future must lie in properly considered design directives coupled with appropriate performance data for all products intended for use within buildings. 6. References [1] Fardell P.J. et al, Cables in concealed spaces - a full scale test facility for standards development, Interflam 96, Cambridge, UK. [2] Hoover J., Caudill E., Chapin T.J., Clarke F., Full scale Fire Research on concealed space communication cables, Interflam 96, Cambridge, UK. [3] Chapin T.J. Tan T.C., Willis A., Pye K., Hoover J.R., Caudill L.M., Full-scale UK fire tests of LAN Data Communication cables in concealed-space applications. Seminar, Loss Prevention Council, Borehamwood, UK, Feb 98. [4] Evans G., Progress on standards for the implementation of the construction products directive, Fire Retardants 98, London Feb 98. [5] Robinson J.E., Wiklund O.A., Trends in Internal Wiring, Plastics in Telecommunications VII, London, Sept 95. [6] Hirvensalo M.W., Robinson J.E., Smith T. & Wiklund O.A., Silicon Stabilised Char as a Flame Barrier, Fire & Materials, Washington, Nov [7] To be published - Sultan IWCS. [8] Heinonen M., Robinson J.E., The impact of the type of insulation on overall fire retardancy, Fire Retardants 98, London. [9] To be published - Robinson J.E., Low cost building wire in large scale fire, Plastics in Telecommunication VII, London Sept 98. [1] To be published - McKeirnan N., Robinson J.E., Rutherford P., Mikkola E., The development of a cost effective Internal Telephone cable having good flame retardancy and the lowest possible halogen content, Plastics in Telecommunication VII, London Sept 98. [11] Building Design Directive, BSI, DD24, Part 1, 1997.