At present, in cable industry, Fire Retardant, Low Smoke Halogen Free (LSZH) OR Low Smoke Fume (LSF), Fire Resistant cables are all described as Fire Retardant & Resistant Cables.
Fire retardant cables are designed for use in fire situations where the spread of flames along a cable route need to be retarded. Due to relative low cost, fire retardant cables are widely used as fire survival cables. No matter the cables are installed in single wire or in bundles, during a fire, the flame spread will be retarded and the fire will be confined to a small area, thus reducing the fire hazard due to fire propagation.
Low Smoke & Halogen Free & Fire retardant ( LSZH )
LSZH cables are not only characterized by its fire retardant performance but also by its halogen free properties, thus offering low corrosivity and toxicity. During a fire, this cable will emit less smoke and acid gases which may damage the human being and expensive equipment. Compared with normal PVC cable, LSZH cable outperforms by its fire retardant properties, low corrosivity and low smoke emission properties, however, normal PVC cables has better mechanical and electrical properties.
Low Smoke Fume (LSF)
The lowhalogen content and low corrosivity of low smoke fume cables lies somewhat in between fire retardant cables and LSZH cables. Low Halogen cable also contains halogen but the content is much less than PVC cables. LSF cable is designed to reduce the spread of fire, toxic gases and smoke during fire. The LSF cable is usually manufactured from flame retardant PVC blended with HCL additive and smoke absorbent. These materials help improve the fire performance of the LSF cables.
Fire resistant cables are designed to maintain circuit integrity of those vital emergency services during the fire. The individual conductors are wrapped with a layer of fire resisting mica/glass tape which prevents phase to phase and phase to earth contact even after the insulation has been burnt away. The fire resistant cables exhibit same performance even under fire with water spray or mechanical shock situation.
Fire Performance Class
The main concern for the cable in its fire survival properties are its flame spread, smoke characterization and and gas toxicity. In American fire standard, the concern lies more on the first two and it differs from the European standard which concerns all these aspects. In USA , it is believed that the fire hazard is mainly due to CO toxic gas emitted and the heat release during the conversion of CO to CO2 during the fire. Therefore, to control the heat release is the most important concern for reducing the fire hazard. However, in European countries, halogen content, the corrosivity of the gases, the smoke density and the toxicity of the gas are equally important factors affecting the safety and survival of human during a fire.
The European Electrical Committee categorized the fire performance of the cables into three classes, namely
IEC60332-1 、 IEC60332-2 和 IEC60332-3 . IEC 60332-1 and IEC 60332-2 are used to assess the flame propagation characteristics of a single wire. IEC 60332-3 is used to assess the flame propagation chacteristics of bundled cables. Comparatively speaking, IEC 60332-3 for bundled cables is more demanding than IEC60332-1 for single wires.
IEC 60332-1/BS 4066-1 ( Flame Test on Single Vertical Insulated Wires/Cables )
This test details a method of test for the assessment of the flame propagation characteristics of a single wire or cable. In this test , a 60cm cable sample is fixed vertically inside a metallic box and a 175mm long flame is applied at 45C from a gas burner placed at 450mm from the top at the upper portion. The specimen is deemed to have passed this test, of after burning has ceased, the charred or affected position does not reach within 50mm of the lower edge of the top clamp which is equivalent to 425mm above the point of flame application. The test method is not suitable for the testing of some small wires due to the melting of the conductors during the time of application of the flame.
IEC 60332-3/BS 4066-3 ( Flame Test on Bunched Wires/Cables )
IEC60332-3C describes a method of type approval testing to define the ability of bunched cables to resist fire propagation . In this test, a cable specimen, consisting of number of 3.5m lengths of cables are fixed to a vertical ladder tray where they are applied with a flame from a gas burner for a specified times under controlled air flow. Four categories (A, B, C & D) are defined and distinguished by test duration and the volume of non metallic material of the sample under test. The cable specimen is deemed to have met the requirements of the standard if, after burning has ceased, the extent of charred or affected portion does not reach a height exceeding 2.5m above the bottom edge of the burner.
UL Fire standard
If a cable can pass a specified UL fire standard, an UL performance verification mark can be applied onto the cable jacket, illustrating both the UL class and the number. There are four primary fire testing standards as follows:
CMP ( Plenum Flame Test/ Steiner Tunnel Test )
Plenum rated cables meet the NFPA -262 standard ( formerly known as UL910 ) which provides the most stringent requirement of all the tests. Cable samples on a horizontal tray in a tunnel type of chamber are burned at 87.9KW (300000 BTU/Hr) for 20 minutes. To qualify for a plenum rating, the cables must have the flame spread of less than 5 feet or 1.5 meters, with a smoke density during the test of (a) 0.5 peak; and 0.15 maximum average.
The CMP cable is usually installed in air ventilation duct and air return widely used in Canada and USA . The fire retardancy properties of CMP cable is much better than LSZH cable complying with IEC 60332-1 and IEC 60332-3.
CMR ( Riser Flame Test )
Riser rated cables meets UL 1666. Cable samples on a vertical shaft are burned at 154.5KW (527500 BTU/Hr) for 30 minutes. To qualify for a riser rating, the cables must have the flame spread of less than 12 feet beyond the ignition point. This test does not look at the smoke density or toxicity. Riser cable is suitable for vertical shafts not defined as an environmental air plenum.
CM ( Vertical Tray Flame Test )
General purpose cables meets UL 1581. Cable samples on a 8 feet vertical tray are burned at 20KW (70,000 BTU/Hr) for 20 minutes. The cable is deemed to pass the test if the flame spread will not extend to the upper portion and extinguish by itself. UL 1581 is similar to IEC 60332-3C, except for that the number of testing samples is different. This test does not look at the smoke density or toxicity. The CMG cables are usually used in runs penetrating single floor . This cable cannot be installed in vertical pathways.
CMG ( Vertical Tray Flame Test )
This general purpose cables also meets UL 1581. CM and CMG are similar, both recognized in Canada and USA . This test does not look at the smoke density or toxicity. The CMG cables are usually used in runs penetrating single floor . This cable cannot be installed in vertical pathways.
The restricted cable meets UL 1581, Limited-use. The test consists of 25 feet long ventilated tunnel. The cable is placed on a ladder inside the tunnel and and the flame of 30,000 BTU/Hr is applied to the cable 15 seconds on and 15 seconds off five times for a total exposure to the flame of 1 minute and 15 seconds. To quality for this test, after the test flame is removed, the cable can flame for not more than 60 seconds and the charred portion will not exceed by 25%. UL 1581, VW-1 is similar to IEC 60332-1, except for the difference in the time for flame applied. This test does not look at the smoke density or toxicity. The CMG cable is suitable for use in. dwellings and for use in raceway . This cable cannot be installed in bundles and must be protected in metal conduit. This type of cable is the minimum requirement in commercial installations.
IEC60754-1/BS6425- 1 （ Emission Of Halogens ）
This specifies a test for determination of the amount of halogen acid gas, other than the hydrofluoric acid , evolve d during combustion of compound based on halogenated polymers and compounds containing halogenated additives taken from cable constructions . Halogen includes Florine, Chlorine, Bromine, Iodine and Astatine. All these elements are toxic by its nature. In this test, when the burner is heated to 800C, 1g sample is placed inside and the HCL is absorbed into water inside the chamber fed with air flow. The water is then tested with its acidity. If the hydrochloric acid yield is less than 5%, the cable is categorized as LSZH. If the hydrochloric acid yield lies between 5% to 15%, the cable is categorized as LSF. IEC 60754-1 cannot be used for measuring the exact HCL yield if it is less than 5% and thus cannot tell if the cable is halogen free or not. To determine if the cable is halogen free, IEC 60754-2 should be used.
IEC6 0754-2 （ Acidity ）
This test specifies a method for the determination of degree of acidity of gases evolved during combustion of cables taken from the cable sample by measuring its pH and conductivity. The specimen is deemed to pass this test if the pH value is not less than 4.3 when related to 1 litre of water and conductivity is less than 10us/min. When the HCL yield lies between 2mg/g and 5mg/g, a cable specimen can pass IEC 60754-1 but its pH value may be less than 4.3 and therefore will not pass the IEC 60754-2 test.
IEC 61034-1&2/ASTM E662 （ Smoke Density ）
This specifies a test for determination of smoke density. The “ 3 metre cube test ” measures the generation of smoke from electric cables during fire. A light beam emitted from a window is projected across the enclosure to a photo cell connected to a recorder at the opposite window. The recorder is adjusted to register from 0% for complete obscuration to 100% luminuous transmission . A 1 metre cable sample is placed in the centre of the enclosure and is applied with a fire. The minimum light transmission is recorded. The result is expressed as percentage of light transmitted. The specimen is deemed to pass this test ( IEC 61034-1&2) if the value is greater than 60%. The higher the light transmittance, the less smoke emitted during a fire.
ISO4589-2/BS2863 （ Oxygen Index LOI ）
This is a test for assessing the oxygen index of the material in accordance with the test method specified in ASTM D 2863-95 “ Measuring the minimum oxygen concentration to support candle-like combustion of plastics ” . At room temperature, when the oxygen content in the air exceeds the oxygen index, the material will burn by itself automatically. The higher the oxygen index, the more retardant the cable is. For example if the oxygen index of a material is 21%, it means that the material will burn by itself even at room temperature because at room temperature the normal oxygen content is 21%, In general, the oxygen index of a fire retardant cable exceeds 33%.
ISO4589/BS2782.1 ( Temperature Index TI for determination of flammability )
This is a test for assessing the performance of a material when it is tested in accordance with BS2782:Part 1: Method 143A and 143B. The oxygen index of a material will drop when the temperature rises. When the temperature rises and the oxygen index drops to 21%, the material will burn automatically. This temperature is defined as temperature index. For example, the temperature index at room temperature is 50% and when the temperature climbs to 150C, the oxygen index drops to 21C and the coal will burn by itself automatically. The temperature index of the coal is defined as 150C. In general, the temperature index of fire retardant cable exceeds 250C.
NES713 ( Toxicity Index ）
This is a test defined by Naval Engineering Standard which is a directed at the analysis of a specified set of gaseous species which are commonly present in the combustion products of materials used in military application and which may cause lethality at the time of a fire. In this test, a 1 g cable specimen are completely burnt inside a sealed chambers of volume 0.7-1m3 using a burner burner fed with air and gas to give a non-l uminous flame. The resulting chamber atmosphere is quantitatively analysed for a specified set of gases. For each gas, the measured concentration (Ci) is scaled up for 100g and the concentration is recalculated as though the combustion products is diffused into a volume of exactly 1m3. The resulting concentration (C8) is expressed as the ratio of critical factor (Cf) which is equal to the concentration of this gas considered fatal to human for 30 minutes exposure. The ratio C8/Cf are summed for all gases detected to give the toxicity index.. The higher the toxicity index, the more toxic the cable material is. In general, the toxicity index of LSZH material is less than 5. LSZH cable will also emit toxic CO and if the cable materials contains P, N or S, the toxic gases generated will even be greater. Thus, LSZH cable cannot be categorized as toxic free. CM, CMR and CMP cables in general contains halogen content which is essential for passing the strict fire retardancy testing. For example CMP is made from FEP which contains Flourine and is much toxic than normal LSZH cable.
IEC Fire Resistance Testing
Fire resistant cables are designed for maintaining circuit integrity during a fire. Both the IEC and the BEC adopted two different standards, namely the IEC 60331 and BS 6387. Comparitively speaking, the fire performance requirement for BS 6387 is more demanding.
IEC 60331 fire performance standard.
IEC 60331-1999 details a method of test for assessing the fire resisting characteristics of a cable. The cable is defined as fire resisting if , under the conditions of this test, it being assumed that the test flame intensity is of sufficient magnitude to destroy the organic material, no failure of any of the 3A fuses occurs and if the withstand voltage on completion is not less than the rated voltage of the cable. The cable sample is deemed to pass this test if no fuse was ruptured nor any lamps extinguished during the 3 hour flame application and on re energizing the cable after 16 hours.
BS 6387 specifies the performance requirements for cables required to maintain circuit integrity under fire conditions. It details the following methods to categorize the cables according to cable withstand capacities.
Resistance to fire alone – the cables is tested by gas burner flame while passing a current at its rate voltage. Four survival categories are defined: Cat A (3hours at 650 ℃ ); Cat B (3 hours at 750 ℃ ; Cat C (3 hours at 950 ℃ ) & Cat S (20 minutes at 950 ℃ ).
Resistance to fire with water spray – a new sample of cable is exposed to flame at 650C for 15 minutes while passing a current at its rated voltage and then the spray is turned on to give exposure to both fire and water for a further 15 minutes. A single survival category W is defined if the cables surpassed the testing requirement.
Resistance to fire with mechanical shock – the final requirement is mechanical shock damage. A fresh sample is mounted on a backing panel in a S bend and is exposed flames while the backing panel is stuck with a steel bar with same diameter as the cable under test every 30 seconds for 15 minutes. The cables will be tested under the following temperatures: X (650 ℃ ), Y (750 ℃ ) and Z (950 ℃ ).