Low smoke zero halogen

Material classification for cable jacketing

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Low smoke zero halogen or low smoke free of halogen (LSZH or LSOH or LS0H or LSFH or OHLS or ZHFR) is a material classification typically used for cable jacketing in the wire and cable industry. LSZH cable jacketing is composed of thermoplastic or thermoset compounds that emit limited smoke and no halogen when exposed to high sources of heat.[1]

Description

The first commercial thermoplastic LSZH material for cable jacketing was invented by Richard Skipper in and patented by Raychem Corporation.[2] This invention resolved the challenge of incorporating sufficient inorganic filler, aluminium trihydrate (ALTH), into an appropriate thermoplastic matrix to suppress the fire and allow a char to be formed, which reduced emission of poisonous carbon gases and also smoke and carbon particles, whilst maintaining electrical insulation properties and physical properties required by the end application. The preferred inorganic filler to achieve flame retardation continues to be aluminium trihydrate (ALTH). In the event of a fire this material undergoes an endothermic chemical reaction

2Al(OH)3 &#; Al2O3 + 3H2O (180 °C)

that absorbs heat energy and releases steam when the compound reaches a certain temperature. It is critical that the decomposition of the polymer(s) used to carry the filler happens at approximately the same temperature. The steam disrupts combustion of the evolved gases and helps form a char layer that protects the remaining material and traps particulates. The high level of filler required (&#; 60%) also replaces the base polymer reducing the total amount of fuel available for combustion.

Low smoke zero halogen cable considerably reduces the amount of toxic and corrosive gas emitted during combustion. When burned, a low-smoke zero halogen cable emits a less optically dense smoke that releases at a lower rate. During a fire, a low-smoke cable is desirable because it reduces the amount and density of the smoke, which makes exiting a space easier for occupants as well as increases the safety of firefighting operations. This type of material is typically used in poorly ventilated areas such as aircraft, rail carriages, tanks, subsea and offshore installations, submarines or ships. It is also used extensively in the rail industry, wherever high voltage or track signal wires must be run into and through tunnel systems. The nuclear industry is another area where LSZH cables have been and will be used in the future. Major cable manufacturers have been producing LSZH cables for nuclear facilities since the early s. Construction of new nuclear plants will almost certainly involve extensive use of LSZH cable. This will reduce the chance of toxic gases accumulating in those areas where personnel are working and the lack of corrosive gases where there are computer controlled systems will reduce the possibility of wires being damaged by fire resulting in a short circuit fault.

Since the s, the wire and cable industry has been using low-smoke, low-halogen materials in a number of applications. The introduction of a thermoplastic LSZH extended its use to accessories such as heat shrink tubing, labelling and fixtures. The objective was to create a wire and cable jacketing system that was not only flame retardant but also did not generate dense, obscuring smoke and less toxic or corrosive gases. In the military field its introduction was accelerated after following the dense black smoke emitted from HMS Sheffield after being hit by an Exocet missile in the Falklands War. Several fires, such as the King's Cross fire in London that killed 31 people in London's underground in , increased the awareness of the contribution that wire and cable jacketing makes in a fire. As a result, there has been an increased use of LSZH cables. With an increase in the amount of cable found in residential, commercial and industrial applications in recent years, there is a greater fuel load in the event of a fire and LSZH systems have a major role to play in protecting the public.

Several standards describe the processes used for measuring smoke output during combustion. For military applications Def Stan 02&#;711 in the UK and ASTM E662 in the US which are both based on an ASTM STP No. 422 pages 166&#;204, modified by AMTE, Portsmouth in the UK[3] and superseded by E662 in the US. During these tests a specified material sample is standardised and then exposed to a radiant heat source; the optical density of the smoke given off is photometrically measured.[clarification needed] There are various means of measuring optical density: peak smoke release rate, total smoke released, and smoke density at various points and durations during the test. Results must be below a certain value and the material must pass the burn test in order for the material to be labelled as low smoke.

These tests are conducted under laboratory conditions and cannot claim to replicate the range of conditions expected in a real fire scenario. However they do provide a measure by which the potential smoke emission of materials can be assessed and dangerous materials identified before proceeding to further testing of preferred materials, if deemed necessary.

References

1. ^MSS Fibre Glossary of Terms
2. ^

   United States Patent

3. ^

   A new approach to testing materials in the NBS smoke chamber, A. Routley and R. Skipper Fire and Materials Volume 4, Issue 2 June Pages 98&#;103

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LSZH or LSOH Cable

It is probably safe to say that the entire construction industry is particularly focused on fire safety right now, and electricians installing cable have a very important role to play.

The need to be vigilant with your product choice has never been higher then, especially when it comes to installing Low Smoke Zero Halogen cables. Use standard PVC cable by mistake, for example, and it will release thick black smoke and toxic gases when reacting to fire &#; a potentially life-threatening error.

Smoke and Fumes Hazard

Smoke and fumes can be more of hazard than flame in the early stages of a fire when occupants are trying to escape &#; particularly in a public building, such as an airport, train station or hospital, where people may not be familiar with the layout of the building or the position of exits.

Low fire-hazard cables have a wide range of common names and rely on a variety of standards to describe their performance during a fire. Prysmian&#;s LSX cable has been specifically designed for use in buildings and can easily replace many standard PVC cables. It is described as a low smoke zero halogen (LSOH) screened wiring cable. But what do these terms actually mean?

Low Smoke

In a building fire the thick black smoke given off by conventional PVC cables can cause asphyxiation, obscure escape routes and make it difficult for fire fighters trying to enter or search the building.

Prysmian has tested some cables claiming to be low smoke that within three minutes caused light transmittance (and hence visibility) to drop to less than 5% within three minutes. If this performance is mirrored in buildings where the cable is installed it would be very difficult for people to find their way out of the building and is likely to greatly increase the stress and sense of panic as they try to do so. 

A cable officially described as &#;Low Smoke&#; should have been tested in accordance with the standard BS EN -2.

The standard uses a test which measures the amount of light transmitted through the smoke produced by burning 1 metre-long samples of cable suspended above a 1litre bath of alcohol in a 3m3 sealed cube. Because the alcohol produces negligible amounts of smoke, any smoke produced is from cable combustion. For a cable to be classed as low smoke, a minimum of 80% of the light must pass through the smoke during the test.

The practical effect of passing the test is that the cable produces very little smoke in a fire: visibility should not be reduced by more than 20%, leaving at least 80% of the light getting through and allowing much greater visibility in the building during the critical period when occupants are trying to escape.

Zero Halogen

When burnt, PVC emits acidic, almost invisible hydrogen chloride gas (HCI).  This horrible gas typically accounts for over 20% of the emissions produced by burning PVC and, when it mixes with water, it turns into equally dangerous Hydrochloric acid.

In any form, HCI is not something you want to deal with.  If inhaled, the gas can irritate the throat lining to such an extent that it becomes very difficult to breathe.  Contact with the eyes or skin can cause debilitating burns, further incapacitating those trying to escape a burning building. HCI can also damage sensitive electrical equipment, even if fire is contained and extinguished.

Afumex LSX cable produces zero halogen, which in industry terms is defined as less than 0.5%.  The test for this characteristic is BS EN -2-1.

Flame Retardant

Flame retardant cable is designed to make sure it does not increase the spread of fire by conducting flames along the surface.  The testing regime makes sure that the cable does not spread fire and, once the source of the flame is removed, the cable must extinguish itself.  The test for this characteristic is BS EN -1-2.

Beware of Fake Cable

Not all low smoke zero halogen cables are the same. Terms such as LSOH and OHLS are specific to individual manufacturers so it is worth checking all of the test standards to which the individual cable complies and be sure that they match the specification required for the building. Also, don&#;t assume cables described as LSF (Low Smoke and Fume) will pass &#;zero halogen&#; tests and have flame retardant characteristics &#; check because some may not, often they just a modified PVC.

The low smoke zero halogen (LSOH) properties of Prysmian&#;s LSX cables makes it ideal for general wiring applications such as lighting and power &#; particularly in public buildings such as multi-storey dwellings, shops, offices and transport hubs such as train stations and airport terminals. 

It is important to remember that while Afumex LSX&#;s low fire hazard LSOH properties make it ideal for non-essential services, such as general lighting and power, installers should continue to use Prysmian FP200 Gold & FP Plus cables for emergency circuits, such as fire alarms and emergency lighting.

For more information see our Low Smoke cables and Fire Resistant cable pages or contact us.

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