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LSZH - Low Smoke Zero Halogen Wire and Cable - Explained

Summary

Even though they are still predominant in the industry, the use of halogenated compounds in wire has decreased over the past several years. Polymers such as PVC are being phased out in certain applications, especially in enclosed high-density cable applications. Relatively new low-smoke and zero (or low) halogen compoundsm, which are typically polyolefin based with a heavy doping of inorganic hydrated minerals, give a cleaner smoke when burned. This mineral doping also reduces certain physical properties so the wire and cable industry has attempted to develop low-smoke and halogen-free compounds that have the same or better functionality than the common halogenated compounds currently in use in industrial applications. However, it is important to understand that smoke production and halogen content are not mutually exclusive. Halogen low-smoke compounds exist as do halogen-free compounds that are not low-smoke. It is always best to consult with wire and cable experts when choosing a cabling solution.

History of LSZH materials?

Since the 1970s, the wire and cable industry has been using low-smoke, low-halogen materials in a number of applications. The objective was to create a wire and cable jacketing that was not only flame retardant but also did not generate dense, obscuring smoke and toxic or corrosive gases. Several notable fires over the years (such as the King's Cross Fire that killed 32 people in London's underground subway in 1987) increased the awareness of the role that wire and cable jacketing plays in a fire and contributed to a greater adoption 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.

So what is LSZH?

LSZH stands for Low Smoke, Zero Halogens. A low-smoke, zero-halogen cable is one which the jacket and insulations materials are made of special LSZH materials. When these cables come in contact with a flame very little smoke is produced making this product ideal for applications where many people are confined in a certain place (office buildings, train stations, airports, etc). While a fire may be very harmful in a building, the smoke can cause more damage to people trying to locate exits and inhalation of smoke or gases. Toxic fumes would present a danger in the event of a fire like mentioned in this text. More people in fires die from smoke inhalation than any other cause. Reducing smoke in general and toxic smoke in particular saves lives.

Today, low-smoke, zero-halogen cables are being used in applications beyond the traditional transit, shipboard, military and other confined-space applications.

What are halogens?

Halogens are elements such as fluorine, chlorine, bromine, and iodine. Halogens are highly reactive and can be harmful to people and animals. Common cable insulations, such as PVC, contains high amounts of halogens. The C in PVC is chloride, which is an ion of chlorine. PVC contains about 29% chlorine by weight. Teflon® FEP and PTFE contains up to 76% fluorine. Teflon, when burned, produces toxic acid.

Halogens, under normal circumstances, are very stable and present no danger. Problems arise when they burn.

A halogen-containing plastic can release hydrogen chloride, hydrogen fluoride, and other dangerous gases when burned. When hydrogen chloride comes in contact with water, it forms hydrochloric acid, which is also dangerous. Beyond beginning toxic to humans and animals, these gases are also highly corrosive to metal.

The concern, then, with common wire and cable insulating materials is that they will emit toxic gases and toxic smoke. These gases become even more harmful when mixed with water, like from a sprinkler system, creating toxic acids.

A LSZH material emits no dangerous gases or smokes when burned. In fact, they mainly contain miniscule trace amounts of halogens - well under 1% - but they essentially are halogen free.

Zero Halogen Cables

A zero-halogen cable does not contain any of the chemical elements defined as halogen. Even though some materials used in wire and cables contain no halogens, many others include a high percentage of halogens with the most common being chlorine, fluorine and bromine. Because halogens are effective fire retardants, they are added to naturally halogen-free materials to allow a cable to pass an industry flame test.

Halogen content

The table here, lists the halogen content in
some typical wire and cable polymers.

Low halogen is not as clearly defined as low smoke. UL does not define an equivalent to the "-LS" rating for halogen-free products, and many materials that are often defined as zero halogen, still contain trace amounts of halogen, even though they are not considered harmful. For example, the military standard "MIL-DTL-24643 dictates a halogen content of less than 0,2 percent by weight. Other standards reference the volume of the acid gas given off, but do not define halogen levels.

A common standard in the U.S is ICEA T-33-655, which covers low-smoke, halogen-free polymeric cable jackets. However, this standard only addresses the cable jacket, not the table insulation, because the jacket is the first part of the cable to undergo combustion. Many cable constructions reference this standard and described as low halogen but still contain halogenated materials. The drawback is that all polymeric materials will eventually combust in a fire scenario, so halogen combustion products will still be present if the cable burns completely.

In recent years, halogens have come under scrutiny for the toxicity and corrosivity of their combustion byproducts. Their reactive nature that makes them effective flame retardants can also present a danger to building occupants by giving of toxic gases when burning and by risking damage to electronic equipment and metallic structures. Standards for corrosivity and toxicity testing exists as well and may be more appropriate indicators of a cables suitability for a certain application than the halogen content.

Despite the concerns related to halogenated materials, the majority of cable in plenus spaces contain halogen. There is now concern that if this material burns, it can represent a significant safety risk. The opposing viewpoint is that while halogen combustion products may be dangerous, the flame retardant properies of halogenated materials make it less likely to burn and, therefore, safer overall.

Flame testing

It is important to seperate the fire performance of wire and cable from the LSZH label. Almost all modern cables are required to pass some type of flame test. A typical LSZH flame test measures five criteria in order to predict how a cable will behave in the event of a fire.

  1. How easily the cable ignites
  2. How fast and far fire will propagate along the cable
  3. How much smoke is generated by the cable when it combusts
  4. The toxicity of the products of combustion
  5. The byproducts' corrosivity

Industry tests exist to measure each of these factors, but there is continuing debate around how best to measure and test wire and cables for fire performance. Developing LSZH compounds and cables that maintain costs and processing characteristics has been a constant challenge for the industry.

There is also ongoing research into correlating small-scale, also called bench tests (e.g. UL94 or cone calorimeter tests) to large-scale fire tests.

One widely used approach is to qualify the wire or cable to the following IEC requirements:

  • IEC 60332-1: Flammability
  • IEC 60754-1 and 60754-2: Acid Gas Generation
  • IEC 61034-2: Smoke Emission

 

Fire Performance Standard

At present, in cable industry, Fire Retardant, Low Smoke Halogen Free (LSZH), Low Smoke Fume (LSF) and Fire Resistant cables are all described as Fire Survival Cables.

Flame Retardant

Flame retardant cables are designed for use in fire situations where the spread of flames along a cable route needs to be retarded. Due to relative low cost, flame 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 & Flame Retardant (LSZH)

LSZH cables are not only characterized by the flame retardant performance, but also by the halogen free properties, thus offering low corrosivity and toxicity. During a fire, LSZH cables will emit less smoke and acid gases which may damaging the human being and expensive equipment. Compared with normal PVC cables, LSZH cables outperform by their flame retardency, low corrosivity and low smoke emission properties, however, normal PVC cables have better mechanical and electrical properties.

Low Smoke Fume (LSF)

The low halogen content and low corrosivity of low smoke fume cables lies somewhat in between that of flame retardant cables and LSZH cables. LSF cables also contain halogen but the content is much less than that of PVC cables. LSF cables are designed to reduce the spread of fire, toxic gases and smoke during fire. The LSF cables are 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 (FR)

Fire resistant cables are designed to maintain circuit integrity of those vital emergency services during the fire. The individual conductors are wrapped with 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 spary or mechanical shock situation.

Fire Performance Class

The main concerns for the cables in their fire survival properties are their flame spread, smoke characterization 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 importance factors affecting the safety and survival of human during a fire.


Halogen Free Flame Retardation Methods

Products containing halogenated polymers (such as polyvinylchloride [PVC] and fluorinated ethylene propylene [FEPI] are inherently flame resistant. When burned, the materials generate free radicals that slow down the combustion process by reacting with the high-energy free radicals. One of the products of this process is a halogen acid gas such as hydrochloric [HCI].

For other materials that are not naturally flame retardant, polymer flame retardation is achieved by using supplementary additives. These supplementary additives, along with synergistic additives, are added to the polymer. Adding inorganic hydrates, such as aluminium trihydrate (ATH) or magnesium hydroxide (MDH) will achieve flame retardation. In the event of a fire, both these materials undergo and endothermic chemical reaction that absorbs heat energy and releases steam when the compound reaches a certain temperature.

  • 2AI(OH) → AL2O3 + 3H2O (230°C)
  • Mg(OH) → MGO + H2O (330°C)

The steam disrupts combustion and a char layer that protects the remaining material and traps particulates then develops. Because these materials replace the base polymer, the total amount of fuel available for combustion is also reduced.

MDH reacts at higher temperature (330°C) than ATH (230°C), which makes processing easier as it allows higher extrusion temperatures. However, MDH is more expensive than ATH, which makes ATH much more common. Recent statistics show ATH represents approximately 50 percent of the European flame retardant market by weight.

The main challenge with mineral-based fillers is the high loading levels required to pass industry flame tests (up to 65-70 percent). This loading can have a negative affect on the cable's physical properties, which typically results in a lower elongation, elongation at break and tensile strengths. Processing can be more difficult with these materials, but many methods of improving the processing exist, including using silicon additives and surface coatings. Manufacturers use a wide variety of compound blends as different polymers process and perform better with some additives than others. Other materials exploited for flame retardation include intumescents, which are materials that undergo and endothermic reaction and swell when exposed to heat to provide a protective layer. Nanocomposite fillers, which are typically a type of clay, are used at lower loadings (~5-10 percent) as a synergistic additive with other flame retardants to improve the processing and flame performance. Synthetic clays further enhance performance, and in the future-carbon nanotubes or other carbon-based nanostructures may be availabe for commercial use.

Fire resistant and Mica tape

Various methods are used to make safety cables fire resistant. One proven method is to apply mica tape insulation layer directly on the metal wire. The layer protects against short circuit in case of fire, and helps to significantly extend the system integrity for emergency power supply, fire alarm and evacuation systems in buildings, tunnels and rolling stock materials or in other safety related applications.

Why is glass mica suitable material as fire barrier?

Mica is a natural sediment which is applied in the form of small plates on glass carrier tape. Mica is highly fire resistant material. Applied on a tape, it allows very small bending radii during manufacturing (coiling) and installation. Mica Tape for fire survival wires & cables is made of mica paper (Phlogopite or Muscovite) and reinforced with fiber glass cloth or Polyethylene film with a small amount of heat resistant Silicon binder. The tapes have excellent flexibility, high tensile strength and long length. It survives temperature up to 1000°C. It is non-toxic, chemically neutral and 100% Halogen free.

A fire resistant cable will typical been described as shown below:

  • Lapped mica/glass tape to be covered by an extruded cross-linked insulation, armoured, LSF sheated. Rated voltage 600/1000V and for instance meet BS 6387 types CWZ or lower temperature performances type A/B/SWX.

Trends in the development of building safety regulations in various countries (e.g. the Construction Products Directive (CPD) in Europe) may mean that use of cables with a defined level of fire resistance in all buildings may be mandated in the future.

Thermoset vs. Thermoplastic LSZH

Thermoset  wire and cable typically offers better performance than thermoplastic wire and cable. A thermoset is a material that assumes its final form after processing. A thermoplastic can be melted and given a new form after processing. Chlorinated thermoset jackets are common in industrial applications due to their desirable physical features and ability to pass the most rigorous flame tests.

LSZH does not have the long track record of performance that chlorinated thermosets possess, and there are questions about the lifespan and performance of these cables.

Recent advances in compounding technology have allowed manufacturers to offer thermoset LSZH cables that pass many of the same tests as chlorinated thermosets, such as the IEEE 1202 and UL VW-1 flame tests. A past problem has been the water absorption tests required in many cable standards. LSZH material typically absorbs a greater amount of moisture than non-LSZH material. Moisture absorption affects the physical and electrical characteristics of wire and cable. New compounds and processing techniques have allowed manufacturers to overcome this problem.

Sustainability

The ultimate goal for a sustainable wire and cable industry is to develop and use materials that safely perform physically and electrically while continously reducing the total amount of energy for a given set of performance criteria. Even though compound selection is important, you must also consider the total processing energy versus field performance. Some studies have shown that halogenated compounds require less overall processing energy. Even if there were processing advantages, the added performance you get from halogenated compounds could potentially outweigh them. Getting a precise measure of the total lifetime environmental impact is difficult. This is why regulations such as the European Union's WEEE (which make producers responsible for recycling any electrical products they put on the market) have become more common in the recent years.

Reprocessing is also another sustainability consideration. It is more difficult to reprocess thermoset materials than thermoplastic materials because they don't remelt (they're basically heat-set, as the name implies). But if you get a greater overall longevity out of a cable population because you used thermoset materials then you could argue the lifetime value of the thermoset material is higher. It is certainly a complex problem, and one that is certain to get more attention in the coming years.

Ultimately, no wire and cable user wants to sacrifice performance, and certainly not safety. Most wire and cable built to industry standards that ensure it can pass various electrical and physical tests. Technically this allows the end-user to select whatever cable design they feel most comfortable with while ensuring the cable is of good quality and will perform at a standardized level. Local building codes such as NFPA 70 (National Electrical Code) used in the United States dictate that cable types installed within a building comply with appropriate fire rating based on its specific use case. For example, communications cabling installed in ducts, plenum and other spaces used for environmental air shall be rated CMP, which means it must pass the difficult NFPA 262 flame test. Currently, there is not a commercial zero-halogen cable that can pass this test and other mechanical tests required by cable standards. Beyond the technical considerations, the end-user must also consider special certifications and allowances that require the use of certain materials. For large cable projects, it is important to consult with wire and cable experts to choose the correct cable system for that application.

Is the LSZH designation limited to wire and cable?

No. LSZH really refers to the plastic material. It can be used in other products, such as shrink and non-shrink tubing.

Applications

The clearest uses for LSZH are confined spaces with large amounts of cables in close proximity to humans or sensitive electronic equipment. Submarines and ships are classic examples, which is why the military was one of the first adopters of LSZH standards. Additionally, mass transit and central office facilities are common applications for LSZH, and many telecommunication standards require LSZH cables.

The use of LSZH cables in Europe has been widespread since the 1980s. It has never achieved such widespread acceptance in the United States, primarily for cost reasons, but also because of performance concerns. Some of the cable designs used in Europe cannot pass U.S. test standards, and the high additive loading needed to pass the U.S. flame testes can lead to reduced physical properties if not done carefully.

Installation at lower temperatures can also be affected. Reduced flexibility due to the high additive loading in the materials can prevent cables from being installed in cold environments. The high mineral content can also result in fractures of the material if the installation is not done carefully. Research of the cracking behavior of LSZH has been done with the goal of improving performance.

One advantage of LSZH is that it typically has a lower coefficient of friction, although lubricant suppliers recommend a special pulling lubricant for low-smoke, zero-halogen jackets. Though there has been a trend toward jackets that do not require lubrication, some installations will still require lube to help with difficult pulls.

There are still questions about the necessity for LSZH cable in some applications. Fires are dangerous, but so is electricity, and if a higher voltage or mission-critical cable is more likely to be damaged during installation or from physical or chemical damage during its lifetime, this could conceivably result in at statistically more dangerous product than a halogenated cable.

Another consideration is the environment in which the cable will be installed. If a fire occurs in an open area in which smoke concentration is not sufficient to obscure escape routes, using a LSZH cable may not be beneficial. There is also the question of the fuel load in a building other than cabling. The smoke being given off by other materials burning can vastly outweigh the contribution of the wire and cable. Of course, this is highly dependent on the installation and the relative amounts of cable present as well as the building's function and contents.

However, there is no question that the amount of cable installed in buidlings has increased as data communication has proliferated. Central office telecommunication facilities were some of the first places that LSZH cables becam common due to the large relative fuel load represented by wire and cable.

Modern data centers contain large amounts of cabling, and are usually enclosed spaces with cooling systems that can potentially disperse combustion byproducts through a large area. In industrial facilities, the relative fuel load of cables will not be at the same level. Other materials burning may also contribute greater amounts of dangerous gases that outweigh the effect of the cables. There have been notable fires where cables burning contributed to corrosion (the Hisdale Office fire is a famous example), but in some instances, better fire response techniques could have prevented this damage.

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 1990s. The expected construction of new nuclear plants in the U.S. in coming years will almost certainly involve some LSZH cable.

One of the most important thing to understand about LSZH cable (and of course cable in generalt) is that no two products are the same and that there are many factors that will define the suitability of the final product to its application. In fact, research done by a major pulling lubricant supplier tested 27 LSZH compounds and found a huge variation in physical properties. So even using material that meets the base requirements of one of the many specifications available may not result in the best material for the application. Understanding the goals, results and  limits of these tests are key to finding the right product. In any case, the trend to consider environmental concerns with a greater weight relative to performance has increased and it can be generally stated that there is an enlarging market for cable that can be demonstrated to be environmentally friendly.

What are the tradeoffs of LSZH cable?

LSZH can be a direct replacement for "generic" PVC-based cables in most applications. The temperature range of LSZH material is a bit more restricted thanfor PVC: -20°C to +75°C for LSZH and -50°C to +90°C for PVC. Applications requiring extended temperature capabilities, wide resistance to chemicals, or other special needs may not be suited to LSZH cable.

LSZH cables are more expensive than PVC counterparts. The safety they offer means they offer more value that goes beyond acquisition costs. These cables can prevent harm to people and also prevent damage to hardware system in the event of a fire.

So the conclusion...

Low-smoke and zero-halogen cable technology has advanced significantly. It is well suited to some, but not all, applications. With further research and investigation into compounds that can pass industry flame tests and offer improved processability, the uses and adoption will increase. If the technology improves to the point where it is equivalent to or exceeds other materials, the industry will continue to see increased adoption of LSZH standards and specifications.


References: Anixter, Alphawire, Addison, Micagroup, Leoni Studer AG and more...

 

Reservation:

 

It is the user's responsibility to ascertain if a particular product is safe and without risk to health and safety by virtue of its location in a hazardous area, i.e. classification of zones, gas groups, ignition temperatures, etc. Both the specifier and user should be thoroughly familiar with standard mentioned on this sites or any document within.

Whilst every care has been taken in the compilation of this document, we regrets that it connot accept responsibility for any errors or omissions contained herein. Readers should not rely upon the information contained in this document or site without seeking specific safety advice and ensuring that their own particular circumstances are in accordance with the matters set out.

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