Get to Know Low-Smoke Zero-Halogen (LSZH) Cables: They Can Save Lives

There is a growing trend toward the use of low-smoke zero-halogen (LSZH) electrical cables that use jacketing materials that are safer when exposed to fire. As their name suggests, they produce less dense smoke and almost no highly toxic gases called halogens, unlike traditional materials such as polyvinyl chloride (PVC) and fluorinated ethylene propylene (FEP).

It might seem logical that designers should always choose such cables, but the decision is far more complicated, making it essential for electrical engineers to understand LSZH cables, where they are suited, and how to choose and apply them.

LSZH cables are not for every application

It’s important to remember that although cables made using halogenated compounds such as PVC and FEP have been singled out as hazardous during a fire, LSZH cables are not a universal solution for replacing them for the following reasons. First, PVC and FEP-based cables have benefits too important to dismiss, and replacing them with LSZH cables may have little benefit in open spaces where smoke and gases can dissipate quickly. In addition, electrical cables are rarely the only source of plastics when fire breaks out, and as PVC and FEP-based cables are fire resistant, their contribution to a fire becomes relatively small, among many.

PVC and FEP-based cables are also less expensive than LSZH cables, deliver excellent electrical performance, are widely available, and have excellent dry and wet electrical characteristics. They are also very flexible, have long lifetimes, resistance to temperature extremes and chemicals, and are very rugged. In short, LSZH cables are best suited for scenarios in which traditional cables could be hazardous. They are not intended to replace traditional cables in every application.

The rise of LSZH cables

The fire and flame characteristics of PVC and FEP in cable jacketing, dielectric, and other components have been known since the 1970s, and cables with alternative materials have been used in military and nuclear systems since about 1980. However, the Kings Cross Station fire in London in 1987, in which more than 30 people were killed and 100 injured, received worldwide attention (Figure 1).

Figure 1: An investigation of the fire at the Kings Cross Station of London’s Underground showed that the burning of large amounts of electrical cable produced dense black smoke and toxic gases, making it extremely difficult for people to escape. (Image source: Wikipedia)

The follow-up investigation revealed that a match dropped on an escalator was the cause with multiple factors allowing it to spread and become deadly. One of these was the presence of large amounts of electrical cable which burned, causing dense black smoke and toxic gases, making it extremely difficult for people to escape. This is cited as the impetus for the development of cables with better and safer performance in a fire.

As a result, the London Underground banned PVC cables, and other members of the European Union began widely using LSZH cables shortly after that. It’s taken a lot longer for the U.S. to embrace such cables for several reasons, including cost, conflicting standards, and controversy over where they should be used. These issues are slowly being resolved, and PVC and FEP-based cables are being replaced in applications that would most obviously benefit from them.

LSZH cable differences

In contrast to PVC jacketing, LSZH cables use jacketing material made from thermoplastic materials that do not create halogens or caustic acids, produce little or no smoke, and significantly reduce flame spreading, all of which can allow people to escape areas of fire more easily, and lesson the already hazardous work of firefighters. LSZH compounds have typically been based on polyolefin and doped with hydrated minerals, the result of which is a white, less dense smoke.

In the years since LSZH cables became available, other compounds have been introduced that produce even better results while maintaining the required electrical performance of the cable. Polymer materials not inherently flame retardant have additives such as inorganic hydrates (aluminum trihydrate or magnesium hydroxide) that optimize flame retardation. This heavy doping typically degrades some physical properties, so the cable industry has developed methods of reducing or eliminating their effects using different compounds.

Confusing requirements

For designers, the situation remains confusing because there are different views about where these cables should be used. There is no single standard or set of standards from which to make informed decisions, and it’s difficult to accurately differentiate LSZH cables from different manufacturers. To appreciate this, consider the many acronyms that have been used over the years to describe electrical cables with some form of “better” performance in fire (Table 1).

Table 1: A lack of coherence produces chaos: some of the many “fire safe” cable designations (Data source: Wikipedia, Anixter, Inc.)

Other examples include the Department of Defense, one of the first agencies to address halogen content in MIL-C-24643, which defines “low” halogen as less than 0.2% by weight. Other standards address the amount of acid gas produced, but not specifically halogen levels.

As it concerns electrical cables used in plenums, the primary issue is that the toxic and corrosive products they can produce will spread throughout a building. The National Electrical Code (NEC) requires cable used in plenum spaces to be rated as “low smoke”, although there is even debate about this, as plenums typically have a lighter fuel load and exposure to sources of ignition. The National Fire Protection Association (NFPA) maintains the standard NFPA 262, addressing cables used in plenum air spaces.

Fortunately, Underwriters Laboratories (UL) addressed low-smoke and halogen-free ratings beginning in 2015, and now offers certification programs for halogen-free (HF) and LSZH cables based on the IEC 62821 and IEC 60754 series of standards. IEC 62821 covers requirements for halogen-free, low-smoke thermoplastic insulated and sheathed cables with voltage ratings up to 750 volts. UL’s HF and LZSH markings can be used on many types of wire and cable, from appliance wiring to communications cable (including fiber optic), flexible cord, and power and control tray cable.

All UL certified HF and LSZH cables also meet other UL general certification requirements as well. UL has also expanded its component recognition program under UL 2885 to include evaluation of halogens. This should be helpful for manufacturers of insulation and jacket compounds and other cable components including fillers, tapes, and wraps.

Where LSZH make sense

Determining whether to use LSZH cable is difficult, especially for those whose diligence leads them to a deep dive into standards, housing codes, and the many documents that address the issue. Fortunately, the need for the protection afforded by LSZH cables is often obvious. In general, places where LSZH cables should be a consideration include areas where many cables are installed in a confined space and are near each other, and where evacuation is limited, ventilation is poor, and high voltage is present.

Good examples include applications in which they were first used, such as surface ships and submarines where escape from fire is difficult and sometimes impossible, and in nuclear facilities where dense smoke and toxic, corrosive gases could be catastrophic.

Other applications are commercial aircraft, transportation stations, some sections of airports, telecommunications switching centers, tunnels, theaters, and nightclubs (Figure 2). This mock-up shows the Station nightclub in Warwick, RI, that was consumed by fire in 2003, killing 100 people and injuring 230 others.

Figure 2: This mock-up of The Station nightclub shows that while there were some evacuation routes there were also places from which it would be extremely difficult to escape, especially for hundreds of people in panic. (Image source: National Bureau of Standards and Technology)

Although there were some evacuation routes, there were also places from which it would be extremely difficult to escape, especially for hundreds of people in panic. Time to egress is critical, as toxic fumes would have built up quickly, as measured in the mock-up (Figure 3).

Figure 3: Gas concentrations at a central point of the nightclub, between the kitchen and the dance floor, for the first 200 seconds after ignition (t = 0). The probe was place 1.5 meters above ground level. (Image source: National Bureau of Standards and Technology)

Most recent applications include data centers, the number of which has grown dramatically. These have forests of cables, as well as massive amounts of cooling infrastructure, in which flame and smoke can rapidly spread. Strangely perhaps, the use of LSZH cables in data centers is not universally required, although their use is becoming more common.

Beyond the obvious are factors that can either substantiate or mitigate the use of LSZH cables. In new buildings, one of the most important considerations is the fuel load contributed by construction materials, which requires knowledge of the materials used and an assessment of the overall environment. Unfortunately, in existing buildings this is often impossible as knowledge of the materials used in construction is difficult and sometimes impossible to obtain.

Performance and cost

It was once true that the electrical and mechanical performance of LSZH cables were inferior to that of their traditional counterparts, but this is far less true today. Cable manufacturers have continually enhanced LSZH products that provide the fire retardant properties of PVC and FEP materials without reducing flexibility, bend radius, cold temperature capability, and electrical performance.

LSZH cables are still typically more expensive because their production requires additional steps and manufacturing time. However, cost could decline as the U.S. market for LSZH cables increases. Applications in cold environments have never been a strong suit of LSZH cables because additives tend to reduce flexibility in general, and at very low temperatures in particular. However, some of the latest LSZH cables mitigate this problem using proprietary techniques.

Choosing the “right” LSZH cable

The proliferation of rules, codes and disparate regulations and standards makes it difficult for cable manufacturers to adequately define whether they are or are not truly LSZH rated. Some data sheets that might otherwise seem to meet these requirements surprisingly often do not state this, but instead provide only cable construction (primarily jacketing), frequently leaving out the dielectric material.

A good example of data sheets that explicitly call out their LSZH attributes are represented by a family of multi-conductor cable from Alpha Wire that includes the 1172L SL005 two-conductor cable through to the 6017L SL005 24-conductor version (Figure 4). These clearly note that they are LSZH rated and the tests they pass, including IEC 60332-1 for flammability, EC 60754-1 and 60754-2 for acid gas generation, and IEC 61034-2 for smoke emission.

Figure 4: The data sheets for the 1172L SL005 two-conductor (left) and the 6017L SL005 24-conductor (right) cables from Alpha Wire avoid confusion by clearly noting that they are LSZH rated and the tests they pass. (Image source: Alpha Wire)

Other manufacturers similarly specify LSZH characteristics, while others bury these features somewhere in their data sheets, something that will probably change if more engineers specify them. If a cable seems desirable but does not clearly indicate its fire associated characteristics, the best way to make a final decision is to contact the manufacturer. The company may also be able to manufacture custom versions with some or all attributes of superior performance in fire. It’s also essential to compare LSZH cables from several manufacturers, select candidates whose fire performance appears satisfactory, and then compare their flexibility, temperature range, bend radius, lifetime, and other key metrics. The “winner” should have both excellent fire related performance and the least possible difference (or degradation) from a PVC cable.

Conclusion

At this point it’s not surprising that although LSZH cables have been available for decades, their use is just now beginning to increase. There has been no alliance or other organization to champion them, little or no coherence about where they should be used, or even how cable manufacturers should describe them on a website or data sheet. Fortunately, this is changing for the better thanks to UL’s decision to add LSZH and LSF to its certification test program. Hopefully, it won’t take another major fatal fire to maintain this momentum.

In the meantime, designers and electrical engineers have LSZH cables to choose from off-the-shelf, and can specify LSZH compliance to various standards for custom applications as needed.