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Low-Smoke, Halogen-Free Cables

Posted on January 16, 2018 by UL

Low-smoke halogen-free (LSHF) cables have been on the market for decades, yet there are still many misconceptions about the application and definition of test requirements/methods for these cables.

Generally speaking, traditional halogenated cables when compared to LSHF cables produce more toxic and corrosive smoke in the event they are burned.

LSHF cables first originated in the 1970s in Europe and the U.S., and began being used in underground structures by London Underground and North Sea offshore drilling platforms in the 1980s.  These cables have traditionally been used in enclosed spaces such as subway systems, tunnels, submarines, ships, and mines. More recently, LSHF cables are being used for life safety and mission-critical applications, such as hospitals and data centers, respectively.

“Traditionally, where these [LSHF] cables are used, the application of these cables were in enclosed spaces,” Robert Bellassai, senior staff engineer at certification company UL, said. “So if you had a space that was not exposed to or installed in outside air, that's where these cables would be used but the use of LSHF cables is rapidly expanding.” Wire and cables, which are essential to any building, structure, or product, generally consist of insulated copper conductors or buffered/unbuffered optical fibers, and are covered by a protective outer jacket.  Cables may also contain other polymetric materials such as fillers, tapes/wraps (PET, fiberglass, polyester, etc.), ripcords, shields (aluminum-PET, copper-PET, etc.), and color concentrates.


In the case of halogenated cables, the polymetric materials they are manufactured with contain one or more halogenated elements such as chlorine, bromine, fluorine, iodine, or — less likely — astatine.

Halogenated elements do serve a purpose when used in polymetric materials. In most applications, they are in the form of chlorinated and brominated flame retardants to inhibit flame propagation of the cable when burned. However, when burned, the cable will emit smoke that is toxic, corrosive, and, most notably and dangerously, contains high amounts of carbon monoxide (CO).

One study from the University of Lancashire in the U.K. found evidence that suggests combustion products can be potentially hazardous to people if they cannot easily evacuate from the area.  The cables studied contained brominated or chlorinated flame retardants which produced significantly higher levels of CO gas during combustion.

Additionally, when mixed with water, hydrogen halides form hydrochloric acid, hydrofluoric acid or hydrobromic acid which can damage and destroy due to the corrosive nature of the combustion product critical structures, components, and equipment.

Different Classifications

Installing LSHF cables significantly reduces the risk of toxic and corrosive smoke being emitted in case of a fire.

Ideal for subterranean and mission-critical applications, such as subway tunnels and data/emergency call centers, LSHF cables can often be called other acronyms, such as LSZH, HFFR, LSF, and LS0H. These different acronyms have been developed by manufacturers to characterize (self-certify) their products for smoke generation and halogen content. 

Each self-certification acronym is associated with industry standards that were not always directly applicable for the characterization of smoke and halogen content. Also, each cable and material manufacturer doesn’t always associate the same standards with the same acronym.

“Low-smoke” refers to the amount of smoke that a complete cable construction produces upon combustion and “halogen-free” refers to the amount of individual halogen elements that are present in each combustible cable component. 

Metro Fire

The dangers of not installing LSHF cables are potentially deadly, as evident by the Jan. 12, 2015 fire at Washington D.C. Metro’s L’Enfant Plaza Station.

There, an electrical malfunction caused a fire which filled up the subway tunnels with toxic smoke. In total, 86 people were injured from smoke-related injuries and one woman died.

D.C. Metro, and other transit agencies, had been advised by the National Transportation Safety Board in July 2014, to replace and update its cables to better protect them from fires. Metro had been in the process of installing new cables for over a year, but had not yet finished when the June 2015 fire broke out.

According to the Washington Post, multiple lawsuits filed by transit riders alleged that Metro created unsafe conditions in the tunnels, in addition to delaying evacuation efforts.

While some suits have been settled out of court, others, including a suit filed by the family of the woman who died, are expected to go to trial in fall 2018.


Installing LSHF cables not only protects employees and customers but can also increase the value of the structure in which they’re installed.

The U.S. Green Building Certification recognizes four levels of LEED certification: Certified (40-49 points), Silver (50-59 points), Gold (60-79 points), and Platinum (80+ points). Having LSHF cables installed counts as points toward certification and can increase the financial value of the property and save owners money in decreased operational costs.

 “...certified LEED green buildings are very efficient, resulting in decreased operational costs due to lower utility and maintenance expenses,” Bellassai said.

The high safety standards associated with LSHF cables are environmentally friendly and cost-effective.

“It does make the building… worth more,” he continued. “…Let's say you were going to rent this green building or you were going to sell this green building, you could actually attain more money. You could attain a higher resell value because now this is a very efficient, money-saving building.”

Additionally, LSHF cables have been proposed to be included in the 2020 version of the National Electrical Code.

The proposal, submitted by Mexichem, seeks to add HF and LSHF optional markings so contractors, architects, specifiers, and users can specify HF and LSHF cables.

If approved by the NFPA for the 202 NEC, this will give Authorities Having Jurisdiction (AHJ) an easy way to identify HF and LSHF products for applications that would benefit from the reduced toxicity and corrosivity of these cables, and from a trusted third party independent test laboratory such as UL.

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