Since the 1970s, fire-retardant, low-smoke zero-halogen (LSZH) wire and cable has been commercially available for shipboard applications, offshore marine platforms, rapid transit and similar applications where people are present in confined areas. When combined with other fire prevention and suppression practices, fire-retardant LSZH cables can minimize fire-related deaths and property damage. However, gases produced by all burning materials—whether LSZH or not—are extremely toxic. Several key advantages and disadvantages of LSZH cables are as follows :
LSZH wire and cable produce less smoke when burned, which permits people to exit a burning building more quickly and results in less soot damage to electronic equipment located near the fire.
Because LSZH releases little or no halogen gas when burned, it reduces the human respiratory system's damage if inhaled. It contributes to less corrosion damage to equipment near the fire.
LSZH jackets have a lower coefficient of friction than some non-LSZH jackets, which can make installation easier.
Because LSZH is more susceptible to jacket cracking caused by pulling lubricants or cable bending, special lubricants1 have been developed to minimize cable damage during installation.
LSZH jacket compounds usually have very high filler content (approx. 50%) to provide the required flame and smoke performance. As a result, most have poorer mechanical, chemical resistance, water absorption and electrical properties than non-LSZH compounds.
The current generation of LSZH cables has not yet established a proven history of long-term performance.
What are Halogens?
Although everyone is familiar with smoke, halogens are not as well understood. When present in a fire, halogens are a class of chemical elements that can form hazardous gasses. For the wire and cable industry, chlorine, fluorine and bromine halogens pose the most concern. Certain polymers contain halogens as part of their basic chemical structure, for example, chlorine in PVC and fluorine in FEP. Ironically, halogens are excellent low-cost fire-retardants, so halogenated fire- retardants are sometimes added to non-halogenated wire and cable polymers to decrease their flammability—resulting in halogenated cables! Fortunately, halogen-free fire retardants are available, but they are not universally used because of their generally higher cost. The typical halogen
Typical Halogen Content of Common Wire and Cable Polymers
XLP (cross-linked polyethylene) <0.02 (% by weight)
FR-XLP (flame-retardant XLP) with halogen-free flame retardants <0.02 (% by weight)
EPR (ethylene propylene rubber) <0.02 (% by weight)
PU (polyurethane) <0.02 (% by weight)
PE (polyethylene) <0.02 (% by weight)
FR-PE (flame-retardant PE) with halogen-free flame retardants <0.02 (% by weight)
FR-XLP with halogenated flame retardants 7–17 (% by weight)
FR-EPR with halogenated flame retardants 9–14 (% by weight)
CSPE (chlorosulfonated polyethylene) 13–26 (% by weight)
CPE (chlorinated polyethylene) 14–28 (% by weight)
PVC (polyvinyl chloride) 22–29 (% by weight)
FEP (fluorinated ethylene propylene) 62–78 (% by weight)
Various abbreviations are used to refer to low-smoke zero-halogen cables around the world. In addition to LSZH, other common names include LSF (low smoke and fume), LSHF (low smoke halogen-free) and LS0H (low smoke zero halogens). These names have no official definition, but numerous test methods have been developed and published by industry organizations to define specific performance levels. A few terms that do have official definitions are described below.
Plenum Rated: In the United States, a cable must pass the smoke emission and fire propagation requirements contained in NFPA 2622 to be plenum rated. This industry-standard specifies maximum permissible values for flame spread and optical density of evolved smoke, but it does not specify halogen content. The NEC3 requires plenum-rated cables such as types CMP and CL2P in certain applications, including ducts, plenums and other spaces for environmental air handling, such as those above-suspended ceilings.
LS Rated: UL standards define LS-rated wire and cable differently, depending on the type of product. For single-conductor wire types that must meet the requirements of UL 443 or UL 834, such as XHHW, RHW, RHW-2, THW and THWN, an LS-rated wire must meet specific values for flame spread, smoke emission and halogen acid gas. However, for other cable types such as TC, AC, ITC, MC, CMR, OFN and CM, an LS-rated cable must comply with only the flame spread and smoke-emission values in UL 16855—a halogen requirement is not specified. In either case, a wire or cable that meets all applicable requirements can be printed LS to indicate compliance—for example, RHW-LS, TC-LS or CM-LS. The NEC permits but does not require the use of LS-rated wire or cable.
ST1 Rated: Industry standards define an ST1-rated cable as a cable that passes the ST1 smoke requirements in standards such as UL 44 and UL 83. These standards specify maximum total and peak smoke-emission values but do not specify halogen content. A wire or cable that meets these requirements can be printed ST1 to indicate compliance. As with LS-rated wires, the NEC permits installing ST1-rated wires but does not require their use.
One term that is not yet well standardized in the industry is zero-halogen (or halogen-free). A few existing standards define a zero-halogen polymer containing less than 0.2 per cent (2,000 ppm) halogen content by weight. However, other standards have significant variations in test methods and pass/fail criteria. Some typical standards used for evaluating the halogen content of wire and cable are listed below :
ASTM D5424 Combustion Gas Analysis (USA)
BSI BS EN 50267-2-1 Gases Evolved During Combustion of Electric Cables (United Kingdom)
CSA C22.2 No. 0.3 Acid Gas Evolution (Canada)
DIN VG 95218-60 Halogen Free Wire & Cable (Germany)
ICEA T-33-655 Low-Smoke, Halogen-Free Polymeric Cable Jackets (North America)
IEC 60754-1 Gases Evolved During Combustion of Materials from Cables (International)
MIL-DTL-24643 Acid Gas Evolution (USA)
MIL-DTL-24643 Halogen Content (USA)
UL 44, Clause 18.104.22.168 Halogen Acid Gas Emission (USA)