1. Understanding Halogen-Free Flame Retardant Technology 了解无卤阻燃技术

Over the past two decades, the cable industry has witnessed a fundamental transformation in safety requirements for confined space installations, particularly in tunnels, underground transit systems, and mining operations. At the center of this evolution lies the shift from traditional halogenated flame retardant cables to halogen-free, low-smoke alternatives. This technological transition represents more than just a materials change—it reflects a deeper understanding of fire safety dynamics and the true risks that cable fires pose to human life, critical infrastructure, and emergency response operations.¹

在过去的二十年中，电缆行业在密闭空间安装（特别是隧道、地下交通系统和采矿作业）的安全要求方面发生了根本性的转变。这一演变的核心是从传统的卤化阻燃电缆向无卤、低烟替代品的转变。

Halogens, defined as Group 17 elements in the periodic table, include fluorine, chlorine, bromine, and iodine. These elements have been widely used in cable manufacturing because they provide excellent flame retardant properties when incorporated into insulation and jacketing materials. Polyvinyl chloride, commonly known as PVC, contains chlorine and has dominated cable construction for decades due to its favorable electrical properties, mechanical durability, and inherent flame resistance. Chloroprene rubber also contains chlorine, while some high-temperature cable materials incorporate fluorine-based polymers such as FEP, PTFE, and ETFE. Bromine-based flame retardant additives are frequently added to enhance fire resistance in various cable compounds.²

However, the very properties that make halogens effective flame retardants become catastrophically dangerous when these materials burn. When halogenated cables are exposed to fire or high temperatures, the halogens undergo combustion reactions that produce halogen acids—specifically hydrochloric acid from chlorine, hydrofluoric acid from fluorine, and hydrobromic acid from bromine. These acids form when halogen gases combine with hydrogen and subsequently react with moisture, whether from the air, firefighting water, or human respiratory systems. The resulting toxic and corrosive fumes create a dual threat: immediate danger to anyone attempting evacuation or rescue, and long-term damage to sensitive electronic equipment, structural reinforcement materials, and building systems.³

[1] HELUKABEL. (2025). Flame Retardancy of Halogen-Free Cables. Retrieved from https://www.helukabel.us/newsroom/flame-resistance-halogen-free-cables

[2] Transfer Multisort Elektronik. (2023). Halogen-free Cables and the Related Regulations. Retrieved from https://www.tme.com/us/halogen-free-cables-regulations

[3] SAB Cable. (2025). In Case of Fire, Choose the Most Protective Halogen-Free Cable. Retrieved from https://sabcable.com/resources/halogen-free-cable

[4] CB Cables. (2025). When to Use Low Smoke Zero Halogen Cables. Retrieved from https://www.cbcables.com/blog/when-to-use-lszh-cables

1.1 What Makes a Cable Truly Halogen-Free 什么使电缆真正无卤

The term “halogen-free” has specific technical definitions established by international standards organizations. According to IEC 61249-2-21, the authoritative international standard for halogen-free materials, a cable component can be classified as halogen-free only when the halogen content of each individual element (fluorine, chlorine, bromine, and iodine) does not exceed nine hundred parts per million by weight, with the total halogen content not exceeding fifteen hundred parts per million. This stringent threshold ensures that even when the material undergoes combustion, the amount of halogen acid gases produced remains below dangerous levels.⁵

根据IEC 61249-2-21（无卤材料的权威国际标准），只有当每个单独元素（氟、氯、溴和碘）的卤素含量不超过百万分之九百（按重量计），且总卤素含量不超过百万分之一千五百时，电缆组件才能被归类为无卤。

Additional testing standards verify halogen-free performance through combustion analysis. DIN EN 60754-1 and VDE 0482-754-1 establish test procedures for measuring the total acidity of gases evolved during cable combustion, with materials required to produce less than five milligrams of halogen acid per gram of material to achieve halogen-free certification. Complementary testing under DIN EN 60754-2 and VDE 0482-754-2 measures the pH value and electrical conductivity of aqueous solutions containing combustion gases, with pH values required to be above 4.3 and conductivity below ten microsiemens per millimeter to confirm low acid gas production.⁶

Modern halogen-free cables achieve flame retardancy through alternative chemistry based on thermoplastic and thermoset polyolefin compounds. These materials incorporate aluminum trihydrate or magnesium hydroxide as flame retardant fillers, which decompose endothermically when exposed to heat, absorbing thermal energy and releasing water vapor that dilutes combustible gases and cools the flame zone. Cross-linked polyethylene, ethylene-vinyl acetate copolymers, and thermoplastic elastomers form the polymer matrix, providing the necessary electrical insulation and mechanical properties while remaining free of halogen elements.⁷

[5] Acquis Compliance. (2023). Understanding Halogen-Free Standards: IEC 61249-2-21. Retrieved from https://www.acquiscompliance.com/blog/halogen-free-standards-iec

[6] HELUKABEL. (2025). Flame Retardancy of Halogen-Free Cables. Retrieved from https://www.helukabel.us/newsroom/flame-resistance-halogen-free-cables

[7] Frigate AI. (2025). Halogen-Free Cables: Flame-Retardant and Low-Smoke Zero Halogen. Retrieved from https://frigate.ai/product/halogen-free-cables

2. Historical Incidents That Drove Regulatory Change 推动监管变革的历史事件

The global movement toward mandatory halogen-free cables in tunnels and confined spaces did not emerge from abstract safety analysis but from tragic real-world incidents that demonstrated the devastating consequences of halogenated cable fires. These catastrophic events fundamentally changed how regulatory bodies, infrastructure operators, and safety engineers approach cable specifications for underground and enclosed environments.

2.1 The King’s Cross Fire and London Underground Tragedy 国王十字火灾和伦敦地铁悲剧

On November 18, 1987, a fire at King’s Cross St. Pancras underground station in London claimed thirty-one lives and injured approximately one hundred people, becoming one of the deadliest peacetime disasters in British history. The fire began beneath a wooden escalator but rapidly intensified due to the combustion of cable insulation materials throughout the station. Post-incident investigation revealed that the dense black smoke produced by burning cables, many of which contained halogenated materials, contributed significantly to the death toll by obscuring escape routes and producing toxic fumes that incapacitated passengers before they could reach safety.⁸

1987年11月18日，伦敦国王十字圣潘克拉斯地铁站发生火灾，造成31人死亡，约100人受伤，成为英国历史上最致命的和平时期灾难之一。调查显示，燃烧电缆产生的浓密黑烟（其中许多含有卤化材料）通过遮蔽逃生路线和产生有毒烟雾严重影响了死亡人数。

The formal inquiry that followed this tragedy led to sweeping changes in fire safety standards for underground transit systems worldwide. One key recommendation focused specifically on cable materials, noting that the contribution of wire and cable jacketing to fire propagation and smoke production had been severely underestimated. As a direct consequence, British standards BS 6724 and BS 7211 were strengthened to explicitly recommend or mandate halogen-free cables in public buildings and transportation infrastructure. Today, halogen-containing cables are completely prohibited in all new London Underground projects and National Health Service hospital installations in the United Kingdom.⁹

[8] Wikipedia. (2025). Low Smoke Zero Halogen. Retrieved from https://en.wikipedia.org/wiki/Low_smoke_zero_halogen

[9] TST Cables. (2025). Requirements and Standards for LSZH Cables in Various Countries. Retrieved from https://www.tstcables.com/lszh-cables-requirements-standards

2.2 The Washington DC Metro Tunnel Incident 华盛顿特区地铁隧道事故

A more recent tragedy occurred in January 2015 when an electrical short circuit in a Washington DC Metro subway tunnel caused legacy power cables to ignite, producing noxious smoke and toxic gases. One passenger died from smoke inhalation, and dozens of others required hospitalization for respiratory injuries. The incident investigation determined that the burning cables, which contained halogenated insulation materials, emitted corrosive hydrogen chloride gas and dense smoke that filled the tunnel and subway cars, preventing safe evacuation despite the relatively small scale of the actual fire.¹⁰

This disaster prompted Metro maintenance officials to accelerate the long-planned replacement of older power cables with low-halogen or halogen-free alternatives throughout the Washington DC transit system. The incident also influenced transit authorities across North America to reassess their cable specifications, with many municipalities updating standards to require halogen-free construction for all new cable installations in underground environments. The tragedy demonstrated that even in modern transit systems with extensive safety protocols, the choice of cable materials remains a critical factor in passenger protection during fire emergencies.¹¹

[10] Texcan. (2025). Choosing Appropriate Cables for Transportation Project Design. Retrieved from https://www.texcan.com/articles/choosing-cables-transportation

[11] Texcan. (2025). Choosing Appropriate Cables for Transportation Project Design. Retrieved from https://www.texcan.com/articles/choosing-cables-transportation

2.3 Military and Naval Lessons: HMS Sheffield 军事和海军教训：谢菲尔德号

The adoption of halogen-free cables in military and naval applications was significantly influenced by the 1982 Falklands War incident involving HMS Sheffield. When the British destroyer was struck by an Exocet missile, the resulting fire produced extraordinarily dense black smoke from burning cables and other materials containing halogenated flame retardants. The toxic and corrosive smoke quickly spread throughout the ship, hampering damage control efforts and contributing to the vessel’s eventual loss. Post-incident analysis revealed that crew members suffered respiratory injuries from the corrosive gases produced by burning PVC-jacketed cables, even in areas distant from the impact zone.¹²

Following this and similar incidents in confined military environments, defense organizations worldwide began mandating halogen-free cable specifications for naval vessels, submarines, armored vehicles, and aircraft applications. The military sector’s early adoption of low-smoke, halogen-free technology helped drive materials research and manufacturing capability improvements that eventually made these cables economically viable for commercial tunnel and building applications.¹³

[12] Wikipedia. (2025). Low Smoke Zero Halogen. Retrieved from https://en.wikipedia.org/wiki/Low_smoke_zero_halogen

[13] Cabling Installation & Maintenance. (2025). UL to Industry: Time to Clarify Some Misconceptions. Retrieved from https://www.cablinginstall.com/standards/ul-misconceptions

3. Global Regulatory Framework and Mandatory Requirements 全球监管框架和强制性要求

The transition from optional to mandatory halogen-free cable requirements has occurred at different paces across various regions and jurisdictions, but the overall trend is unmistakably toward stricter regulations that eliminate halogenated cables from tunnel, underground transit, and public building applications. Understanding this evolving regulatory landscape is essential for infrastructure developers, mining operators, and cable specifiers working on international projects or planning installations that must remain compliant for decades of operational life.

3.1 European Union Construction Products Regulation 欧盟建筑产品法规

The European Union has established the world’s most comprehensive and stringent framework for halogen-free cable requirements through the Construction Products Regulation, officially designated as EU Regulation 305/2011. Implemented in 2017, this regulation mandates that all power and communication cables used in permanent building installations must undergo fire classification testing according to EN 50575 standards and bear the CE conformity marking demonstrating compliance with established performance criteria.¹⁴

欧盟通过建筑产品法规（正式名称为EU 305/2011法规）建立了世界上最全面和最严格的无卤电缆要求框架。该法规于2017年实施，要求所有用于永久性建筑安装的电力和通信电缆都必须按照EN 50575标准进行防火分类测试。

The EN 50575 classification system categorizes cables into seven performance classes ranging from Aca (highest performance) to Fca (lowest performance), with specific classes designated for different application scenarios. For underground transit systems including subways, metro railways, and vehicular tunnels, European regulations explicitly require cables meeting Cca, Dca, or higher classifications, all of which mandate halogen-free construction to limit smoke production and toxic gas emission. Germany, France, the Netherlands, Sweden, Italy, and other EU member states strictly enforce these CPR low-smoke halogen-free standards, with non-compliant cables prohibited from sale or installation in covered applications.¹⁵

Additional European standards provide detailed technical requirements beyond the basic CPR framework. EN 50305 specifies maximum corrosivity levels by requiring combustion product pH values to remain above 4.3, ensuring that acid gas production stays within safe limits even during extended fires. The stringent emphasis on corrosivity testing reflects European infrastructure operators’ concern about protecting sensitive electronic equipment in railway signaling systems, telecommunications facilities, and tunnel ventilation control systems from acid damage that could compromise functionality long after fire suppression.¹⁶

[14] TST Cables. (2025). Requirements and Standards for LSZH Cables in Various Countries. Retrieved from https://www.tstcables.com/lszh-cables-requirements-standards

[15] Transfer Multisort Elektronik. (2023). Halogen-free Cables and the Related Regulations. Retrieved from https://www.tme.com/us/halogen-free-cables-regulations

[16] TST Cables. (2025). Requirements and Standards for LSZH Cables in Various Countries. Retrieved from https://www.tstcables.com/lszh-cables-requirements-standards

3.2 Chinese GB Standards and New Certification Requirements 中国GB标准和新认证要求

China has implemented comprehensive halogen-free cable standards through the GB (Guobiao or National Standard) system, with recent regulatory updates significantly strengthening enforcement mechanisms. GB 31247-2014 establishes the classification system for flame-retardant and fire-resistant wires and cables, dividing products into A, B, C, and D grades based on performance characteristics. The highest flame-retardant classification, designated B1 grade, explicitly requires cables to demonstrate low smoke properties with light transmittance of at least sixty percent, halogen-free construction with halogen acid gas emission below five milligrams per gram, and low toxicity during combustion.¹⁷

The general technical requirements for low-smoke halogen-free cables are specified in GB/T 19666, which applies across all scenarios within Chinese jurisdiction and establishes baseline performance criteria that all compliant cables must meet. More specific application requirements are mandated by GB 50217, the Code for Design of Power Engineering Cables, which identifies compulsory scenarios where halogen-free cables must be used. These mandatory applications include all subway and metro railway systems such as those in Beijing, Shanghai, Guangzhou, and Shenzhen; high-speed rail installations; data centers and telecommunications facilities; hospitals and medical centers; and super high-rise buildings including landmark structures like the Shenzhen Ping An Finance Center.¹⁸

A critical regulatory change takes effect on July 1, 2025, when new Chinese regulations will require all flame-retardant wires and cables to pass mandatory CCC (China Compulsory Certification, also known as 3C certification) before they can be legally sold or installed. Products failing to meet the established standards will be prohibited from the market entirely. This certification requirement ensures that imported and domestically produced cables undergo rigorous testing by accredited laboratories before being approved for use in Chinese infrastructure projects, significantly reducing the risk of substandard or counterfeit products entering the supply chain.¹⁹

[17] CB Cables. (2025). When to Use Low Smoke Zero Halogen Cables. Retrieved from https://www.cbcables.com/blog/when-to-use-lszh-cables

[18] CB Cables. (2025). When to Use Low Smoke Zero Halogen Cables. Retrieved from https://www.cbcables.com/blog/when-to-use-lszh-cables

[19] TST Cables. (2025). Requirements and Standards for LSZH Cables in Various Countries. Retrieved from https://www.tstcables.com/lszh-cables-requirements-standards

3.3 North American Approach: Project-Based Requirements 北美方法：基于项目的要求

Unlike the European Union’s comprehensive regulatory framework or China’s national standards system, the United States has not adopted federal mandates requiring halogen-free cables in all tunnel and underground applications. However, this does not mean that American tunnel projects can freely use halogenated cables. Instead, halogen-free requirements are typically imposed through project specifications, transportation authority standards, and recommendations from influential safety organizations like the National Fire Protection Association.²⁰

NFPA 130, the Standard for Fixed Guideway Transit and Passenger Rail Systems, strongly recommends the use of low-smoke, halogen-free materials in confined spaces including subway tunnels, elevated rail systems, and underground stations. Similarly, NFPA 101, the Life Safety Code, provides guidance on cable materials for buildings and structures where limited egress routes make smoke and toxic gas production particularly dangerous. While these NFPA standards are not legally binding federal requirements, they are frequently adopted by state and local authorities having jurisdiction, and many transit agencies reference them directly in procurement specifications.²¹

Major infrastructure owners and technology companies have independently adopted halogen-free cable requirements as corporate standards. For example, Google and Microsoft explicitly require LSZH cabling in all data center tender documents and construction specifications, recognizing that the equipment protection benefits and improved personnel safety justify any additional material costs. Transit authorities operating major subway systems in cities like New York, Boston, San Francisco, and Chicago have progressively updated specifications to require halogen-free cables for new installations and major renovation projects, even absent federal mandates.²²

[20] Galaxy Wire & Cable. (2025). What Makes a Cable Low Smoke Zero Halogen and When is it Required? Retrieved from https://www.galaxywire.com/what-makes-cable-lszh

[21] Top Cable. (2025). Fire Resistant Cables vs Halogen-Free Cables. Retrieved from https://www.topcable.com/blog/fire-resistant-halogenfree

[22] Wesco. (2025). Cable Materials Evolution: The Shift to Halogen Free. Retrieved from https://www.wesco.com/knowledge-hub/cable-materials-halogen-free

4. Why Tunnels and Mining Operations Require Halogen-Free Cables 为什么隧道和采矿作业需要无卤电缆

The physical characteristics of tunnels and underground mining environments create uniquely dangerous conditions during fire emergencies that make halogen-free cables not just preferable but essential for protecting human life and preserving critical infrastructure. Understanding these environmental factors helps explain why regulatory bodies worldwide have prioritized tunnel applications when implementing mandatory halogen-free requirements.

4.1 Confined Space Smoke Accumulation 密闭空间烟雾积聚

Tunnels and underground mining galleries are characterized by limited ventilation and restricted air circulation compared to above-ground structures. During a cable fire, smoke and combustion gases rapidly accumulate in these confined spaces, with no natural escape path to allow toxic fumes to dissipate harmlessly into the atmosphere. Standard tunnel ventilation systems, while designed to provide fresh air during normal operations, often prove inadequate to clear smoke during active fires, particularly when ventilation equipment itself becomes damaged by heat or corrosive gases.²³

隧道和地下矿井巷道的特点是与地面结构相比通风有限和空气循环受限。在电缆火灾期间，烟雾和燃烧气体迅速在这些密闭空间中积聚，没有自然逃逸路径允许有毒烟雾无害地消散到大气中。

Research on smoke behavior in tunnel fires has demonstrated that the smoke layer descends from the ceiling much more rapidly in confined underground environments than in conventional buildings with windows and multiple ventilation openings. When traditional PVC or other halogenated cables burn in a tunnel, the dense black smoke they produce can fill the entire cross-section of the tunnel within minutes, reducing visibility to near zero and making evacuation or rescue operations virtually impossible. In contrast, halogen-free cables produce significantly less smoke with much lower optical density, maintaining light transmittance above eighty percent even during active combustion. This improved visibility can mean the difference between successful evacuation and tragedy in underground emergency scenarios.²⁴

[23] SAB Cable. (2025). In Case of Fire, Choose the Most Protective Halogen-Free Cable. Retrieved from https://sabcable.com/resources/halogen-free-cable

[24] Remee Products. (2025). Benefits of Low Smoke Zero Halogen Cables. Retrieved from https://remee.com/low-smoke-zero-halogen-cable-benefits

4.2 Extended Evacuation Distances and Limited Egress Routes 延长的疏散距离和有限的出口路线

Unlike conventional buildings where occupants are typically within a hundred meters of an exit, tunnel users and mining personnel may be located several kilometers from the nearest safe egress point when a fire begins. Highway tunnels, railway tunnels, and mine shafts often extend for many kilometers with emergency exits spaced at intervals of five hundred to one thousand meters or more. This reality means that evacuation times measured in tens of minutes are normal and expected, rather than the two- to three-minute evacuation windows used in building fire safety calculations.

The extended evacuation timeline makes smoke toxicity and visibility even more critical than in surface structures. Occupants attempting to evacuate through a smoke-filled tunnel for ten or fifteen minutes face cumulative exposure to toxic gases that would be immediately fatal if the cable materials produce high concentrations of hydrogen chloride, hydrogen fluoride, or carbon monoxide. Halogen-free cables address this challenge by limiting toxic gas production to levels below fifty parts per million, allowing human exposure for the extended periods necessary to complete tunnel evacuation without causing irreversible respiratory damage or incapacitation.²⁵

Mining operations present additional complications due to complex tunnel networks with multiple branches, shafts at various elevations, and equipment that may obstruct emergency egress routes. Underground coal mines and metal mines often have limited escape ways that must serve as the only evacuation path for hundreds of workers distributed across kilometers of underground workings. In these environments, cable fires that produce dense smoke or toxic gases can trap miners underground for extended periods while rescue teams attempt to establish safe evacuation routes, making halogen-free cable construction an essential safety measure.²⁶

[25] Texcan. (2025). Choosing Appropriate Cables for Transportation Project Design. Retrieved from https://www.texcan.com/articles/choosing-cables-transportation

[26] Remee Products. (2025). Benefits of Low Smoke Zero Halogen Cables. Retrieved from https://remee.com/low-smoke-zero-halogen-cable-benefits

4.3 Protection of Critical Safety and Control Systems 保护关键安全和控制系统

Modern tunnels and mining operations rely heavily on electronic systems for ventilation control, emergency lighting, communication, fire detection, and emergency response coordination. These systems must remain operational during fire emergencies to support evacuation and firefighting efforts. However, the corrosive acid gases produced when halogenated cables burn can damage or destroy sensitive electronic equipment within minutes of exposure, rendering critical safety systems inoperable precisely when they are most needed.

Tunnel ventilation control systems, emergency lighting inverters, fire alarm panels, and communication equipment contain numerous metal components, printed circuit boards, and connector systems vulnerable to acid corrosion. By eliminating halogen content from cable insulation and jacketing materials, halogen-free cables prevent the formation of corrosive acid gases even when exposed to fire, allowing critical systems to continue functioning during emergencies and minimizing post-fire cleanup and restoration costs.²⁸

[27] Wesco. (2025). Cable Materials Evolution: The Shift to Halogen Free. Retrieved from https://www.wesco.com/knowledge-hub/cable-materials-halogen-free

[28] Frigate AI. (2025). Halogen-Free Cables: Flame-Retardant and Low-Smoke Zero Halogen. Retrieved from https://frigate.ai/product/halogen-free-cables

5. Technical Standards and Performance Testing 技术标准和性能测试

Ensuring that cables marketed as halogen-free and low-smoke actually meet safety performance requirements demands rigorous testing according to internationally recognized standards. Cable manufacturers, testing laboratories, and certification bodies employ multiple test methods to verify compliance with halogen content limits, smoke production characteristics, and flame retardant properties.

5.1 Key International Testing Standards 关键国际测试标准

The IEC 60754 series of standards establishes test methods for measuring gases evolved during combustion of cable materials. IEC 60754-1 specifies procedures for determining halogen acid gas content through chemical titration, while IEC 60754-2 measures the pH value and electrical conductivity of aqueous solutions containing combustion gases. These tests provide quantitative data on the amount of acidic gases produced when cable insulation burns, verifying that materials meet halogen-free classification criteria.²⁹

IEC 60754系列标准建立了测量电缆材料燃烧过程中释放的气体的测试方法。IEC 60754-1规定了通过化学滴定测定卤素酸气含量的程序，而IEC 60754-2测量含有燃烧气体的水溶液的pH值和电导率。

Smoke density testing follows procedures outlined in IEC 61034, which measures light transmittance through smoke produced by burning cable samples in a standardized test chamber. Cables are burned under defined conditions, and photometric equipment measures the percentage of light that can pass through the resulting smoke layer. Low-smoke cables must demonstrate light transmittance of at least sixty percent after three minutes of combustion, ensuring that visibility remains adequate for evacuation purposes even when cables are burning.³⁰

Flame retardancy is evaluated using various test methods depending on the cable type and installation configuration. IEC 60332-1 assesses single vertical cable flame propagation, verifying that individual cables are self-extinguishing and will not sustain burning when the ignition source is removed. For cables installed in bundles or on cable trays—the typical configuration in tunnels and mining applications—IEC 60332-3 vertical flame spread testing provides more realistic evaluation of fire behavior, measuring how far flames propagate along a multi-cable sample before self-extinguishing.³¹

[29] UL Solutions. (2025). Halogen Free and Low Smoke Halogen Free Cable Certification. Retrieved from https://www.ul.com/services/halogen-free-lshf-cable-certification

[30] Teknor Apex. (2025). Flame Retardant Wire and Cable Jacketing Compounds Receive Halogen-Free Assessments from UL. Retrieved from http://www.teknorapex.com/news/flame-retardant-halogen-free-ul

[31] Frigate AI. (2025). Halogen-Free Cables: Flame-Retardant and Low-Smoke Zero Halogen. Retrieved from https://frigate.ai/product/halogen-free-cables

5.2 Certification and Compliance Verification 认证和合规验证

Third-party certification from recognized testing laboratories provides independent verification that cables meet established halogen-free and low-smoke standards. Organizations such as UL Solutions, Underwriters Laboratories, and various national testing authorities offer optional certification programs specifically for halogen-free and low-smoke halogen-free cables. UL’s Material Recognition service under UL 2885 standard evaluates individual cable components including insulation compounds, jacket materials, fillers, and tapes for halogen content and combustion performance, while complete cable assembly certification programs verify that finished cables meet all applicable requirements.³²

The certification process involves comprehensive testing of cable samples, factory inspections to verify manufacturing quality control systems, and ongoing surveillance testing of production to ensure continued compliance. Certified cables receive distinctive markings or labels that allow purchasers, inspectors, and regulatory authorities to quickly verify compliance. For example, UL-certified halogen-free cables bear optional “-HF” surface markings, while cables meeting both halogen-free and low-smoke requirements display “-LSHF” markings according to IEC 62821 standards.³³

[32] UL Solutions. (2025). Halogen Free and Low Smoke Halogen Free Cable Certification. Retrieved from https://www.ul.com/services/halogen-free-lshf-cable-certification

[33] Cabling Installation & Maintenance. (2025). UL to Industry: Time to Clarify Some Misconceptions. Retrieved from https://www.cablinginstall.com/standards/ul-misconceptions

6. Engineering Considerations and Material Trade-offs 工程考虑因素和材料权衡

While the safety benefits of halogen-free cables are unquestionable in tunnel and underground applications, engineers must understand certain material property differences when transitioning from traditional halogenated cables to halogen-free alternatives. Proper cable selection and installation practices ensure that halogen-free cables deliver reliable long-term performance while maximizing safety advantages.

6.1 Mechanical Properties and Installation Requirements 机械性能和安装要求

Halogen-free cable compounds contain high percentages of inorganic filler materials such as aluminum trihydrate or magnesium hydroxide that provide flame retardancy through endothermic decomposition. These mineral fillers, while essential for fire performance, can affect mechanical properties compared to traditional PVC formulations. Halogen-free jackets may exhibit slightly reduced flexibility and cold-temperature performance, with some formulations becoming stiff or prone to cracking when installed in very cold environments or subjected to tight bending radii.³⁴

无卤电缆化合物含有高百分比的无机填料材料，如水合氧化铝或氢氧化镁，通过吸热分解提供阻燃性。这些矿物填料虽然对防火性能至关重要，但与传统PVC配方相比可能影响机械性能。

To address these characteristics during installation, cable manufacturers provide specific minimum bend radius specifications that must be observed to prevent jacket cracking. Installation teams should pay particular attention to ambient temperature during cable pulling operations, as halogen-free cables may require heated storage or pre-warming when installed in cold weather conditions below minus ten degrees Celsius. Modern advanced thermoplastic elastomer and thermoplastic polyolefin formulations have significantly improved the mechanical performance of halogen-free cables, with premium grades achieving flexibility and low-temperature performance approaching traditional PVC while maintaining excellent fire safety characteristics.³⁵

[34] Wesco. (2025). Cable Materials Evolution: The Shift to Halogen Free. Retrieved from https://www.wesco.com/knowledge-hub/cable-materials-halogen-free

[35] Wesco. (2025). Cable Materials Evolution: The Shift to Halogen Free. Retrieved from https://www.wesco.com/knowledge-hub/cable-materials-halogen-free

6.2 Cost Considerations and Lifecycle Value 成本考虑和生命周期价值

Halogen-free cable materials typically cost thirty to fifty percent more than equivalent PVC-jacketed cables on a purchase price basis. This cost premium reflects the specialized polymer formulations, higher filler content, and more complex manufacturing processes required to achieve halogen-free performance while maintaining necessary electrical and mechanical properties. For large tunnel or mining projects involving hundreds of kilometers of cable installation, this price difference can represent a significant budget impact that requires careful justification.³⁶

However, lifecycle cost analysis that includes potential fire damage, business interruption, equipment replacement, and liability costs presents a very different picture. The devastating equipment damage caused by corrosive gases from halogenated cable fires, as demonstrated in incidents like the AT&T Hinsdale fire, can result in losses measuring hundreds of millions of dollars and service interruptions lasting months. For critical infrastructure applications like subway systems, highway tunnels, and major mining operations, the potential cost of a single catastrophic fire far exceeds the additional cable material cost investment in halogen-free construction.³⁷

Many infrastructure owners have concluded that specifying halogen-free cables throughout a tunnel or underground facility represents the most cost-effective approach to risk management, eliminating the possibility of mixing cable types and ensuring uniform fire safety performance. The relatively modest initial cost premium becomes insignificant when amortized over the decades-long service life of tunnel infrastructure, while the safety and equipment protection benefits provide value throughout the installation’s operational lifetime.³⁸

[36] CB Cables. (2025). When to Use Low Smoke Zero Halogen Cables. Retrieved from https://www.cbcables.com/blog/when-to-use-lszh-cables

[37] Wesco. (2025). Cable Materials Evolution: The Shift to Halogen Free. Retrieved from https://www.wesco.com/knowledge-hub/cable-materials-halogen-free

[38] Top Cable. (2025). Fire Resistant Cables vs Halogen-Free Cables. Retrieved from https://www.topcable.com/blog/fire-resistant-halogenfree