Currently, the cable industry commonly refers to cables with certain fire performance characteristics, such as flame retardant, halogen-free low smoke, low halogen low smoke, or fire resistant, collectively as fire-resistant cables.

(1) Flame Retardant Cables
Flame retardant cables are designed to delay the spread of flame along the cable, preventing the fire from expanding. Due to their low cost, they are widely used in fire-resistant cable applications. Whether in single cable or bundled installation conditions, these cables can confine the spread of flame within a certain range when burned, thereby avoiding major disasters caused by cable fire spread and improving the fire safety level of cable lines.

(2) Halogen-Free Low Smoke Flame Retardant Cables
Halogen-free low smoke cables not only exhibit excellent flame retardancy but are also made of materials free from halogens. When burned, they produce low corrosivity and toxicity, with minimal smoke generation, thereby reducing harm to people, instruments, and equipment, which facilitates timely rescue during a fire. Although halogen-free low smoke flame retardant cables offer excellent flame retardancy, corrosion resistance, and low smoke density, their mechanical and electrical properties are slightly inferior to those of ordinary cables.

(3) Low Halogen Low Smoke Flame Retunrdant Cables
The hydrogen chloride emission and smoke density indicators of low halogen low smoke flame retardant cables fall between those of flame retardant cables and halogen-free low smoke flame retardant cables. Low halogen cables also contain halogens, but in lower amounts. These cables not only provide flame retardancy but also emit less smoke and lower hydrogen chloride when burned. Such cables are typically made from PVC-based materials, combined with efficient flame retardants, hydrogen chloride absorbers, and smoke suppressants. Thus, this flame retardant material significantly improves the combustion performance of ordinary flame retardant PVC materials.

(4) Fire Resistant Cables
Fire resistant cables can maintain normal operation for a certain period under flame combustion, ensuring circuit integrity. These cables produce low levels of acidic gas and smoke during combustion, significantly enhancing fire resistance. Particularly under conditions such as water spraying and mechanical impact during a fire, the cables can still maintain circuit operation.

(5) Flame Retardant Cable Standards and Ratings
The main technical indicators related to fire safety in cables are the flame retardancy of CO² cables, smoke density, and gas toxicity. U.S. fire standards focus more on the first two issues, while Europe and the U.S. hold completely different views on fire safety. The traditional U.S. perspective believes that the root cause of fire lies in the generation of carbon monoxide poisoning and the subsequent heat release when CO converts to CO₂ during combustion. Therefore, controlling the heat release during combustion can reduce fire hazards. Europe has long believed that the amount of halogen acid released, gas corrosivity, smoke density, and gas toxicity during combustion are the main factors determining whether people can safely escape a fire scene.

1) IEC Flame Retardant Ratings
To evaluate the flame retardancy of cables, the International Electrotechnical Commission (IEC) has established three standards: IEC 60332-1, IEC 60332-2, and IEC 60332-3. IEC 60332-1 and IEC 60332-2 are used to evaluate the flame retardancy of a single cable under inclined and vertical installation conditions (corresponding to Chinese standards GB 12666.3 and GB 12666.4). IEC 60332-3 (corresponding to Chinese standard GB 12666.5-90) is used to evaluate the flame retardancy of bundled cables under vertical burning conditions. In comparison, the requirements for flame retardancy under vertical burning of bundled cables are much higher.

• IEC 60332-1/BS 4066-1 Flame Retardant Rating (Single Wire or      Cable Vertical Burning Test): This is the      flame retardant standard for a single cable. The test specifies that a 60      cm long sample is vertically fixed in an open metal box, and a propane      burner with a flame length of 175 mm contacts the cable at a 45-degree      angle from a position 450 mm away from the upper fixed end of the sample.      If the burned portion of the sample does not exceed 50 mm from the lower      fixed end, the test is passed.

• IEC 60332-3/BS 4066-3 Flame Retardant Rating: This is the flame retardant standard for bundled cables. The      test specifies that bundled cable samples of 3.5 m length are fixed on a      ladder-shaped test rack with iron wire, and the number of samples is      determined based on the non-metallic material requirements for different      classifications. The samples are vertically hung on the back wall of the      combustion furnace, and air is introduced through the intake port on the      bottom plate. A propane flat burner contacts the sample with a 750°C      flame. Under forced air blowing (airflow discharge 5 m³/min, wind speed      0.9 m/s), the cables must not ignite within 20 minutes of vertical      burning, and the flame must self-extinguish within 2.5 meters. IEC 60332      is divided into Class A, B, C, and D to evaluate the flame retardancy      performance.

2) UL Flame Retardant Standards
If any cable listed by UL is tested and verified to meet a certain fire rating, it can be printed with the UL identification, fire rating, and approval number.

• Plenum Grade - CMP (Airflow Burning Test/Steiner Tunnel Test): This is the highest requirement among UL fire standards for      cables, applicable to safety standard UL 910. The test specifies that      multiple samples are laid on the horizontal air duct of the device and      burned with an 87.9 kW gas Bunsen burner (300,000 BTU/hr) for 20 minutes.      The pass standard is that the flame must not extend beyond 5 feet from the      front end of the gas Bunsen burner flame. The peak optical density must      not exceed 0.5, and the average density must not exceed 0.15. Such CMP      cables are typically installed in ventilation ducts or air return plenum      systems used by air handling equipment and are recognized in Canada and      the United States. FEP/PLENUM materials that meet UL 910 standards have      better flame retardancy than low-smoke zero-halogen materials that meet      IEC 60332-1 and IEC 60332-3 standards and produce lower smoke      concentration when burned.

• Riser Grade - CMR (Vertical Burning Test): This is a commercial-grade cable in UL standards, applicable      to safety standard UL 1666. The test specifies that multiple samples are      laid on a simulated vertical shaft and burned with a specified 154.5 kW      gas Bunsen burner (527,500 BTU/hr) for 30 minutes. The pass standard is      that the flame must not spread to the upper part of a 12-foot-high room.      Riser-grade cables have no smoke concentration specifications and are      generally used for vertical and horizontal wiring between floors.

• Commercial Grade - CM (Vertical Burning Test): This is a commercial-grade cable in UL standards, applicable      to safety standard UL 1581. The test specifies that multiple samples are      laid on an 8-foot-high vertical bracket and burned with a specified 20 kW      ribbon torch (70,000 BTU/hr) for 20 minutes. The pass standard is that the      flame must not spread to the upper end of the cable and must      self-extinguish. UL 1581 is similar to IEC 60332-3C, except for the number      of cables laid. Commercial-grade cables have no smoke concentration      specifications and are generally only used for horizontal wiring on the      same floor, not for vertical wiring between floors.

• General Grade - CMG (Vertical Burning Test): This is a general-grade cable in UL standards, applicable to      safety standard UL 1581. The test conditions for commercial grade and      general grade are similar, and both are recognized for use in Canada and      the United States. General-grade cables have no smoke concentration      specifications and are generally only used for horizontal wiring on the      same floor, not for vertical wiring between floors.

• Residential Grade - CMX (Vertical Burning Test): This is a residential-grade cable in UL standards, applicable      to safety standard UL 1581, VW-1. The test specifies that the sample is      kept vertical and burned with a test torch (30,000 BTU/hr) for 15 seconds,      then stopped for 15 seconds, repeated five times. The pass standard is      that the residual flame must not exceed 60 seconds, the sample must not be      burned more than 25%, and the cotton placed at the bottom must not be      ignited by falling objects. UL 1581-VW-1 is similar to IEC 60332-1, except      for the burning time. This grade also has no smoke or toxicity      specifications and is only used in home or small office systems where a      single cable is installed. Such cables should not be used in bundles and      must be installed in conduits.

3) Smoke Density, Halogen Content, and Toxicity Ratings

• IEC 60754-1/BS 6425-1 Determination of Halogen Gas Content: This is the specification for hydrogen chloride emission      concentration in IEC and BS standards. Halogens include fluorine,      chlorine, bromine, iodine, and the radioactive volatile element astatine,      which are highly toxic. The test specifies that when the combustion      furnace is preheated to 800°C, a built-in 1.0g sample is pushed into the      furnace, and the airflow rate is used to dissolve HCl into water, after      which the halogen acid content of the aqueous solution is measured. If the      halogen acid emission of the cable material during combustion is less than      5 mg/g, it can be called a halogen-free cable. If the halogen acid      emission is greater than 5 mg/g but less than 15 mg/g, it can be called a      low-halogen cable. It is worth noting that the IEC 60754-1 method cannot      be used to measure materials with an HCl content of less than 5 mg/g,      meaning it cannot determine whether they are "halogen-free." To      determine whether they are completely halogen-free, the IEC 60754-2 method      can be used.

• IEC 60754-2 Measurement of Gas Acidity: This is the specification for the corrosiveness of combustion      gases in IEC standards. This test measures the acidity of halogen acid      gases produced by materials during combustion by determining the pH value      and conductivity of the aqueous solution. The test specifies that the      combustion furnace is preheated to 800°C, and a quartz tube with a      built-in sample is pushed into the furnace while timing begins. In the      first 5 minutes of sample combustion, the pH value and electrical      conductivity are measured every minute, and for the next 25 minutes, they      are measured every 5 minutes. Generally, halogen-free cable materials have      a pH value greater than 4.3 and a conductivity of less than 10 μS. The      lower the pH value, the higher the acidity of the halogen acid gas emitted      by the material. It is worth noting that when the HCl content is greater      than 2 mg/g but less than 5 mg/g (i.e., meeting the requirements of IEC      60754-1), the pH value of its aqueous solution is also less than 4.3,      meaning it does not meet the requirements of IEC 60754-2.

• IEC 61034-1/ASTM E662 Smoke Density:      This is the specification for smoke density in IEC and ASTM standards. The      test consists of a 3m³ cube and a photometric measurement system with a      light source. A rectangular tray containing alcohol is used as the      combustion source. A blower with a power of 10-15 m³/min ensures uniform      distribution of smoke and prevents flame vortices on the tray. When the      alcohol burns, a recorder connected to the light source records the      reduction in light. Smoke density is measured by light transmittance. If a      light transmission value of 60% can be achieved, the cable material meets      the low-smoke standard. The higher the light transmittance, the less smoke      the material releases during combustion.

• ISO 4589-2/BS 2863 Oxygen Index:      This is the specification for oxygen index in ISO and BS standards. It      refers to the minimum oxygen concentration in air at room temperature      required to support combustion of a material. The higher the oxygen index      value, the more flame-retardant the material is. If a material has an      oxygen index of 21%, it means the material will ignite spontaneously at      normal room temperature since the oxygen content in air under normal room      conditions is 21%. Generally, flame-retardant cables have an oxygen index      greater than 33%.

• ISO 4589-3/BS 2782.1 Temperature Index: This is the specification for temperature index in ISO and BS      standards. The oxygen index of a material decreases as the temperature      increases. When the temperature rises and the oxygen index of the material      drops to 21%, the material will ignite spontaneously. This temperature is      called the temperature index. For example, the oxygen index of coal at      room temperature is 50%, but when the temperature rises to 150°C, the      oxygen index drops to 21%, and the material will ignite immediately. Thus,      the temperature index of this material is 150°C. Generally,      flame-retardant cables have a temperature index greater than 250°C.

• NES 713 Toxicity Index: This is the      specification for the toxicity of gases produced by cable materials during      combustion in the British Naval Engineering Standard (NES). Toxicity      refers to the property of destroying the structure or disrupting the      function of an organism. The toxicity index represents the total toxicity      performance of all gases produced during material combustion. The test      specifies that the combustion furnace is preheated to 800°C, and the toxic      substances contained in the cable material are burned separately. The      airflow rate is then used to collect each toxic gas, and chemical analysis      is performed to calculate the content of each toxic substance. This index      is expressed numerically. The higher the toxicity index, the higher the      toxicity of the gases released by the material. Generally, halogen-free      cable materials have a toxicity index of less than 5. It is worth noting      that low-smoke zero-halogen materials also produce toxic CO when burned.      If the material contains P, N, or S, even more toxic gases are generated.      Therefore, halogen-free cables cannot be called non-toxic cables but      should be called low-toxicity cables.

CM, CMR, and CMP cables, as they need to meet strict UL fire standards, often use materials containing halogens. CM and CMR cables are generally based on polyvinyl chloride (PVC), and PVC materials contain chlorine. CMP cables are generally based on Teflon (polytetrafluoroethylene), and FEP materials contain fluorine. The gas toxicity produced by such halogen-containing cables is several times greater than that of halogen-free cables, posing significant potential safety hazards in fires. It may lead to most casualties in a fire scene not being burned to death but suffocated by toxic gases.

4) IEC Fire Resistance Ratings
Fire-resistant cables refer to cables that can maintain normal operation for a certain period under flame combustion, i.e., maintaining circuit integrity. To evaluate the fire resistance performance of cables, the International Electrotechnical Commission and the British Electrotechnical Committee have formulated the IEC 331 and BS 6387 standards, respectively. In comparison, BS 6387 has much higher requirements for fire resistance capability.

• IEC 60331 Flame Retardant Rating:      In IEC 60331-1999, the fire temperature requirement is 750°C/3h level,      indicating that it does not break down when burned horizontally at 750°C      under 300 volts for 3 hours.

• BS 6387 Flame Retardant Rating: BS      6387 requires passing horizontal burning test, water spray test, and      mechanical impact vibration burning test.

• Horizontal burning tests are classified as Class A 650°C/3h,       Class B 750°C/3h, Class C 950°C/3h, and Class S 950°C/3min.

• Class A: Does not break down when burned horizontally at        650°C under 300 volts for 3 hours.

• Class B: Does not break down when burned horizontally at        750°C under 300 volts for 3 hours.

• Class C: Does not break down when burned horizontally at        950°C under 300 volts for 3 hours.

• Class S: Does not break down when burned horizontally at        950°C under 300 volts for 3 minutes.

• Water spray burning test is classified as Class W, indicating       that it does not break down when burned for 15 minutes under 300 volts       and then sprayed with water while burning for another 15 minutes.

• Impact vibration burning tests are classified as Class X       650°C/15min, Class Y 750°C/15min, and Class Z 950°C/15min.

• Class X: Does not break down when burned at 650°C under 300        volts while being mechanically impacted every 30 seconds for 15 minutes.

• Class Y: Does not break down when burned at 750°C under 300        volts while being mechanically impacted every 30 seconds for 15 minutes.

• Class Z: Does not break down when burned at 950°C under 300        volts while being mechanically impacted every 30 seconds for 15 minutes.
         The highest grade model required by BS 6387 is CWZ.