Achieving the Highest Fire Safety Standards: An In-Depth Look at B1/d0 Grade Flame-Retardant Cables

In today's world, where electrical fires pose a significant threat to life and property, the fire safety performance of cables is paramount. Among the various fire safety standards, the B1/d0 grade represents a high level of flame retardancy, particularly concerning the emission of flaming droplets and particles during combustion. This article delves into the intricacies of achieving this standard, drawing insights from a recent research paper titled "Research on Flaming Droplets / Particles of d0 Grade for B1 Flame-Retardant Cables" by Sun Kai, Duan Chunlai, and Hong Senlin, published in the October 2024 issue of "Wire & Cable" journal. This paper meticulously analyzes the design, materials, and manufacturing processes crucial for producing B1-grade flame-retardant cables that meet the stringent d0 classification, meaning they produce no flaming droplets or particles during burning.

Understanding the Significance of B1/d0 Grade

Before diving into the specifics, it's essential to understand what the B1 and d0 classifications entail. These classifications are part of the GB 31247-2014 standard, which categorizes the burning behavior of cables and optical fibers.

• B1 Grade: This indicates a certain level of flame retardancy, meaning the cable has a limited contribution to fire propagation. It specifies requirements for flame spread, heat release rate, total heat release, smoke production, and vertical flame spread.
• d0 Grade: This is the highest classification for flaming droplets and particles. It signifies that during the standard fire test, the cable does not produce any flaming droplets or particles that could ignite surrounding materials and contribute to the spread of fire.

Achieving the d0 grade for B1 flame-retardant cables is a significant technical challenge, but it is increasingly becoming a requirement for cables installed in high-risk areas such as subway stations, airports, high-rise buildings, shopping malls, schools, stadiums, exhibition halls, and hospitals – places with high population density. The GB 51348-2019 "Standard for Electrical Design of Civil Buildings" also explicitly outlines the required fire performance grades for cables in various building types, often specifying B1/d0 for critical applications.

Key Factors Influencing the B1/d0 Performance

The research paper highlights three primary factors that significantly influence the ability of B1-grade flame-retardant cables to achieve the d0 rating: the sheath material, the cable structure, and the production process.

1. The Crucial Role of Sheath Material

The outer sheath of the cable acts as the first line of defense against fire. The paper emphasizes that for a cable to achieve the d0 rating, the sheath material must possess two critical properties: excellent combustion shelling and self-extinguishing capability.

• Combustion Shelling (Char Formation): This refers to the ability of the sheath material to form a dense, protective char layer when exposed to fire. This char layer acts as a barrier, preventing the flame from penetrating deeper into the cable, protecting the insulation, and significantly reducing the release of flammable gases that can lead to dripping. The paper notes that a higher residual carbon content, fewer surface cracks, smaller and evenly distributed internal pores in the char layer contribute to better char integrity and hardness, making it less likely to break and drip.

• Self-Extinguishing Property: Even with good char formation, the sheath material must also be able to self-extinguish quickly once the external flame source is removed. This prevents the cable from continuing to burn and potentially producing droplets. The research suggests that adding flame retardants to the sheath material formulation helps release inert gases during combustion, effectively displacing oxygen and suppressing the flame. A lower burning rate and a smaller charred distance are indicators of good self-extinguishing properties.

The paper presents a visual comparison of two different B-grade sheath materials. One from Company A demonstrates superior char formation and achieves the B1/d0 rating, while a commercially available B-grade sheath only reaches B2/d2 due to significant dripping during burning. This clearly underscores the importance of selecting the right sheath material with optimized char-forming and self-extinguishing characteristics.

2. The Impact of Cable Structure

Beyond the sheath material, the internal structure of the cable also plays a vital role in preventing the formation of flaming droplets. The paper discusses several aspects of cable structure:

• Outer Sheath Extrusion Method: The study compares two extrusion methods for the outer sheath: extruded tube and extrusion. The extruded tube method, while more common, involves stretching the sheath during production to fit the cable core. This stretching can create internal stresses in the polymer chains, which are released as the cable heats up during a fire. This stress release can lead to axial shrinkage and rupture of the sheath, compromising its protective function and increasing the risk of dripping. Additionally, the extruded tube method can sometimes leave gaps between the sheath and the inner layers, allowing hot air to expand during a fire, potentially causing the sheath to detach and crack. In contrast, the extrusion method, where the sheath is directly molded onto the core, generally results in better resistance to dripping. However, this method has drawbacks such as difficulty in controlling the core diameter, higher material consumption, increased cost, and slower production rates. The paper suggests that if the extruded tube method is used, optimizing the die size to minimize stretching and employing a powerful vacuum device to ensure tight adhesion between the sheath and the inner layers can mitigate these issues.

• Core Filling and Wrapping Tapes: For multi-core low-voltage power cables, filling the gaps within the core (e.g., between insulated conductors) with non-combustible or highly flame-retardant materials (oxygen index ≥ 50) is crucial. Materials like glass fiber, rock wool, or inorganic paper ropes are more effective than polypropylene fillers or flame-retardant non-woven fabrics in preventing the spread of flame within the core and reducing the likelihood of molten insulation dripping. Similarly, wrapping the core with one or more layers of high-density, small-weave glass fiber tape provides an additional barrier. Two or more layers of wrapping tape are found to be more effective in containing the molten insulation and gaseous byproducts generated during combustion, thus reducing damage to the outer sheath and the potential for dripping. For armored cables, the paper recommends wrapping two layers of tape over the metal armor to improve the adhesion of the outer sheath and prevent it from detaching during a fire.

3. The Significance of Production Process Control

The research highlights that even with the right materials and cable design, stringent control over the production process is essential to achieve the B1/d0 rating. One key aspect discussed is the control of the bonding seam of the outer sheath. B-grade sheath materials typically have a high flame-retardant filler content, which can make them less fluid than ordinary halogen-free materials. If the extruder head, screw, and die are not thoroughly cleaned before starting production, residual material can accumulate at the bonding seam. During a fire, this weak point in the sheath may not form an effective char layer, leading to axial cracking and lifting, exposing the insulation and potentially causing increased heat release and dripping. The paper presents a case study where failure to clean the equipment resulted in significant dripping and failure to meet the B1/d0 requirements. Therefore, meticulous cleaning of production equipment before starting the extrusion process is crucial.

Table: Fire Performance Requirements for Building Cables (Based on GB 51348-2019)