In recent years, the proportion of power system fires globally resulting in power outages caused by power cable fires has been increasing. Therefore, in locations like power plants and substations, where a large number of electrical equipment and densely laid cables are located, the use of flame-retardant cables to prevent the spread of fire and fire-resistant cables to ensure uninterrupted power supply to critical circuits has become increasingly important.
When is flame-retardant cable used?
1. Flame-retardant cable refers to a cable that, under specified test conditions, burns within a specified range after the test fire source is removed, and any residual flames or burns self-extinguish within a specified timeframe. Its fundamental characteristic is that, while it may be damaged and rendered inoperable in a fire, it can prevent the spread of fire. In simple terms, in the event of a cable fire, the fire is confined to a local area, preventing it from spreading, thus protecting other equipment and preventing further damage.
2. Structural Features of Flame-Retardant Cables
The structure of flame-retardant cables is essentially the same as that of conventional cables. The difference lies in the fact that the insulation layer, sheath, outer sheath, and auxiliary materials (taping and filling) are all or partially flame-retardant.
3. Classification of Flame-Retardant Cables
Based on the different flame-retardant materials used, flame-retardant cables are divided into two categories: halogen-containing flame-retardant cables and halogen-free low-smoke flame-retardant cables.
Halogen-containing flame-retardant cables, in particular, have excellent flame-retardant properties because their insulation, sheath, outer sheath, and auxiliary materials (taping and filling) are all or partially made of halogen-containing polyethylene (PVC) flame-retardant material. However, when cables burn, they release large amounts of smoke and halogen acid gases, which are corrosive to surrounding electrical equipment. Rescue workers must wear gas masks to approach the scene to extinguish the fire. Cable combustion poses a threat to surrounding electrical equipment and rescue workers, hindering firefighting and rescue efforts, and leading to serious secondary hazards. Halogen-free, low-smoke, flame-retardant cables use, in whole or in part, halogen-free cross-linked polyethylene (XLPE) flame-retardant material for their insulation, jacket, outer sheath, and auxiliary materials (taping and filling). This not only offers superior flame retardancy but also produces no halogen acid gases when the cable burns, resulting in minimal smoke generation and low levels of corrosive gases. Low-halogen, low-smoke flame-retardant cables were developed as a compromise between flame retardancy and reduced halogen acid gas generation. Their halogen content is approximately one-third that of halogen-containing flame-retardant cables, and their smoke generation is close to the generally accepted "low smoke" level.
According to the "Wire and Cable Fire Test Methods," flame-retardant cables evaluated using the bundled combustion test are divided into three categories: A, B, and C. Category A cables undergo the most stringent testing conditions, offer superior performance compared to categories B and C, and are also the most expensive. To achieve flame retardancy or improve the flame retardancy level of cables, researchers have developed novel flame-retardant cables with extruded "oxygen-barrier" or "fire-barrier" fillings or wrapped with flame-retardant tape (fire-barrier). These cables are called "oxygen-barrier" cables. The main principle is the addition of metal hydrates to the material, which can achieve low-halogen and low-smoke, halogen-free and low-smoke, and flame-retardant properties, respectively. However, the so-called "oxygen-barrier" and "fire-barrier" layers are only one means of achieving flame retardancy. Ultimately, they are still flame-retardant cables, and the highest flame retardancy level they can achieve is Class A. Therefore, flame-retardant cables cannot be called "oxygen-barrier" cables; instead, "oxygen-barrier" cables can be considered a type of flame-retardant cable.
4. Issues to note when selecting: Designers should design, select and install flame-retardant cables based on their specific characteristics, and should pay attention to the following issues:
(1) Since halogen-containing flame-retardant cables (including flame-retardant cables and low-halogen, low-smoke flame-retardant cables) release corrosive halogen acid gases when burning, which greatly hinders firefighting work, thereby delaying firefighting time and exacerbating the spread of fire, halogen-free, low-smoke flame-retardant cables should be designed and selected as much as possible in public places with high population density. Any type of flame-retardant cable can be selected in work areas with low population density. (2) Flame-retardant cables are divided into three categories: A, B, and C. Among them, category A flame-retardant cables have better performance than categories B and C and are also the most expensive. Designers should indicate the category of flame-retardant cables when providing order lists.
(3) It is impossible to distinguish the categories A, B, and C of flame-retardant cables from the appearance, and the only guarantee of supply is by the manufacturer. Therefore, cable manufacturers should be carefully selected when designing and selecting.
(4) Compared with halogen-containing flame-retardant cables, halogen-free low-smoke flame-retardant cables have the advantages of low corrosion and low smoke, but their electrical and mechanical properties are significantly reduced. Therefore, when laying cables, halogen-free low-smoke flame-retardant cables should have a larger bending radius than halogen-containing flame-retardant cables.
(5) When designing cable laying, it is not advisable to lay non-flame-retardant cables and flame-retardant cables side by side, and it is not advisable to lay flame-retardant cables of different flame-retardant categories side by side.
When should fire-resistant cables be used?
1. Fire-resistant cables refer to cables that can maintain normal operation for a certain period of time when the sample is burned in the flame under specified test conditions. Its fundamental characteristic is that the cable can still maintain normal operation of the line for a period of time under burning conditions. In layman's terms, in the event of a fire, the cable will not burn immediately, and the circuit is relatively safe. Therefore, the main difference between fire-resistant cables and flame-retardant cables is that fire-resistant cables can maintain normal power supply for a period of time when a fire occurs, while flame-retardant cables do not have this characteristic. This characteristic determines the crucial role that fire-resistant cables play in modern urban and industrial buildings. Once a fire breaks out, power supply circuits for control, monitoring, guidance, and alarm systems must remain operational. Therefore, these cables are primarily used in power supply circuits connecting emergency power supplies to user firefighting equipment, fire alarms, ventilation and smoke exhaust systems, guide lights, emergency power outlets, and emergency elevators.
2. Structural Features of Fire-Resistant Cables: The structure of fire-resistant cables is essentially the same as that of conventional cables. The difference lies in the use of copper conductors (copper has a melting point of 1083°C) for their conductors, which have high fire resistance properties. A fire-resistant layer is added between the conductor and the insulation. This layer is wrapped with multiple layers of mica tape. Because the allowable operating temperatures of different mica tapes vary significantly, the mica tape is crucial to the cable's fire resistance.
3. Classification of Fire-Resistant Cables: In China, conventional fire-resistant cables are classified into Class A and Class B. Class B cables can withstand flames of 750°C to 800°C at rated voltage for at least 90 minutes without breaking down (i.e., a 3A fuse will not blow). By improving the manufacturing process for the fire-resistant layer and adding additional layers, Class A fire-resistant cables have been developed. These cables can withstand flames of 950°C to 1000°C and rated voltage for at least 90 minutes without breaking down (i.e., a 3A fuse will not blow). Class A fire-resistant cables offer superior fire resistance to Class B cables. Furthermore, mineral insulated cables (MI cables) are a superior type of fire-resistant cable. They are made of a copper core, copper sheath, and magnesium oxide insulation. The fire-resistant layer of these cables is composed entirely of inorganic materials, while the fire-resistant layer of conventional fire-resistant cables is a composite of inorganic and general organic materials. Therefore, MI cables offer superior fire resistance and do not produce corrosive gases due to decomposition during combustion. MI cables offer excellent fire resistance and can operate at temperatures of 250°C for extended periods. They are also explosion-proof, corrosion-resistant, have a high current carrying capacity, are radiation-resistant, have high mechanical strength, are compact, lightweight, have a long lifespan, and are smokeless. However, it is expensive, the process is complex, and the construction is difficult. This type of cable with good fire resistance can be used in oil filling areas, important wooden public buildings, high-temperature places, and other places where fire resistance requirements are high and the economy is acceptable.
4. Issues to note when selecting
Based on the specific characteristics of fire-resistant cables, designers should pay attention to the following issues when designing and selecting:
(1) When fire-resistant cables are used in cable tunnels and cable mezzanines with dense cables, or in flammable places such as oil pipelines and oil depots, Class A fire-resistant cables should be used first. Except for the above situations and when the number of cables is small, Class B fire-resistant cables can be used. (2) Fire-resistant cables are mostly used as emergency power supply circuits and are required to work normally during fires. Since the ambient temperature rises sharply during fires, in order to ensure the transmission capacity of the line and reduce the voltage drop, for circuits with long power supply lines and strictly limited allowable voltage drop, the cross-section of the fire-resistant cable should be enlarged by at least one level.
(3) Fire-resistant cables cannot be used as high-temperature resistant cables.
(4) To reduce the probability of cable joint failure in fire accidents, the number of joints should be minimized during installation to ensure that the line can operate normally in a fire. If branch wiring is required, the joints should be properly fireproofed. Some electrical designers do not clearly distinguish between flame-retardant and fire-resistant cables, and do not fully understand their structures and characteristics. As a result, they are unable to correctly design and select these two types of cables according to power supply requirements, and cannot correctly guide the installation of these two types of cables during on-site design agency or supervision work. Therefore, a comprehensive and correct understanding of flame-retardant and fire-resistant cables is the first condition for their correct selection.
