According to national standards, bundled cables are classified into three flame-retardant grades: A, B, and C, based on the volume of non-metallic materials per meter.
my country currently does not have detailed regulations regarding the flame-retardant grades used for high and low voltage cables in projects. However, in practice, choosing a higher flame-retardant grade is more advantageous in terms of reducing the probability of fire and minimizing losses from failures. For the same type of cable, the higher the flame-retardant grade, the higher the price. The price difference between Class A and Class C flame-retardant cables is approximately 15%-20%. Therefore, when selecting the flame-retardant grade for high and low voltage cables in a project, comprehensive considerations must be taken into account, including the number of cables in the project, installation density, fire probability, and safety requirements.
Regarding the selection of smoke density during cable combustion, national standards stipulate that high and low voltage cables are considered low-smoke if their light transmittance exceeds 60% in combustion tests; halogen-free if the halogen acid content of the evolved gas is less than 5 mg/g; and low-halogen if the halogen acid content is between 5 and 100 mg/g.
However, it's important to note that during high- and low-voltage cable combustion tests, a 60% low-smoke transmittance can only be achieved if the halogen acid content of the emitted gases is less than 5 mg/g. Low-halogen products with a transmittance of 30%-50% do not meet the low-smoke requirement. Therefore, the term "low-smoke, low-halogen, flame-retardant cable" is not standard. The correct term should be "low-smoke, halogen-free, flame-retardant cable" or "low-halogen flame-retardant cable."
The outer sheath of cross-linked polyethylene insulated flame-retardant cables is generally made of environmentally friendly materials such as halogen-free polyolefin or low-halogen polyvinyl chloride to prevent the harmful effects of halogen acid gases released during cable combustion.
For low-smoke, halogen-free, flame-retardant cables, strict requirements must be met for the pH value, conductivity, and transmittance of the gases emitted during combustion. For low-halogen flame-retardant cables, the acid gas content and transmittance of the outer sheath must be measured during combustion. For power supply systems for urban viaducts and other rail transit systems, low-halogen outer sheath materials should be used.
