In the fields of building electrical engineering, fire protection engineering, and municipal infrastructure, fire-resistant cables are the core material for ensuring fire safety and are the "lifeline" of all fire protection power supply systems. Many users often confuse fire-resistant cables with ordinary flame-retardant cables, leading to incorrect engineering selection and failure in acceptance. This article explains in a simple and understandable way what fire-resistant cables are, their working principle, classification, differences from flame-retardant cables, applicable scenarios, and construction precautions, helping everyone comprehensively master the knowledge of fire-resistant cables.
I. Basic definition of fire-resistant cables
Fire-resistant cables are a special type of cable with special high-temperature resistance and fireproofing capabilities. They refer to special power cables that can maintain normal power supply and insulation performance for a certain period of time under the conditions of the specified flame burning temperature and burning time as stipulated in national standards.
When ordinary cables catch fire, the insulation layer melts, carbonizes, and falls off, directly causing a short circuit and power outage, and the equipment stops working completely. However, fire-resistant cables can temporarily maintain the circuit's continuity during a fire, ensuring the continuous operation of fire-fighting equipment, and providing critical time for personnel evacuation, fire suppression, and emergency rescue.
In summary: Ordinary cables lose power upon fire, while fire-resistant cables do not lose power upon fire.
II. Working principle of fire-resistant cables
Fire-resistant cables can withstand high-temperature combustion because the outer layer of the conductor is increased with one or more layers of mica tape fire-resistant layers. Mica is a natural inorganic mineral material with non-combustibility, high-temperature resistance, and stable insulation properties.
The principle of the cable's combustion process is as follows: First, the outer sheath and the ordinary insulation layer of the cable are completely burned away by the flame; the multiple layers of wrapped mica tape will form a hard and dense high-temperature-resistant insulation protection layer on the surface of the conductor, effectively isolating the high temperature and flame, protecting the copper conductor from short circuits and breakdown, and ensuring continuous power supply within the standard burning time.
It should be noted that fire-resistant cables are not "never burning out", they only maintain power supply within the specified temperature and time limit as per national standards, and will fail after exceeding the limit burning time.
III. Classification of fire-resistant cables
According to domestic electrical standards, fire-resistant cables are mainly divided into two fire-resistant grades: A grade and B grade. The two grades have different tolerance temperatures and application scenarios:
1. A-grade fire-resistant (the highest grade)
Can withstand 950℃ to 1000℃ high-temperature flame combustion, with a continuous fire resistance time of 90 minutes. It is suitable for first-class fire protection key places such as high-rise buildings, subways, tunnels, hospitals, and large complexes, and is the preferred choice for high-standard engineering selection.
2. B-grade fire-resistant (conventional grade)
Can withstand 750℃ to 800℃ flame combustion, with a continuous fire resistance time of 90 minutes. It is mostly used in ordinary public buildings and general fire linkage circuits.
In industry models, NH is the exclusive code for fire-resistant cables, and common models include NH-YJV, NH-BV, etc.
IV. Core differences between fire-resistant cables and flame-retardant cables
In engineering applications, most people confuse fire-resistant cables (NH) with flame-retardant cables (ZR), but they have completely different functions and cannot be substituted for each other:
1. Flame-retardant cables (ZR)
The core function is to prevent the spread of fire. Flame-retardant cables do not easily continue to burn and can extinguish themselves when away from the fire source, preventing the fire from spreading along cable trays and pipelines on a large scale. However, during the fire combustion process, flame-retardant cables will directly lose power and cannot guarantee the operation of fire-fighting equipment.
2. Fire-resistant cables (NH)
The core function is to ensure continuous power supply during a fire. The cable can be burned by the flame, but it maintains the circuit's continuity within the specified time limit, ensuring the normal operation of key equipment such as fire pumps, smoke exhaust fans, emergency lighting, and alarm hosts.
In one sentence: Flame-retardant is to prevent spread, fire-resistant is to ensure power supply; fire protection circuits must use fire-resistant cables, and flame-retardant cables must not be used as substitutes.
V. Main application scenarios of fire-resistant cables
Any equipment lines that need to continue working during a fire and cannot lose power must use fire-resistant cables:
- Lines of fire emergency lighting system and evacuation indication system;
- Control and power supply circuits of fire pumps, smoke exhaust fans, and positive pressure supply fans;
- Lines of automatic fire alarm system and fire linkage control system;
- Fireproof curtains, fire elevators, emergency broadcasting, etc., dedicated lines for fire protection;
- High-rise buildings, underground garages, shopping malls, hospitals, schools, subways, tunnels, etc., crowded places;
- Substations, computer rooms, public infrastructure, etc., important power supply locations.
Ⅵ. Precautions for Selecting and Construction of Fire-resistant Cables
- The inner mica fire-resistant layer of fire-resistant cables is extremely vulnerable to damage. During construction, violent bending, squeezing, and dragging are strictly prohibited to avoid cracking and detachment of the fire-resistant layer, which may lead to the failure of fire resistance performance.
- It is recommended to independently install in conduits or independent bridge frames for separate laying, and try not to mix with ordinary cables to avoid damage to the structure due to high temperature and squeezing.
- During procurement, it is necessary to verify the product test reports and qualifications. Class A and Class B fire-resistant cables must be selected according to design requirements strictly, and downgrading use is strictly prohibited.
- The advantages of fire-resistant cables are only applicable to short-term high-temperature fire environments. During daily operation, the temperature still needs to follow the specifications of ordinary cables and must not be over-temperature or overloaded.
