When designing life safety and critical circuit systems, one of the most vital questions is: how long will the cables continue to function in a fire? The answer isn’t a single number—it depends on the cable’s construction, compliance standards, and installation environment. At JZD Cable, we provide a range of fire resistant cables engineered to meet specific duration requirements. This guide breaks down the survival times, from the standard 90 minutes to over 3 hours, based on GB (Chinese National Standard) classifications and real-world performance.
Part 1: GB Fire Resistance Ratings and Minimum Duration (GB/T 19216)
In China, the primary standard for fire resistant cable testing is GB/T 19216. It classifies cables into two main categories, both defining a minimum circuit integrity of 90 minutes under specific flame temperatures.
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Grade
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Flame Temperature
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Required Power Supply Duration
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Pass/Fail Criteria
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|---|---|---|---|
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Class A
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950 ~ 1000°C
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≥ 90 minutes
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Circuit remains intact; a 3A fuse does not blow.
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Class B
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750 ~ 800°C
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≥ 90 minutes
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Circuit remains intact; a 3A fuse does not blow.
|
Key Takeaway: The 90-minute rating is the minimum guaranteed performance under controlled laboratory conditions. In a real fire scenario—with temperature fluctuations, water spray, vibration, and dense cable grouping—the actual functional duration is often shorter than 90 minutes.
Part 2: Actual Fire Survival Time by Cable Type
1. Conventional Fire Resistant Cables (e.g., NH-YJV, NH-BV)
- Construction: Conductor wrapped with fire-resistant mica tape, plus conventional insulation and sheathing.
- GB Standard: ≥ 90 minutes (Class A or B).
- Real-World Engineering Performance:
- Single cable, no spray, no vibration: Can approach the 90-minute mark.
- Densely packed in trays, with spray or vibration: Typically lasts approximately 45 to 60 minutes.
2. Mineral Insulated (MI) Cables (e.g., BTTZ, RTTZ)
- Construction: Copper conductor + inorganic Magnesium Oxide (MgO) insulation + copper sheath.
- Fire Resistance Limit: 180 to 360 minutes at 950~1000°C.
- Key Features: This construction offers true fire resistance, is non-combustible, and produces no toxic smoke. MI cables are the standard solution for critical 3-hour fire survival power circuits.
3. Long-Duration Fire Resistant Cables (180-Minute Grade)
- Standards: GB/T 31248, XF/T 306, etc.
- Test Condition: 950°C flame plus water spray plus mechanical shock, for 180 minutes.
- Application: Mandatory for 3-hour fire-fighting power supply in super high-rise buildings, large complexes, subways, and tunnels.
Part 3: Key Factors That Affect Fire Survival Duration
Understanding the variables that impact performance is crucial for specification and safety.
- Cable Type: This is the most significant factor.
- Mica-Tape Based (NH): 90-minute grade.
- Mineral Insulated (MI): 180-360 minute grade.
- Fire Rating Class:
- Class A (950-1000°C) offers a higher temperature threshold than Class B (750-800°C) for the same duration.
- Installation & Environment:
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- Single, spaced cables perform better than cables densely packed in a ladder or tray.
- Cables in dry conditions outlast those exposed to water spray.
- Static installations maintain integrity longer than those subject to vibration or shock.
- Cables inside conduits or closed protection fare better than those openly installed.
Conclusion: Choosing the Right Cable for Your Required Duration
Selecting the appropriate fire resistant cable is a critical safety decision. For general 90-minute circuit integrity requirements, standard Class A or B NH cables are suitable, but always consider derating for harsh real-world conditions. For mandatory 3-hour fire-fighting power supplies or applications where failure is not an option, Mineral Insulated (MI) Cables or certified 180-minute long-duration cables are the necessary choice.
At JZD Cable, we manufacture and supply a comprehensive range of fire resistant cables, from standard NH types to high-performance MI cables, ensuring your projects meet the exact safety and code requirements. Contact our team today to discuss your specific application and get a quote for cables you can trust in an emergency.
