Fire protection technology for power cables
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The flammability and risk associated with power cables during a fire are significant, particularly in critical infrastructure such as power plants and large substations. In the event of a fire, these cables can easily spread flames and damage other circuits, leading to widespread electrical failures. As a result, fire protection technology for power cables is essential to ensure the safe and reliable operation of electrical systems.
Power cables play a crucial role in electrical engineering by transmitting and distributing electrical energy. They consist of an insulated core, an insulating sheath, and a protective layer, designed to withstand various challenging installation environments. These cables are engineered with excellent insulation, waterproofing, and mechanical properties. However, when a cable experiences an overcurrent or overheating, its performance degrades, potentially leading to a fire. This paper explores effective fire protection technologies for power cables.
**1. Create a good operating environment to avoid accelerated aging and damage of cable insulation**
Cable trenches and tunnels must have proper drainage systems, such as water collection pits and shallow drains, to prevent moisture buildup. In some cases, automatic pumping systems should be installed to maintain dry conditions. The slope of the trench and tunnel should be maintained to prevent water, corrosive substances, or flammable materials from entering. Natural ventilation is preferred in cable tunnels, but if temperatures exceed 40–50°C due to normal load, a combination of natural and mechanical ventilation may be required. Ventilation fans should be connected to fire detectors to automatically shut down airflow in case of a fire. Additionally, cable tunnels should not be used as air intakes for ventilation systems.
Fire doors should remain open unless necessary, and cables should be fully enclosed with fire barriers. Gaps in cable trench covers must be sealed to prevent interference with ventilation and heat dissipation. However, complete enclosure can hinder routine inspections and delay the detection of faults.
According to statistics, about 70% of cable accidents are caused by failures at the cable ends. Therefore, strict quality control is essential in the production of cable joints. The service life of the joint should match that of the cable, and its voltage rating must meet or exceed that of the connected cable. The insulation between the two sides of the joint should be at least twice the level of the cable's protective layer. The joint design should be compatible with environmental conditions and not restrict the cable’s current-carrying capacity. Fireproof tape should be applied within 2–3 meters of the joint for additional fire resistance.
Monitoring and managing the condition of cable ends is vital for fire prevention. Terminal joints should not be placed in trenches or tunnels, while intermediate joints must be registered and tested regularly. If abnormal temperature, odor, or smoke is detected, the cable should be removed immediately to prevent a fire during operation.
A sufficient distance should be maintained between intermediate joints, and multiple joints should not be placed in the same location. Sealing measures must be strictly implemented between the joints and other cables.
**2. Strengthen the preventive test of cables**
After installation, a DC withstand voltage test (preventive test) should be conducted on the cables and their accessories. The results should be compared with those of the same cable or historical data to identify trends. If any of the following occurs during the test, it may indicate a defect in the cable insulation:
- Leakage current increases rapidly with rising voltage or extended testing time.
- The leakage current value is higher than that of the same cable.
- There is a clear upward trend compared to previous measurements.
- The imbalance coefficient between the three phases is high.
All such anomalies should be thoroughly analyzed. If the test method is questionable, the test voltage or duration can be adjusted to assess whether the cable is suitable for continued operation.
The DC withstand voltage test standards for power cables can be found in Chapter 17 of the "Testing Standard for Electrical Equipment Installation Engineering" (GB50150-91). For commonly used plastic-insulated power cables, the test voltage is typically four times the rated voltage, with a duration of 15 minutes.
**3. Strengthen management and monitoring of cable head quality**
**4. Prevent ignition from other equipment**
Equipment such as oil-filled devices, coal handling systems, and gas engines can pose a fire risk to cables. Proper measures must be taken, such as sealing cable trench covers near oil-filled equipment to prevent oil from igniting the cables. Regular cleaning of coal dust around cables is necessary to avoid spontaneous combustion. Cables near explosion-proof doors should be protected with fireproof boxes to prevent damage from pressure release.
**5. Use sealing, blocking, coating, and wrapping to prevent cable fires**
Fireproof coatings offer advantages like thin application and good flame retardancy, but they also have drawbacks, such as poor adhesion, uneven thickness, and peeling. Many commercially available products are ineffective or counterfeit. Solvent-based coatings, while common, have issues with toxicity, cost, and environmental impact. Their resistance to cold, water, and oil is limited, and they tend to crack after curing, making them unsuitable for flexible applications.
Therefore, relying solely on fire-retardant coatings is not advisable. Instead, sealing, blocking, and separation methods are more effective. These techniques should ensure that a single cable does not ignite others. All cables entering trenches, tunnels, trays, or jackets must be tightly sealed to prevent the spread of fire.