Power Cable Fire Protection Technology

According to incomplete statistics, there have been more than 100 major accidents caused by cable fires spreading across the country in the past ten years, and more than 320,000 meters of cables have been burned. The restoration and reconstruction work is expensive and time-consuming, and the loss of power supply alone amounts to more than 100. 100 million yuan. The cable fire accident has its special hazards, that is, the failure of the control circuit causes the accident to expand and even damage the main equipment, and it is difficult to repair, and production cannot be resumed for a long time.

Power cables are an important part of the normal operation of power plants and substations. The widespread distribution of cables in power plants and substations, the flammability of cables, the serial ductility of cable fires, and the seriousness of the consequences of cable fires make the fire protection of cables highly valued by power departments, firefighting agencies and scientific research institutions. In view of the frequent occurrence of cable fire accidents, it is necessary to discuss the fire safety measures for cables.

1. Create a good operating environment to avoid accelerated aging and damage of cable insulation
Cable trenches and cable tunnels must have good drainage facilities, such as shallow drainage ditches and water collection wells, and can effectively hold water. If necessary, install automatic start and stop pumping devices to prevent water accumulation and keep the interior dry. The longitudinal drainage slope of cable trenches and tunnels should not be less than 1% to 2%, and should be at least greater than 0.5% to prevent water, corrosive gas or liquid, and flammable liquid or gas from entering. Cable trench, cable tunnel. The cable tunnel should be naturally ventilated, but when the normal load of the cable causes the air temperature in the tunnel to be higher than 40℃~50℃, a combination of natural exhaust and mechanical exhaust can be used for ventilation. The fan of the ventilation system should be interlocked with the fire detector to ensure that the air supply can be automatically stopped in the event of a fire in the tunnel. The cable tunnel shall not be used as the air inlet of the ventilation system.

 

Avoid placing the cable fire door in a normally closed state, using a fire barrier to completely seal the cable, and filling and sealing all gaps in the cable trench cover, which affect the ventilation and heat dissipation of the cable. Moreover, completely enclosing the cable also makes normal inspection of the cable impossible, and the cable failure cannot be found in time.

In addition, there must be complete facilities for preventing the intrusion of rats and snakes to prevent small animals from destroying the insulation cable and causing accidents.

2. Strengthen the preventive test of cables
The preventive test of cables should not only look at the qualified and unqualified test data, but also compare and analyze the data. It can be compared with the test data of the same cable or with the historical test data of this cable to explore the law of the test data. For example, when the DC withstand voltage test is performed, if the measured leakage current value rises faster with the increase of the test voltage value or the increase of the pressure time, or the value increases more than the same cable, or if the The measured data shows an obvious upward trend, or the leakage current imbalance coefficient between the three is relatively large, etc., which should be carefully analyzed. If it is not caused by improper test method, the test voltage can be appropriately increased or the test time can be extended to determine whether the cable meets the conditions for continued operation.

3. It is necessary to strengthen the management and operation monitoring of the production quality of cable heads
According to statistics, cable fire and explosion accidents caused by cable head failure account for about 70% of the total cable accidents. Therefore, it is necessary to strictly control the quality of the materials and workmanship of the cable head. It is required that the service life of the made cable head should not be less than the service life of the cable. The rated voltage level and insulation level of the connector shall not be lower than the rated voltage level and insulation level of the connected cable. The withstand voltage between the insulation pads on both sides of the insulation head shall not be less than 2 times the insulation level of the cable sheath. The form of the connector should be compatible with the environmental conditions set, and should not affect the flow capacity of the cable. Within the range of 2 to 3 meters on both sides of the cable head, fire-resistant tapes shall be used for fire-resisting and flame-retardant cable treatment.

Generally speaking, the cable head is the weak link of the cable insulation, so strengthening the monitoring and management of the cable head is an important part of cable fire protection. The terminal cable head must not be placed in the cable trench, cable tunnel, cable trough box, or cable interlayer. The intermediate cable heads placed in the cable trenches, cable tunnels, cable trough boxes, and cable interlayers must be registered and monitored using a variety of monitoring equipment. When the cable head is found to have abnormal temperature rise or smell or smoke, exit the operation as soon as possible to avoid the cable head from catching fire during operation.

There should be sufficient safety length between the middle cable heads. Two or more cable heads should not be placed in the same position. Tight sealing measures should be taken between the cable heads and other cables.

4. Prevent other equipment from catching fire and igniting the cable
Oil-filled electrical equipment, coal conveying, pulverizing systems, and steam engine oil systems can be ignited by cables. Corresponding measures must be taken for them; the cable trench cover near the oil-filled electrical equipment should be sealed to prevent the oil from flowing into the cable trench to ignite the cable when the equipment fails and catches fire. The dust accumulation on the electric blanket near the coal conveying and pulverizing system should be cleaned regularly to prevent the spontaneous combustion of the pulverized coal from igniting the cable. The cables facing the explosion-proof door of the pulverizing system must be packed in a fire-proof box to prevent the fire-proof door from igniting the cable. The cables under the head of the turbine should be packed in a fire-proof trough box.

5. Use measures such as sealing, blocking, coating, isolation, and wrapping to prevent the cable from spreading

Fire-retardant paint has the characteristics of thin coating, does not affect normal heat dissipation, and can play a good heat insulation and flame retardant effect, but it also has many defects.
Therefore, it is not advisable to apply a large number of fire-retardant coatings to the cable to be flame-retardant. Instead, a large number of methods of sealing, blocking and isolation should be used. When using sealing, blocking, and isolation methods, the following issues should be noted:

(1). Use the methods of sealing, blocking and separating to ensure that the fire of a single cable does not extend to multiple cables. Cables enter cable trenches, cable tunnels, and cable trough boxes; the nozzles of the cable interlayer should be tightly fireproofed to prevent a single cable or a small number of cables from igniting a large number of cables. A fire wall and fire door shall be installed every 60 to 100 meters in cable trenches and cable tunnels. Fire barriers shall be arranged in layers in the shafts. Fire barriers shall be arranged between power cables and control cables. The control cables shall be fully fireproofed. Flame-retardant cables are used to ensure that the main equipment can safely stop running in any emergency. Important cable channels should be equipped with automatic alarm and automatic fire extinguishing devices, such as water spray and water mist fire extinguishing devices, to achieve early detection and early fire fighting.

(2). The tightness and thickness of the fire-proof sealing must be guaranteed. If the fire-proof sealing is not tight, the sealing effect will be lost, especially where the cables are concentrated, it is best to use soft blocking materials to ensure tight sealing. During maintenance and inspection, the damaged blockage should be restored in time. The thickness of the blocking material is not enough, and the fire will pass through the blocking material and burn after the cable catches fire. The thickness of the plugging material should be proportional to the number of cables on the plugging surface, the more the number of cables, the thicker the plugging. The fire door should have a device that automatically closes after a fire occurs.

(3). The fire blocking layer must have sufficient mechanical strength. Because the cable catches fire, especially the electrical short circuit, it will cause the rapid expansion of the air to produce a certain impulse, destroy the fireproof sealing layer with low mechanical strength, and make the fireproof sealing useless.

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