Analysis of flammability of power cables

The flammability and stringency of power cables, in the event of a fire, are serious, especially in power plants and large substations, in the event of a fire, affecting other circuits. Therefore, the fire protection technology of power cables is one of the important technologies for the normal operation of electrical systems.

Power cables are an important part of electrical engineering and are used to transmit and distribute electrical energy. The power cable is composed of an insulated core wire, an insulating sheath and a protective layer. In order to adapt to various complicated laying environments, the power cable is designed to have good insulation performance, waterproof performance and mechanical properties. However, when an overcurrent occurs in the cable loop and the cable overheats, the performance of the cable changes, which in turn causes a fire. This paper intends to discuss the fire protection technology of cables.

1. Create a good operating environment to avoid accelerated aging and damage of cable insulation.

Cable trenches and cable tunnels should have good drainage facilities, such as water shallow trenches and water collecting wells, and can effectively drain water. If necessary, set up automatic water pumping and stop pumping devices to prevent water supply and keep the interior dry. The cable trench and the longitudinal direction of the tunnel maintain the drainage slope. Prevent water, corrosive gases or liquids, and flammable liquids or gases from entering cable trenches and cable tunnels. The cable tunnel should be naturally ventilated. When the normal load of the cable is used to make the air temperature in the tunnel higher than 40~50 °C, the natural exhaust and mechanical exhaust can be combined to ventilate. The fan of the ventilation system should be interlocked with the fire detector to ensure automatic wind stop when the tunnel fires. The cable tunnel should not be used as the air inlet of the ventilation system.

The cable fire door should be avoided in the normally closed state, the cable should be completely enclosed by the fire barrier, and the gap of the cable trench cover should be filled and closed to affect the ventilation and heat dissipation of the cable. Moreover, the cable is completely enclosed, which makes the normal inspection of the cable impossible, and the cable failure cannot be found in time.

According to statistics, cable fires and explosions caused by cable head failures account for about 70% of the total cable accidents. Material and process quality must be produced in strict air cable heads. It is required that the service life of the cable head produced cannot be lower than the service life of the cable. The rated voltage level and insulation level of the street shall not be lower than the rated voltage level of the connected cable and its insulation level. The withstand voltage between the insulating mats on both sides of the insulating head shall not be less than 2 times the insulation level of the cable protective layer. The form of the joint should be compatible with the environmental conditions set and will not affect the flow capacity of the cable. Within the range of 2 to 3 m on both sides of the cable head, fireproof tape should be used for fire retardant treatment.

Generally speaking, the cable sees the weak link of cable insulation, so strengthening the monitoring and management of the cable head is an important part of cable fire prevention. Terminal cable heads must not be placed in cable trenches, cable tunnels, cable trays, cable mezzanine intermediate cable heads must be registered, and use a variety of testing equipment for testing. When the cable head is found to have abnormal temperature rise or odor or smoke, the emergency will be withdrawn early to prevent the cable from being fired during operation.

A sufficient safe length distance should be ensured between the intermediate cable heads. Two or more cable heads should be placed in the same position, and strict sealing measures should be taken between the cable head and other cables.

2. Strengthen the preventive test of cables

After the cable and cable accessories are installed, the DC withstand voltage test [preventive test] should be performed. The experimental data were compared and analyzed. That is, it can be compared with the experimental data of the same cable, and can also be compared with the historical data of the cable to explore the law of the experimental data. If the measured value of the leakage current is one of the following when the DC withstand voltage test is performed, the cable insulation may be defective, reflect the defective part, and be disposed of:

(1) As the test voltage value increases or the pressurization time increases, it rises faster.

(2) The value is increased more than the same cable.

(3) Compared with the previous measured data of this cable, it has a clear upward trend.

(4) The leakage current imbalance coefficient between the three phases is large.

All of the above situations should be carefully analyzed. If the non-test method is improperly obtained, the test voltage or the test time may be appropriately increased to judge whether the cable meets the continuous operation conditions.

The DC voltage withstand test standard for power cables can be found in Chapter 17 of the "Testing Standard for Electrical Equipment Installation Engineering Electrical Equipment Handling" (GB50150-91). The commonly used plastic insulated power cable DC withstand voltage test value is 4 times the cable rated voltage test time is 15 minutes.

3, strong management and operation monitoring of cable head production quality

4, to prevent other equipment from igniting the ignition cable

There are oil-filled electrical equipment and coal handling, milling system, and gasoline engine system that can ignite the cable. They must take corresponding measures: the cable trench cover near the oil-filled electrical equipment should be sealed, and the oil flow to the cable trench to ignite the cable when the textile equipment fails. The accumulated powder on the cable near the coal conveying and milling system should be cleaned regularly to prevent the pulverized coal from naturally igniting the cable. The cable facing the anti-riot door of the milling system should be packaged with a fireproof box to prevent the explosion-proof door from operating and igniting the cable.

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

Fireproof coatings have the characteristics of thin coating, no effect on normal heat dissipation, and good flame retardant effect, but also many defects. Including the improper selection of fireproof coatings, the market is not effective in the practical application of counterfeit and shoddy products, and the surface of the spitting materials has peeling off, uneven coating thickness, and cut corners.

The fire retardant coatings used in cable engineering are mainly solvent-based. Solvent-based solvents have problems such as toxicity, price, transportation safety and ring pollution. The application effects of cold resistance, water resistance and oil resistance are not ideal. In particular, the bending resistance is not good, and the flexibility of the coating after curing is also poor. It is only suitable for the cable that is erected, and the coating is easy to be cracked and peeled off.

Therefore, the cable should not be flame retarded in a large number of ways to apply fire-retardant coatings, but should be used in a large number of sealing, blocking, and separation methods, the following issues should be noted:

(1) The method of sealing, blocking and separating should ensure that a single cable does not ignite multiple cables. Cables entering cable trenches, cable tunnels, cable trays, and cable jackets should be tightly sealed to prevent a single cable or a small number of cables from igniting and igniting a large number of cables. A firewall and fire door shall be provided every 60 to 100 m in the cable trench and cable tunnel. Fire barriers shall be arranged in layers in the shaft. Fire barriers shall be provided between the power cables and the control cables. The control cables shall be fully fireproofed or adopted. Flame-retardant cable to ensure that the main equipment can be safely stopped in any emergency.

Important cable passages should be equipped with automatic alarms and automatic fire extinguishing devices, such as water spray and water mist fire extinguishing devices, to achieve early detection and early rescue.

(2) The tightness and thickness of fireproof sealing must be guaranteed. If the fireproof sealing is not tight, the sealing function will be lost. Especially in the place where the cable is concentrated, the soft blocking material should be used to ensure the sealing is tight. When the maintenance inspection is carried out, the damaged plugging should be restored in time. The thickness of the plugging material is not enough. After the cable is fired, the fire will pass through the blocked material and burn. 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.

(3) The fireproof sealing layer should have the mechanical strength of the seat. Because the cable is ignited, especially the electrical short circuit, it will cause the rapid expansion of the air to generate a certain impulse, destroying the fireproof sealing layer with low mechanical strength, and making the fireproof sealing lose its effect.

(4) Fireproof plugging materials commonly used are fire rock wool, asbestos board, asbestos rope, fireproof clay and advanced high-tech materials. The construction unit should select suitable materials according to the actual conditions of each project and different fire prevention areas. And blocking methods. The main parts of the blockage are in the high and low voltage configuration electric room, distribution shaft, fire control room, cable tray, and distribution trunk line through the national wall of the slab.

In addition, it is necessary to have a perfect anti-rat and snake-injection facilities to prevent small animals from damaging cable insulation and causing accidents.