- Jul 28, 2020-
Due to the flammability and string ductility of cables, once a fire occurs, the damage is serious, especially in power plants and large substations, once a fire occurs, it will spread to other circuits. Therefore, the fire protection technology of power cables is one of the important technologies for the normal operation of electrical systems.
Power cable is an important part of electrical engineering, and its function is to transmit and distribute electrical energy. Power cables are composed of insulated core wires, insulated sheaths and protective layers. In order to adapt to various complex laying environments, power cables are designed to have good insulation, waterproof and mechanical properties. However, when an overcurrent occurs in the cable loop and the cable is overheated, the various properties of the cable will change, and a fire will follow. This article 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 must have good drainage facilities, such as shallow water ditches and water collection wells, which can effectively drain water. If necessary, install automatic start and stop pumping devices to prevent water supply and keep the interior dry. The longitudinal drainage slope of the cable trench and tunnel shall be maintained. Prevent water, corrosive gas or liquid and flammable liquid or gas from entering the cable trench and cable tunnel. Cable tunnels should be naturally ventilated. When the air temperature in the tunnel is higher than 40-50°C under the normal load of the cable, 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 tunnel can automatically stop the wind when a fire occurs. The cable tunnel shall not be used as the air inlet of the ventilation system.
Avoid methods that affect cable ventilation and heat dissipation, such as keeping the cable fire door in a normally closed state, completely enclosing the cable with a fire barrier, and filling and sealing the gap of the cable trench cover. Moreover, the complete closure of 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 entry of rats and snakes to prevent small animals from destroying the cable insulation and causing accidents.
2. Strengthen the preventive test of cables
After the cable and cable accessories are installed, the DC withstand voltage test [preventive test] should be carried out. And compare and analyze the experimental data. That is, it can be compared with the experimental data of the same cable, and it can also be compared with the historical experimental data of this cable to explore the laws of the experimental data. If during the DC withstand voltage test, the measured leakage current value has one of the following conditions, the cable insulation may be defective, reflecting the defective part, and processing it:
(1) Rise faster with the increase of test voltage value or increase of pressure time;
(2) Compared with the same cable, the value increases more;
(3) Compared with the previous measured data of this cable, there is an obvious upward trend;
(4) The leakage current unbalance coefficient between the three phases is relatively large.
All of the above conditions should be carefully analyzed. If it is not caused by improper test methods, the test voltage or the test time can be appropriately increased to determine whether the cable meets the conditions for continued operation.
3. Strong management and operation monitoring of cable head production quality
According to statistics, cable fire and explosion accidents caused by cable head failure account for about 70% of the total cable accidents. The materials and workmanship quality of the hollow cable head must be strictly. It is required that the service life of the made cable head cannot be less 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 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 protection layer. The joint form should be compatible with the set environmental conditions, and will not affect the flow capacity of the cable. Within the range of 2～3m on both sides of the cable head, fire-resistant tapes shall be used for fire-resisting and prolonged burning.
Generally speaking, cables see through the weak links of cable insulation, so strengthening the monitoring and management of cable heads 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, cable interlayer. The middle cable head must be registered, and a variety of testing equipment should be used for testing. If abnormal temperature rise or smell or smoke is found in the cable head, exit the operation as soon as possible to avoid the cable throwing fire during operation.
There should be enough safe length between the middle cable heads, two or more cable heads should be placed in the same position, and tight sealing measures should be taken between the cable heads and other cables.
4. Prevent other equipment from catching fire and ignite the cable
Oil-filled electrical equipment, coal conveying, pulverizing systems, and gasoline engine systems can ignite the cable. Corresponding measures must be taken against them: the cable trench cover near the oil-filled electrical equipment should be sealed, and the oil will flow into the cable trench to ignite the cable when the textile equipment fails and catches fire. The dust accumulation on the cables near the coal conveying and pulverizing systems should be cleaned regularly to prevent the coal dust from igniting the cables naturally. The cables facing the anti-riot door of the pulverizing system should be packed in a fire-proof box to prevent the operation of the anti-explosion door from igniting the cable.
5. Use measures such as sealing, blocking, coating, and wrapping to prevent the cable from spreading
Fire-retardant coatings have the characteristics of thin coating, do not affect normal heat dissipation, and have good heat insulation and flame retardant effects, but they also have many defects. Including improper selection of fire retardant coatings, counterfeit and inferior products in the market for ten days; poor use in actual applications, peeling off the surface of the spray material, uneven coating thickness, and cutting corners.
The fireproof coatings used in cable engineering are mainly solvent-based. Solvent-based coatings have problems with solvent toxicity, price, transportation safety, and environmental 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 fixed and erected cables, and the coating is easy to dry and peel off.