Fire hazards and precautions of laying wires and cables in high-rise buildings

- May 16, 2018-

With the rapid development and application of high technology in today's world, in modern buildings, especially in intelligent high-rise buildings, various electrical and electronic systems are becoming more and more complex, and the lines are spread out vertically and horizontally, bringing great opportunities for office automation and agile communication. Convenience, at the same time, also highlights the increase in the number of electrical fire accidents year after year, which has brought negative effects to high-rise managers, users and maintenance personnel, adding to many annoyances.

Wire and cable are composed of conductors, insulation layers and protective layers. Under certain working conditions, they can safely operate and transmit electric energy. However, when they are under overload, short circuit, local overheating and other fault conditions and external heat, they can cause damage to the insulation material. , make the insulation resistance drop lose insulation ability, even burn. Cable fire is a vicious accident in the power system. When a fire accident occurs, the fire is fierce, the combustion is rapid, and the smoke is harmful. In addition, due to the narrowness of the active area around the cable, it is difficult to save, the repair time is long, and the loss is serious. Its occurrence will bring huge losses to the national economy and the lives and property of the people. Therefore, we should attach great importance to the fire protection of power system cables.

Fire hazards in laying wires and cables in power systems

In the cable manufacturing process, the electrical insulation performance of the cable body is reduced, the insulation is destroyed, and hidden fire hazards are buried. The main reasons are as follows: the insulation medium contains impurities, moisture and bubbles; the thickness of the insulation layer is not uniform; the surface between the main insulation and the conductor and the shielding layer is rough, there are burrs, there is an air gap, resulting in uneven distribution of the electric field intensity in the insulation layer.

The distance between the cable and the heat pipe is too short or the cable is overloaded for a long time. If the temperature is too high, the insulation material will be burned due to aging and overheating, which will cause short-circuit auto-ignition, resulting in a decrease in insulation performance and breakdown. Due to the thermal expansion and contraction phenomenon of the cables on the cable trays, the cable and the cable edges are mechanically damaged by the edges of the cable trays, destroying the cable's protective layer and insulation layer, and reducing the electrical insulation performance.

The cable head and the terminal box are spontaneously ignited. Because the manufacturing process is rough, the stripped insulator is exposed to the air for a long time, which causes the insulator to get wet, causing it to explode during operation. The surface of the cable head is damped or accumulates, and the porcelain bushing of the cable head is broken and the distance between the lead wires is too small to cause the wire to catch fire.

The crimping of the middle connector of the cable is not tight, the welding is not firm, and the oxidation occurs during operation. The amount of insulating material injected into the cable middle joint box does not meet the requirements, or there are pores in the box during pouring; The cable box is poorly sealed or damaged cracks are immersed in moisture. Insulation breakdown, fire explosion.

The cable tunnel is piled up with debris, and the cable or cable support is covered with excessively thick ash. The cable tunnel has flammable gas leaking flammable liquids and it ignites through high temperature or open flame, causing a cable fire or explosion.

The dielectric coefficient and volume resistivity of the wrapping-type insulating material that are widely used now are lower than the dielectric coefficient and volume resistivity of the cable body. Under the action of a strong electric field, the increase of internal dielectric loss and leakage current of the insulation material will cause the cable accessory to generate heat. In addition, the junction between the cable terminator and the middle connector conductor is manually crimped by a crimper at the site. The contact resistance at the crimped connection is generally greater than the resistance of the cable conductor core, and it is difficult to achieve the resistance with the cable conductor core. In the same way, it will also cause hidden dangers for cable and cable fire accidents.

When the oil-immersed cable is laid, the difference between the height and the height is large, oil leakage occurs in the oleophobic or cable head, and the insulating oil at the high cable end is lost. The thermal resistance increases the coking of the insulation and breaks down the fire.

Human factors are also another important cause of wire and cable fire accidents. For example, operators do not follow regulations to regulate operations. Misoperation causes cables and cables to be short-circuited and overloading occurs, exacerbating the heat generation of electric wires and cable surfaces and causing electrical fire accidents.

Analysis of Fire Hazard and Causes in the Laying of Electric Wires and Cables

In high-rise buildings, because wires and cables are laid in bundles on a wire rack, once wires and cables cause a fire, the consequences are disastrous. Because the insulating layers used for electric wires and cables are all flammable rubber, polyvinyl chloride, and other hydrogen polymer organic compounds, the amount of heat generated during incineration is 19,000 to 46,000 kJ/kg. The melting point of the copper core of wire and cable is about 1038°C, and the melting point of aluminum core is 658°C. However, the melting point of wire and cable insulation is much lower than these values. For example, the melting point of PVC plastic is only 120°C. When the crowded wire and cable accident hot spot melt temperature reaches 800 to 1100°C, the threat to high-rise buildings is particularly significant. In the accident, the smoke is large, it is difficult to observe the ignition source, and the accident handling is very difficult. In recent years, the fire disasters in the Sichuan Department Store and the Kowloon Tower in Hong Kong are all caused by wires and cables.

An electrical fire accident can also cause other chain incidents. In general, plastic wires and cables not only burn and spread quickly, but also produce a large number of toxic and harmful gases, such as hydrogen chloride and carbon monoxide, which put people in a state of suffocation and poisoning and face the danger of death. At the same time, when the halide gas is extinguished, a strong acid mist is generated by the combustion reaction of the halide gas, resulting in “secondary pollution”, which causes the erosion of strong acid in modern intelligent high-level electrical and electronic equipment, and the consequences are also very serious.

Precautions against fire hazards in the laying of wires and cables in power systems

We must attach great importance to ideological considerations and stringently design, manufacture, install, maintain, overhaul, etc., implement the whole process of management, and improve the safety and reliability of cables. Wire and cable design selection should be appropriate. The high-rise construction of the wire and cable should be handled skillfully. Between the building, floors and floors, floors and walls, racks and floors, between elevator shafts and racks, and control rooms, fire prevention materials must be strictly used, and they should be tightly sealed so that when the wires and cables escape, the flames will not rush toward the fire. Other orientations. Specially for the bundled groups of wires and cables, it is necessary to indicate the trend and strictly isolate and close the electrical bridge boxes, settling boxes, and joint boxes to prevent accidents from spreading after the fire accident.

Keep cables away from heat and sources of ignition. When laying cables, the cable ducts should be as far away from the steam and oil pipes as possible, and they should maintain a certain safety distance between them. Flammable gas or flammable liquid pipe trenches shall not be laid. If laying in the heat pipe trenches, heat protection measures should be taken to the cables and the cables should not be aerialed in places where there is a risk of explosion or fire.

Fire-proof treatment shall be carried out on fire-prone or fire-spreading parts. In the laying of cables, inflammable and explosive materials shall be isolated. In the cable sections that are susceptible to external influences, necessary fireproofing shall be carried out. In order to effectively prevent the spread of fire and expand and reduce the loss of fire, firewalls and fire blocking sections can be installed in cable trenches, cable tunnels, etc. to control the fire in a certain cable section to reduce the scope of fire, for the vicinity of oil-filled electrical equipment. Cable trenches should be well sealed. Holes in the cable compartment leading to the control room, all wall holes in the shaft, and cable perforations in the floor must be sealed with refractory materials. The fire resistance limit should not be less than 1 h to prevent cable fires. Non-fire areas spread and expand.

It must be at the junction of different factories or workshops, outdoors into the interior, the busbars of the power distribution room, the junction of different voltage distribution devices, the connection of the cables of different units and main transformers, and the long distance cableway every 100m. All firewalls and fire blocking sections are provided; firewalls with doors are provided at the connection between the tunnel and the main control, central control and network control rooms. When a fire breaks out, the fire doors can be closed automatically or remotely; the tunnels at the fences of the plant area should also be equipped with Locked fire door.

To set up automatic alarm and fire extinguishing devices. To effectively fight the initial fire of the cable, an automatic alarm and fire extinguishing device can be set up at the appropriate location of the cable sandwich and cable tunnel so that the initial fire of the cable can be extinguished in the bud.

In order to reduce the hidden dangers of cable fire, we must pay full attention to the three aspects of design, operation and construction and take effective measures. All relevant units must clarify the responsibility system for cable fire prevention and regularly organize personnel to inspect and implement the cable fire prevention measures. In short, as long as we can do everything possible to minimize the possibility of cable fire and prolonged combustion, cable fire accidents will be effectively controlled.