With the transformation of urban overhead lines, the lines of 10kV-220kV voltage level are changed to cables, which provides the possibility for large-scale construction of urban power cable tunnels. This kind of cable tunnel project is basically in the central area of the city, once a fire occurs, it will endanger the safety of the city, so active measures should be taken to prevent the occurrence of cable tunnel fire.
Cables in cable tunnels produce large amounts of smoke and flammable gases when they burn. The space of the cable tunnel is long and narrow, the flame spreads and burns rapidly along the cable, and the smoke generated, toxic gases, high temperature and high heat accumulate in the tunnel, which not only corrodes and destroys electrical equipment, but also is very harmful to the human body, threatens the safety of human life, and is difficult to extinguish the fire. Moreover, once the cable is ***, it will cause more serious fire and power outage accidents, the consequences are very serious, and the repair work is also very difficult. In view of the severity of the consequences of cable fires, it is necessary to carry out an in-depth analysis of the fire prevention, ventilation and fire protection measures of cable tunnels.
The causes of urban cable tunnel fires can be summarized in the following aspects:
1) Poor quality of cable intermediate joints, poor quality of crimping joints, poor crimping joints, excessive contact resistance, and overheating of cable heads caused by long-term operation and burning through insulation. The crimping of the middle joint of the cable is bad, which can only be found in operation, and the longer the operation time, the more likely it is to overheat and burn through the accident.
2) Grounding fault current causes disaster.
3) Long-term overload operation, moisture and heat, etc., the insulation layer will be destroyed, and short circuit will occur, causing cable fire.
In order to make the cable tunnel work for a long time, safely and effectively, it is necessary to take reasonable measures to prevent the occurrence of fire according to its characteristics, so as to prevent problems before they occur.
When a fire occurs in the tunnel, the division of the fire partition is conducive to the evacuation of people, and the fire partition of the cable tunnel is usually divided by the firewall, which effectively reduces the impact of the fire of a certain partition on other areas and reduces the loss of other partitions.
Technical Regulations for the Design of Urban Power Cable Lines" DL T-2016 in 94.2. Provisions: The cable tunnel is located in the power plant and substation, and the fire separation distance is not more than 100m; The cable tunnel is located outside the power plant and substation, and the fire spacing is not more than 200m.
Power Engineering Cable Design Standard GB50217-2018 in 70.2. Provisions: long-distance cable trenches, tunnels and overhead bridges should be separated by about 100m, or at the ventilation section of the tunnel, and about 200 places outside the factory and station.
When the cable tunnel is located in the power plant or substation, the maximum fireproof partition should not exceed 100m, and when the cable tunnel is located outside the power plant or substation, the maximum fireproof partition should not exceed 200m.
Power Engineering Cable Design Standard GB50217-2018 in 56.6. Provisions: The longitudinal length along the tunnel should not be less than 2 safety holes; The spacing of safety holes in tunnels in industrial plants or substations should not be greater than 75m;
According to the experimental data, in a dense smoke environment, people can walk at a distance of about 30m when they lower their heads and cover their noses. Therefore, if conditions permit, it is recommended to make a fire partition at an interval of about 75m between the cable tunnels in the factory area and substation station.
Power Engineering Cable Design Standard GB50217-2018 in 70.3. Provisions: Except for the firewall part that leads to the main control room, the factory wall or the long-distance tunnel separated by the ventilation section, the fire door can not be set up in other cases.
It is recommended that for the sake of safety, a fire door can be added to each fire partition, under normal circumstances, the fire door is open to play the role of ventilation, once a fire occurs, the fire detector will send out an alarm and the fire door will be automatically closed, which will play the role of controlling the fire in the zone. The fire door adopts a Class A fire door, and the fire resistance limit is not less than 200h。
Fire Protection Standard for Design of Thermal Power Plants and Substations GB50229-2019 in 114.1. Provisions: Cable trenches or cable tunnels with a length of more than 100m shall take flame retardant or separation measures to prevent the spread of cable fires, and the fire resistance limit shall not be less than 200h firewall or partition;
Power Engineering Cable Design Standard GB50217-2018 in 70.3. Stipulation: The fire resistance limit of fireproof plugging components such as fireproof plugging, firewall and fireproof section shall not be lower than the fire resistance limit of penetrating components (such as building walls, floor slabs, etc.), and shall not be less than 1h.
Fire Door GB12955-2008 in 45.1.2 Grade A fire door fire resistance and thermal insulation 150h, fire integrity 150h。
Brush fireproof paint on the cables of power cable tunnels, and apply fireproof coatings, fireproof tapes or fire retaining plates on all cables in the section of not less than 1m on both sides of each fire door (firewall) and fire protection partition; In the part with fire hazard (such as cable joint), about 3m on both sides of the cable and other cables laid in parallel adjacent to the range, it is advisable to use fireproof coating or fireproof tape to prevent flame retardation. The main purpose of these methods is that if one cable catches fire and does not burn to other cables, in addition to partitioning firewalls and fire doors, fire partitions should also be added between power cables and control cables. The fireproof plug must have a certain thickness and be really tight. Especially in the dense area of the cable, soft plugging material can be used for tight plugging, and after the daily maintenance of the cable, the damaged place must be re-blocked.
Power Engineering Cable Design Standard GB50217-2018 in 70.3. Provisions: The holes where the cables penetrate the partition wall and floor slab, and the cable pipe holes in the working well should be fireproofed; The branches of the main passages of public cable trenches, tunnels and bridges, and the entrances of cable trenches, tunnels and overhead bridges to control or distribution devices, and the walls of the factory are provided with fireproof separation; The control cable and non-flame retardant communication optical cable laid in the same channel of the power cable shall be inserted into the flame retardant pipe or fire-resistant cable trough box, or the fireproof sealing plate shall be set between the power cable and the control cable;
Technical Regulations for the Design of Urban Power Cable Lines" DL T-2016 in 94.2. Provisions: When the cable passes through the floor slab, wall or cabinet hole and both ends of the pipeline, it should be plugged with fireproof plugging material. The fireproof plugging material should be dense and free of pores, and the thickness of the plugging material should not be less than 100mm.
It is very necessary to set up the automatic fire alarm system in the cable tunnel, because the development speed of the tunnel cable forming fire is relatively slow and the time is longer, it is completely possible to find the hidden danger in time through the automatic fire alarm system to prevent the occurrence of fire.
The fire alarm device of the urban power cable tunnel generally adopts the distributed optical fiber cable temperature measurement system, which not only has the fire alarm project, but also can form a linkage system with other fire prevention measures. It is to lay the temperature-sensing optical cable in parallel with the cable, and the temperature-sensing optical cable transmits back the temperature signals of each position of the measured object in the form of light waves, and finally extracts and displays them. When the temperature sensing cable detects that the cable temperature is high, it will automatically start the exhaust fan for ventilation and heat dissipation; When a fire signal is detected, the system will control the opening and closing of the fire doors at both ends of the fire partition and the closing of the exhaust fans. (For specific requirements, please refer to 12 in the "Code for Design of Automatic Fire Alarm System" - GB50116-2013.)3 cable tunnels).
Fire extinguishers, yellow sand boxes and other fire-fighting equipment should be set up at the entrance and exit of the cable tunnel, the joints and each fireproof partition. (Code for Design of Power Cable Tunnels, DL T5484-2013, 9.)2.8）
Code for Design of Building Fire Extinguisher Configuration》GB50140-2005 Zhongde Cable Tunnel belongs to Class E fire (live fire): fire with live combustion of objects Ammonium phosphate dry powder fire extinguisher, sodium bicarbonate dry powder fire extinguisher, halide alkane fire extinguisher or carbon dioxide fire extinguisher shall be used, but carbon dioxide fire extinguisher equipped with metal horn spray canister shall not be selected. Cable tunnels are classified as medium hazards in industrial buildings, and fire extinguisher configurations can be calculated according to the formulas in the code.
The main feature of flame retardant cable is that the ignition point of the outer sheath of the cable is high, when the cable is on fire, the main insulation of the cable is protected from damage within a certain period of time, the cable power supply is extended and the cable fire range is shortened, but the flame retardant cable can only play a role in preventing combustion, and can only be used as an auxiliary measure for cable fire prevention. Generally, flame-retardant cables have low smoke and low toxicity, and are more suitable for use in occasions where people are patrolling; Fire-resistant cables can keep the circuit energized at a specified temperature and time, and are more suitable for important circuits. For urban power cable tunnel engineering, flame-retardant cables should be basically used, and fire-resistant cables should be selected in important circuits. (For details of the selection of flame-retardant and fire-resistant cables, please refer to 70.5-7.0.7）
In the accidents of cable ** and fire, about 70% of the accidents are caused by the failure of the cable head, which shows how important the quality control of the cable head production is for cable fire prevention. The service life of the cable head must be equal to or longer than the service life of the cable itself. The insulation performance and voltage rating of the cable gland must be equal to or higher than the insulation performance and voltage rating of the cable itself. The withstand voltage between the insulating pads should be equal to or higher than twice the insulation performance of the cable sheath. In the section of about 3m of the cable on both sides of the cable joint, it is advisable to use fireproof coating or fireproof tape to prevent flame retardation; It is recommended to install a portable dry chemical fire extinguisher at the location of each cable termination. Power Engineering Cable Design Standard GB50217-2018 in 70.4. It is stipulated that the non-flame retardant cable joint is 3m on both sides, and if the flame retardant cable specification is not required, the electrical profession is generally listed 15m）
Technical Regulations for the Design of Urban Power Cable Lines DL T5221-2016 in 94.4. Provisions: In tunnels, cable interlayers and shafts where cable inlet and outlet lines are particularly concentrated, fixed fire extinguishing devices such as wet automatic sprinkler fire extinguishing, water spray fire extinguishing or gas fire extinguishing can be added. Based on the actual situation of the on-site visit and understanding, I personally suggest that the automatic fire extinguishing system is generally not used for the urban open-cut power tunnel. Here's why:
1) The safety exit setting of the urban power tunnel by the cut-and-cover method can basically meet the escape needs of personnel, and the setting of the fireproof partition reduces the financial loss in other partitions.
2) the automatic fire alarm system of the urban power cable tunnel can carry out accurate temperature prediction before the fire occurs, if the cable encounters local accidents in the process of operation, fire hazards, the distributed optical fiber temperature detection system can ensure that the real-time information is transmitted to the monitoring terminal before the cable burns, and the fire doors at both ends of the ventilation partition can be considered to be closed, and the ventilation equipment is opened, and the intensity of mechanical exhaust is strengthened in this ventilation partition, and the heat dissipation conditions of the cable are improved, so as to achieve the purpose of reducing the temperature of the cable. If necessary, the dispatch center can transfer the load of this circuit cable according to the urgency and severity of the situation, and fundamentally eliminate the hidden danger of the accident.
3) All flame-retardant cables are used in the urban power cable tunnel, which reduces the probability of fire.
4) In terms of cost, the cost of building a separate set of automatic fire extinguishing system is higher. For example, foam fire extinguishing systems are large in long-distance urban power tunnels and have high operating and maintenance costs. (A set of foam fire extinguishing system made by the State Grid Company is about 500,000, excluding pipeline laying, the longest coverage of a single set of equipment is 200m, and the equipment size is about 10mx6m, considering that the cost of a single kilometer is relatively high).
For the underground excavation method of urban power cable tunnel, the specific problem should be analyzed, and according to the structure of the cable tunnel and the specific construction plan, it is necessary to determine whether it is necessary to set up an automatic fire extinguishing system. And it must be clarified in coordination with the local fire audit department.
QGDW 11187—2014 Design Guidelines for Cut-and-Cover Cable Tunnels 》In tunnels with voltage levels of 500kV and above, fixed fire extinguishing devices can be added.
Q GDW 11186—2014 Design Guidelines for Underground Excavated Cable Tunnels》In tunnels where the cable inlet and outlet lines are particularly concentrated or the voltage level is 330kV and above, you can choose to add one of the fixed fire extinguishing devices such as aerosol fire extinguishing device, wet automatic sprinkler fire extinguishing, water spray fire extinguishing, gas fire extinguishing or sprinkler pipe, focusing on the fire extinguishing in the cable joint area.
The design of urban cable tunnels should not only do a good job in fire prevention and fire protection measures, but also need to ensure the normal operation of cable tunnels, personnel maintenance, and ventilation measures in the event of fires; For details on how to do a good job in the ventilation design of urban cable tunnels in the design and operation stage, see Fire Protection, Fire Protection and Ventilation Design of Urban Cable Tunnels (Part II).
References. 1 DL T 5484-2013 Code for the design of power cable tunnels;
2 GB 50217-2018 Standard for the design of cables for power engineering.
3 DL T 5221-2016 Technical Regulations for the Design of Urban Power Cable Lines.
4 GB 50016-2014 Code for fire protection in building design.
5 GB 50229-2019 Fire protection standard for the design of thermal power plants and substations.
6 DL T 5035-2004 Technical specification for the design of heating and air conditioning of thermal power plants.
7 GB 50116-2013 Code for the design of automatic fire alarm systems.
8 GB 23864-2009 Firestop Blocking Material.
9 GB 50019-2015 Design Code for Heating, Ventilation and Air Conditioning of Industrial Buildings.
10 GB 12955-2008 Fire Door.
11 GB 50140-2005 Code for design of fire extinguishing configuration in buildings.
12 Q GDW 11186—2014 Design Guidelines for Underground Cable Tunnels.
13 Q GDW 11187—2014 Design Guidelines for Cut-and-Cover Cable Tunnels.
14 State Grid Corporation of China Power Transmission and Transformation Engineering General Design Cable Line Fascicle (2017 Edition).
15 Zhao Hui, Ventilation Design of Urban Power Tunnels, North China Electric Power Technology, 2009
16 Zhao Hui, Design Values of Fire Protection Partition and Ventilation Partition of Urban Power Cable Tunnel, Power Supply and Consumption, 2009
17 Zhao Hui, Fire Protection and Fire Protection Design of Urban Power Cable Tunnels, North China Electric Power Technology, 2010
18 Peng Chen, Fire Protection and Fire Protection Design of Urban Power Cable Tunnels, Electric Power Technology and Management, 2014
19 Gao Yi, Analysis of Fire Protection Technical Scheme for Power Cable Tunnels, Power and Energy, 2013