Analysis and engineering application of fire protection performance of mining cable

Author：hlyb Add Time：2016-03-31 17:24:01

The increase of wires and cables, the concentration of laying and the poor quality of construction exacerbate the risk of wire and cable fire. Therefore, to prevent wire and cable fire in practical engineering application, we must start from controlling risk factors, apply relevant specifications and take corresponding fire prevention measures.

1 fire causes and characteristics of wires and cables

It is mainly due to overload, short circuit, excessive contact resistance and external heat source. Short circuit, local overheating and other fault states and the causes of fire caused by wires and cables under the action of external heat. Insulation data: the insulation resistance decreases, the insulation capacity is lost, and even burns, leading to fire. The main characteristics of wires and cables in fire are:

In case of fire, 1 the fire temperature is generally 800 ℃ ~ 1000 ℃. Conductors and cables will soon lose their insulation capacity, which will lead to secondary electrical accidents such as short circuit, resulting in greater losses;

2. The conductor and cable have large overload capacity under the specified allowable current carrying capacity;

The conductor and cable will cause the insulation data to melt and burn in an instant, 3 under the short-circuit state. And ignite the surrounding combustibles.

2. Analysis on fire resistance of wires and cables

2.1 fire prevention mechanism analysis

2.1.1 flame retardant mechanism

The flame retardant located in the condensed phase decomposes endothermic, 1 under the thermal action of combustion reaction. It slows down the temperature rise in the condensed phase and delays the thermal decomposition rate of the data;

The chain reaction free radical blocker is released, and 2 flame retardant is decomposed after heating. The branch of flame and chain reaction is interrupted, and the speed of gas phase reaction is slowed down;

The formation of coking layer or foam layer strengthens the role of these layered crust in blocking heat transfer. 3 catalyze the thermal decomposition of solid products in condensed phase.

The flame retardant has endothermic phase transition, 4 under the action of heat. Physically prevent the temperature rise in the condensed phase.

2.1.2 fire resistance mechanism

To reduce the heat generated by the polymer, 1 add some additives to the insulation and sheath data of wires and cables. Prevent polymer decomposition or promote the carbonization of insulation and sheath materials to form a maintenance layer;

After the insulation and sheath layers are burned and corroded by fire, a layer of mica glass ribbon and other inorganic insulation materials shall be added at the 2 wire core* The mica refractory tape wrapped on the conductor is maintained and continues to be energized, so as to maintain normal operation for a certain time in case of fire.

2.1.3 mechanism of mineral insulated cable

Aloh3 is 34.6% under high temperature operation. The absorption effect of metal hydrate makes the cable flame retardant. For example, aloh3 and MgOH are used as flame retardants. Mgohz is 31%, see reaction formula 1 and reaction formula 2. The reaction is decomposed into endothermic reaction, which can inhibit the combustion of polymers. 2A IOH3--*A lz03+3H20-2648KJ1MgOH2--MgO+H20-93．3KJ2

2.2 classification and specification test of combustion characteristics of wires and cables

It can be divided into ordinary wires and cables, flame retardant wires and cables, fire-resistant wires and cables, halogen-free low smoke wires and cables and mineral insulated cables. Wires and cables according to their own combustion characteristics.

1 flame retardant wires and cables refer to wires and cables that are difficult to catch fire and have the ability to prevent or delay flame spread. The commonly used standard test is gbt18380.3, which is equivalent to iec60332-1999

Wires and cables that can still adhere to line integrity. The commonly used specification test is gbt12666.6, which is equivalent to iec60331-21-19992. Fire resistant wire and cable refers to the flame combustion at the specified temperature and time.

The smoke generated during combustion is less, and there are wires and cables to prevent or delay the spread of flame. There are three common standard tests: gbt17650.2 is equivalent to iec60754-2gbt17651.2 is equivalent to iec61034-2 and gbt18380.3 is equivalent to ec60332-3. On the basis of the above, the flame-retardant and fire-resistant type also needs to meet the requirements of adhering to line integrity. 3 halogen-free low smoke wires and cables are divided into flame-retardant type and flame-retardant and fire-resistant type. Flame retardant refers to halogen-free materials. At the same time, gbt12666.6 is added to the common standard test, which is equivalent to iec60331

It will not cause fire due to short circuit. In addition to gbt12666.6 fire resistance test, 4 mineral insulated cables have the properties of non combustion, smoke-free and non-toxic in flame. In addition, according to the actual situation of fire, refer to the requirements of British BS code for cable test with the ability to resist spray water and mechanical impact (heavy object falling). At the same time, refer to the national specification mineral insulated cables and terminals with rated voltage of 750V and below gbl3033-1991

Application of 3 wires and cables in 4 "1 - in fire protection engineering

3.1 selection of wires and cables

3.1.1 selection principle

It only needs to meet the use under non fire conditions, general power system. Therefore, it can be selected according to general requirements. The power supply line of fire-fighting electrical equipment must meet the continuous power supply time of fire-fighting equipment in case of fire, so as to ensure the integrity of the line and the normal operation of the system. At the same time, the fire risk of wires and cables shall also be considered to prevent them from becoming a fire source due to short circuit and overload. Under the action of external fire, it shall not contribute to the spread of fire, which can effectively reduce the harmful gas decomposed by organic insulation layer and prevent the occurrence of "secondary disaster".

3.1.2 selection of non fire electrical lines

Ordinary wires can be used; In case of direct buried laying and concealed laying through pipes, 1 ordinary equipment lines are laid through pipes. Ordinary cables can be used;

Flame retardant wires and cables shall be used; 2. When wires and cables are laid in bundles.

Wires and cables used in secondary places should be halogen-free and low smoke type; 3 wires and cables used in special and first-class places shall be halogen-free and low smoke type.

The main power supply lines for particularly important loads in special and first-class places are 4 used for wood structure public buildings. Mineral insulated cables should be used.

3.1.3 selection of fire electrical circuit

Fire resistant wires or mineral insulated cables shall be used; When the wires are laid in the metal trunking in an open manner or through metal pipes or flame retardant rigid plastic pipes in a concealed manner, 1. When the power consumption time of fire-fighting equipment is long (≥ 1H) and the distribution line is laid in the open or in the dark, flame-retardant and fire-resistant wires shall be used; when the wires are laid in the fire-resistant metal trunking or through the metal pipe coated with fire-retardant coating, flame-retardant wires can be used; the main power line from the substation (or general distribution room) to the fire-fighting equipment shall be flame-retardant and fire-resistant cables or mineral insulated cables;

Ordinary wires and cables can be used; 2. When the power consumption time is long (≤ 1H), mineral insulated cables can be used for open laying of distribution lines; when the buried wall (maintenance layer thickness shall not be less than 30mm, concealed laying, fire-resistant cable bridge laying or mica insulated fire-resistant cable closed metal bridge laying.

When the mica insulated fire-resistant cable is laid in the open, 3 the power consumption time is short (< 20min). The distribution line passes through the metal pipe or closed metal trunking and is coated with fireproof paint. Ordinary wires and cables can be used.

3.2 electrical wiring measures for fire fighting equipment

3.2.1 wiring principle

To ensure the integrity and fire resistance of distribution lines. On the premise of meeting relevant technical regulations, fire-fighting equipment shall adopt fire-resistant and heat-resistant wiring design. The selection of conductor section shall be relaxed appropriately to prevent the increase of conductor resistance and voltage drop due to the rise of ambient temperature in the process of fire, which will affect the function of fire-fighting equipment.

3.2.2 basic wiring measures

Ordinary wires and cables shall be laid through metal pipes or flame-retardant rigid plastic pipes (oxygen index 30) and buried in non combustible structures. 1 when the lines are laid in a concealed manner, and the protective layer shall be laid in a concealed manner (thickness of 30ram)

Wear metal pipe or metal fire retardant coating to improve the fire resistance of the line. 2 when the line is laid in the open. Or flame retardant wires and cables and copper trunking shall be directly used for protection, and leather fireproof cables shall be used and laid in the closed trunking with fire prevention measures in the cable shaft or ceiling;

The two shall be separated by fire-resistant materials; 3. When the insulation layer and sheath of the distribution line are non flame retardant cables and laid in the well (metal trunking shall be used for sealing at the same time), it can not be protected by metal pipes; However, when it is laid in the same shaft as the flame retardant cable.

Flame retardant (oxygen index 50) rigid plastic pipe and plastic trunking can be used for wiring; 4. The fire electrical circuit in the building ceiling adopts metal pipe or metal trunking for wiring; the flame retardant material is in the ceiling.

The distribution line protected by metal pipe can be laid close to the outer wall of air duct; When there are combustibles in the ceiling, 5 distribution lines shall not pass through the inner cavity of the air duct or be laid on the outer wall of the air duct. The distribution line shall be protected by metal pipe.

3.3 fire prevention measures in engineering application

The wiring of fire-fighting equipment subsystem is very key and is widely used in engineering. The fire protection function of the system in the whole project is guaranteed. The following introduces the wiring protection of fire equipment subsystem by taking the wiring of automatic fire alarm system and fire elevator as an example.

1. Wiring protection of automatic fire alarm system

Higher fire protection requirements shall be considered. The transmission line of automatic fire alarm system shall be protected by metal pipe, flame retardant treated (oxygen index 35%) hard plastic pipe or closed fire trunking; When the fire control, communication and alarm lines are concealed, flame-retardant wires shall be laid in the non combustible structural layer through the maintenance Pipe (the thickness of the maintenance layer is 30mm or treated according to the basic wiring measures (1 and (2); the trunk line of the bus system. Fire resistant cables can be laid in the fire-resistant cable bridge or copper sheet fire-resistant cables can be selected;

The route is long and complex. In order to improve the reliability of power supply, 2 fire elevator distribution line fire elevator is generally distributed from the substation on the ground floor of high-rise building to the elevator machine room on the top floor. Fire resistant cables shall be used for distribution lines of fire elevators as far as possible; When there are special requirements for power supply reliability, one of the two special distribution lines can choose copper sheet fireproof cable; The vertically laid distribution line shall be set in the electrical shaft as far as possible, and the basic wiring measures shall be considered.

3.4 fire prevention measures for cable laying

3.4.1 fire prevention principle

The basic principle of fire prevention in cable laying is

No delayed combustion after ignition; 1. Prevent the cable from catching fire.

2. Take effective anti blocking and fire-fighting measures along the cable path or flammable section;

3. Make the cable itself flame retardant.

3.4.2 fire prevention measures

Fireproof sealing measures shall be taken. 1. When the cable is laid under the following conditions.

Super high-rise buildings shall be blocked on each floor. ① when cables pass through different fire compartments; ② The cable is vertically laid along the shaft and crosses the floor. Other buildings can be blocked every 23 floors; ③ Cable tunnel, cable trench and partition wall between cables; ④ Where the fire resistance limit of crossing is not less than LH partition wall; ⑤ Crossing the partition wall of the building; ⑥ To the entrance of the building, or to the channel of the power distribution room and control room; ⑦ The cable is led to the opening of electrical cabinet, panel or control panel and console;

2 keep away from heat source and fire source

Cables shall not be laid; If it is laid in the thermal pipe trench, thermal insulation measures shall be taken. Electrical cables shall not be laid in the environment with explosion and fire hazards; The cable trench shall be as far away from steam and oil pipelines as possible; Combustible gas and combustible liquid pipe trench.

3. Set fire alarm system

Select appropriate alarm probe and alarm system suitable for the characteristics of cable layer. At present, linear (or cable type) temperature detectors are widely used in cable trenches and pipeline wells; According to the actual situation.

4 high pressure water spray fire extinguishing

It is difficult to use general fire-fighting data, such as cable corridors and areas with dense cables. High pressure water spray should be adopted. High sensitivity monitoring and control system is needed to make water spray extinguishing work in a timely and effective manner. This method is effective in large buildings and cable tunnels;

5. Strengthen the ventilation of cable layer (well)

Vents shall be set at the outer wall of the cable layer * as far as possible (the specific setting of vents can be combined with the breakthrough in fire fighting). At the same time, a mechanical smoke exhaust system with uninterrupted power supply shall be established to make active use of natural ventilation conditions. In order to open the smoke exhaust fan through automatic alarm linkage in the early stage of fire.

4 Analysis of engineering application problems

Only the fire resistance characteristics are tested according to gbl2666.6. Superior fire resistance characteristics. 1 at present, there is no national specification or industry standard in China. Can't explain that the fire-resistant cable is of good quality and its electrical performance? How mechanical? Moreover, there is a certain gap between the fire resistance test results of a single fire source in the laboratory and the actual fire influencing factors. Therefore, the fire-resistant cables should be comprehensively inspected and unified specifications should be formulated;

The use requirements of various distribution wires and cables shall be detailed, and 2 the current specifications on the distribution lines of fire-fighting equipment. Supplement the contents of new fire-resistant cables and give more choices. In particular, mineral insulated cables shall be clearly specified in the specification and can be used as distribution cables of fire-fighting electrical equipment.

5 Conclusion

The correct selection and reasonable wiring of wires and cables are directly related to the fire safety of buildings. Standardization and standardization should be strengthened in process application. To sum up, it is analyzed. Formulate a scientific and specific scheme to provide basis for wire and cable fire prevention and electrical wiring of fire-fighting equipment in engineering design, so as to make it safe, reliable, technologically advanced, economical and reasonable, effectively prevent fire caused by wire and cable, and reduce casualties and property losses caused by fire.

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Analysis and engineering application of fire protection performance of mining cable