Causes and characteristics of fire caused by wire and cable（二）

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2 Analysis of fire protection function of wire and cable

2．1 Analysis of the mechanism of fire prevention

2．1．1 Flame retardancy mechanism

（1）Under the thermal effect of incineration reaction, the flame retardant in the agglomerate phase differentiates and absorbs heat, which slows down the temperature rise in the agglomerate phase and delays the heat differentiation rate of the data.

（2）After the flame retardant is heated and differentiated, the chain reaction radical blocker is released, which interrupts the branch of flame and chain reaction and slows down the gas phase reaction speed.

（3）The formation of solid products, coking layers or foam layers in the catalytic coagulation phase enhances the effect of these layered hard shells on heat transfer.

（4）Under the thermal effect, the flame retardant has endothermic phase transformation, which physically blocks the temperature rising in the agglutination phase.

2．1．2 Fire resistance mechanism

（1）Take part in some additives in the insulation and sheath data of wire and cable, reduce the heat of the polymer, avoid the polymer differentiation or promote the insulation and sheath data to form the maintenance layer.

（2）On line core, a layer of mica glass ribbon and other inorganic insulation material is added. After the insulation and sheath are burned, the mica refractory belt wrapped in the conductor is maintained and continuously energized, so that the normal operation at the time of ignition is maintained.

2．1．3 Mechanism of insulating cable in mineral deposits

The absorption effect of metal hydrate is used to make the cable flame retardant. For example, using Al (OH) 3 and Mg (OH): as a flame retardant, Al (OH) 3 is 34.6%, Mg (OH) Z is 31%, (repercussion 1 and reverberant 2) under high temperature operation, and the reaction is differentiated into endothermic reaction, so it can suppress the incineration of polymer.2AI（OH）3--*Alz03+3H20-2648KJ （1） Mg（OH）2--～MgO+H20-93．3KJ （2）

2．2 Classification and standard experiment of the incineration characteristics of wire and cable

According to its own incineration characteristics, wire and cable can be divided into general wire and cable, flame retardant wire and cable, fire resistant wire and cable, halogen free low smoke wire and cable, and mineral insulated cable.

（1）Flame retardant wires and cables refer to wires and cables that are difficult to catch fire and have the ability to avoid or delay flame propagation. The standard experiment is GB / T18380.3 (equivalent to IEC60332-1999).

（2）Fire-resistant wires and cables refer to wires and cables that can still adhere to the integrity of the lines at regular temperatures and incinerated fires. The standard experiment is GB / T12666.6 (equivalent to IEC60331-21-1999).

（3）Halogen free low smoke wire and cable can be divided into two types: flame retardant type and flame retardant type. Flame retardant refers to cables and wires that contain no halogen, less smoke and dust during incineration and have the ability to prevent or delay flame spread. The commonly used standard experiments are GB / T17650.2 (equivalent to IEC60754-2), GB / T17651.2 (equivalent to IEC61034-2) and GB / T18380.3 (equivalent to EC60332-3) three items. On the basis of the above, fire retardant and refractory needs to meet the requirements of adhering to the integrity of the line. A commonly used standard experiment has been added to GB / T12666.6 (equivalent to IEC60331).

（4） Mineral insulated cables have non flammable and smoke-free functions in flames, and they do not cause fires due to short-circuit. In addition to the actual fire conditions of the GB / T12666.6 fire test, the standard requirements of anti spray water and resistance to mechanical bump (heavy falling) should be used in accordance with the actual situation of the fire. Together, it can refer to the national standard "extra voltage 750V and below mineral insulated cables and terminals" (GBl3033-1991).

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