Fire performance analysis and engineering applicat

Fire performance analysis and engineering application of wires and cables

at present, the fire prevention measures required by China's wire and cable design regulations are only passive fire protection after wires and cables are on fire. In the actual construction and operation, the increase of wires and cables, the concentration of laying, and the poor quality of construction have aggravated the risk of wire and cable fire. Therefore, to prevent wire and cable fires in practical engineering applications, we must start with controlling risk factors, drawing various curves and printing, applying relevant standards, and taking corresponding fire prevention measures

1 fire causes and characteristics of wires and cables

the causes of fire caused by wires and cables are mainly due to overload, short circuit, excessive contact resistance and the effect of external heat sources. Under the fault conditions of short circuit, local overheating and the effect of external heat, the insulation resistance of insulating materials decreases, loses the insulation capacity, and even burns, thereby causing a fire. The main characteristics of wires and cables in fire are:

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

(2) the conductor and cable have large overload capacity under the specified allowable current carrying capacity

(3) under the short-circuit state, the conductor and cable will instantly cause the insulation material to melt and burn, and ignite the surrounding combustibles

2 analysis of fire protection performance of wires and cables

2.1 analysis of fire protection mechanism

2.1.1 flame retardant mechanism

(1) under the thermal effect of cutting off the power supply of the combustion reaction, the flame retardant in the condensed phase decomposes and absorbs heat, slowing down the temperature rise in the condensed phase and delaying the thermal decomposition rate of the material

(2) after thermal decomposition, the flame retardant releases a chain reaction free radical blocker, interrupting the branches of flame and chain reaction, and slowing down the gas phase reaction speed

(3) catalyzing the thermal decomposition of solid products in condensed phase, and the formation of coking layer or foam layer strengthens the role of these layered hard shells in hindering heat transfer

(4) under thermal action, the flame retardant has endothermic phase transition, which physically prevents the temperature rise in the condensed phase

2.1.2 fire resistance mechanism

(1) add some additives to the insulation and sheath materials of wires and cables to reduce the heat generated by the polymer, prevent the polymer from decomposing or promote the carbonization of insulation and sheath materials to form a protective layer

(2) add a layer of inorganic insulating materials such as mica glass ribbon at the core. After the insulation and sheath layers are burned by fire, they continue to be powered on by the protection of mica fire-resistant tape wrapped on the conductor, so as to maintain normal operation for a certain period of time in case of fire

2.1.3 mechanism of mineral insulated cable

use the absorption effect of metal hydrate to make the cable flame retardant. For example, using Al (OH) 3 and Mg (OH): as flame retardants, Al (OH) 3 is 34.6% and Mg (OH) Z is 31% under high temperature operation (see reaction formula 1 and reaction formula 2). The reaction decomposes into endothermic reaction, which can inhibit the combustion of polymers

2ai (OH) 3--*alz03+3hkj (1)

Mg (OH) 2--~mgo+h.3kj (2)

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

wires and cables 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 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 the spread of flame. The commonly used standard test that can be checked carefully is GB/t18380.3 (equivalent to IEC)

(2) fire resistant wires and cables refer to wires and cables that can still maintain line integrity under the flame combustion at the specified temperature and time. The standard test of common product unit weight: 23.2g (box)/15.6g (cover) is GB/t12666.6 (equivalent to IEC):

(3) halogen free low smoke wires and cables are divided into flame retardant and flame retardant fire resistant types. Flame retardant refers to wires and cables that contain no halogen, produce less smoke and dust when burning, and prevent or delay the spread of flame. The commonly used standard tests include GB/t17650.2 (equivalent to IEC), GB/t17651.2 (equivalent to IEC) and GB/t18380.3 (equivalent to IEC). On the basis of the above, the flame-retardant and fire-resistant type also needs to meet the requirements of maintaining the integrity of the line. At the same time, GB/t12666.6 (equivalent to iec60331) is added to the commonly used standard test

(4) mineral insulated cable has the performance of non combustible, smoke-free and non-toxic in the flame, and it will not cause fire due to short circuit. In addition to GB/t12666.6 fire resistance test, the commonly used standard test should also refer to the standard requirements of British BS standard for the test of cables with the ability to resist sprinkler water and mechanical impact (heavy object falling). At the same time, refer to the national standard mineral insulated cables and terminals with rated voltage of 750V and below (GBL)