Fireproof wires and cables are the general name of wires and cables with fireproof performance. They are usually divided into flame-retardant wires and cables and fire-resistant wires and cables. For some designers just engaged in electrical design, it is essential to understand some cable characteristics in the industry. Let’s talk about the difference between flame-retardant cable and fire-resistant cable, as well as the matters needing attention in selection!
(1) Main uses of fire-resistant and flame-retardant cables
1) Main purpose of fire-resistant cable
It is mainly used in high-rise buildings, underground railways, nuclear power plants and important industrial and mining enterprises related to fire safety and fire rescue, such as power supply lines from emergency power supply to indoor fire-fighting equipment, fire alarm equipment, signal lamps, ventilation and smoke exhaust equipment, emergency elevators, etc.
Service characteristics: fire resistant cable (wire) is a kind of cable that can keep the line running normally for a certain period of time under the condition of combustion. The difference between it and ordinary flame-retardant wires and cables is that it can still transmit electric energy in case of fire, which is conducive to evacuating people and rescuing important equipment.
2) Main uses of flame retardant cables
It is widely used in large buildings, tunnels, petrochemical industry, power stations, mines and other places with high demand for fire prevention. In case of fire, it can prevent the fire from spreading on the cable, avoid the expansion of fire accidents and reduce losses.
Operating characteristics: the long-term allowable operating temperature of cables (wires) is consistent with the models of ordinary products in corresponding standards. A comprehensive and correct understanding of flame-retardant and fire-resistant cables is the primary condition for their correct selection.
(2) Differences between flame retardant wires and cables and fire-resistant wires and cables
1) Flame retardant wire and cable
Flame retardant refers to blocking and delaying the spread of flame along wires and cables so that the fire does not expand. This type of cable has self extinguishing performance after fire.
Flame retardant cables can be divided into three grades according to the national experimental standard (gb12666-99): Zra, ZrB and ZrC.
In general product naming, Zra is usually represented by gzr, which is called high flame retardant cable or oxygen barrier cable or high flame retardant oxygen barrier cable. ZrC represents Zr in general flame retardant products.
Flame retardant cable is self extinguishing while maintaining the electrical and physical and chemical properties of ordinary cable, that is, it is not easy to burn. Or when the cable is on fire for some reason or ignited by an external fire source, the cable will not continue to burn after the fire is extinguished, or the combustion time is very short (within 60 minutes), or the flame extension length is very short.
Compared with halogen-containing flame-retardant cables, halogen-free flame-retardant cables have the advantages of low toxicity, low smoke and halogen-free. The flame retardant level is not high, basically class C. The reason is that the energy efficiency of oxygen swallowing fire extinguishing of halogen radical is greater than that of degradation temperature of non halide flame retardant.
The oxygen barrier flame retardant technology is mainly to fill a layer of inorganic metal hydrate between the cable insulation core and the cable outer sheath. It is a non-toxic, odorless and halogen-free white colloid. The flame-retardant cable with oxygen barrier layer can be applied to low, medium and high voltage power cables, control cables and communication cables without changing the cable structure and original material parameters, and make the cable flame-retardant class A.
2) Fire resistant wire and cable
Fire resistance means that it can maintain the operation for a certain time under the condition of flame combustion, that is, to maintain the integrity of the circuit. This type of cable has the power supply capacity for a certain time in the flame.
Fire resistant cables can be divided into two grades according to the national experimental standard (gb12666-99): NHA and nhB; In general product naming, Nha is usually represented by GNH, which is called high fire-resistant cable. NhB is expressed as NH in general refractory products.
The fire-resistant cable can work continuously (transmit current and signal) in case of fire, and whether it is delayed or not is not included in the assessment. The flame-retardant cable will stop working soon in case of fire. Its function is that it is flame-retardant, does not spread and can self extinguish.
The fire-resistant cable shall maintain normal operation for 180 minutes in the flame combustion at 750 ~ 800 ℃.
Fire resistant cables are actually another type of fire resistant cables. It is made of copper as the conducting wire core, seamless copper tube as the sheath and inorganic mineral magnesium oxide as the insulating material.
As the whole is made of inorganic materials, the whole fireproof cable will neither burn nor delay combustion. And can withstand fire. The test shows that after burning in the flame of 800 ~ 900 ℃ for 2 hours, the cable has been running normally. After burning in the flame of 1000 ℃ for 30 minutes, it is still intact and continues to operate normally. Generally, the normal working temperature is 250 ℃, which can operate safely and reliably.
In addition, this kind of fireproof cable is also called mineral cable (internationally known as ml cable)
(3) Problems needing attention in the selection of flame retardant and fire-resistant cables
1) Problems needing attention in selecting flame retardant cables
The designer shall design, select and lay the flame-retardant cable according to the specific characteristics of the flame-retardant cable, and pay attention to the following problems:
1. Since halogen-containing flame-retardant cables (including flame-retardant cables and low halogen and low smoke flame-retardant cables) will release corrosive halogen acid gas during combustion, which will greatly hinder fire fighting work, thus delaying fire fighting time and aggravating fire spread, halogen-free low smoke flame-retardant cables should be designed and selected as far as possible in public places with high population density. Any flame retardant cable can be selected in the operation area with small population density.
2. Flame retardant cables are divided into classes a, B and C3, of which class a flame retardant cables have better performance and the most expensive price than class B and C. The designer shall indicate the category of flame retardant cable when providing the order list.
3. It is impossible to distinguish the A, B and C categories of flame retardant cables from the appearance, and only the manufacturer can guarantee the supply. Therefore, the cable manufacturer should be carefully selected in the design and selection.
4. Compared with halogen-containing flame-retardant cables, halogen-free low smoke flame-retardant cables have the advantages of low corrosion and low smoke, but their electrical and mechanical properties are significantly reduced. Therefore, during cable laying, halogen-free low smoke flame-retardant cables should have a larger bending radius than halogen-containing flame-retardant cables.
5. During cable laying design, it is not suitable to lay non flame retardant cables and flame retardant cables in parallel, and it is not suitable to lay flame retardant cables of different flame retardant categories in parallel.
2) Problems needing attention in selecting fire-resistant cable
According to the specific characteristics of fire-resistant cables, designers should pay attention to the following problems in design and selection:
1. When fire-resistant cables are used in cable tunnels and cable interlayer with dense cables, or in flammable places such as oil pipes and oil depots, class a fire-resistant cables shall be selected first. In addition to the above conditions and when the number of cables is small, class B fire-resistant cables can be used.
2. Fire resistant cables are mostly used as the power supply circuit of emergency power supply, which is required to work normally in case of fire. Due to the sharp rise of ambient temperature in case of fire, in order to ensure the transmission capacity of the line and reduce the voltage drop, for the circuit with long power supply line and strictly limited allowable voltage drop, the section of fire-resistant cable shall be enlarged by at least one gear.
3. Fire resistant cables cannot be used as high temperature resistant cables.