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Fire Test of Single and Bundled Cables under Flame Conditions
The insulation and jacket materials (including part of shield materials) of most of cables are organic polymers, such as PVC, PE, PU. Severe burning will occur when these materials encounter flame, and release massive amount of smokes and toxic, combustion-supporting gas. The purpose of the flammability test of cable under fire conditions is to evaluate the capability of the cables to suppress flame to propagate  under defined conditions, that is the capability that the non-metallic materials such as the insulation and jacket materials of cables self extinguishing in a period of time after the fire source removed or retard the flame propagation.  
 
The flammability test of cables include the flammability test of single cable and that of bundled cables, the standards about flammability test of single cable include: GB/T18380.1-2001(equivalent to IEC60332-11993) “Test on electric cables under fire conditions, Part 1” , a flammability test standard for generic cable with insulation; and GB/T18380.2-2001(equivalent to IEC60332-21989) “Test on electric cables under fire conditions, Part 2”,  a flammability test standard for the fine wire with inner conductor less than 0.8 mm, or the small size stranded wire with the cross-section less than 0.5 mm2. The standards about flammability test of bundled cables include: GB/T18380.3-2001(equivalent to IEC60332-21992) “Test on electric cables under fire conditions, Part 3” , a type approval test for wires and cables developed starting from the safety in the practical installation places of the wires and cables.
 
Even the insulated wires and cables that pass the vertical burning test of single cable, they may not have very good fire retardance under bundled installation status, this is the purpose that takes the flammability test of bundled cables as the type approval test of the cables. Because the flame propagation along the bundled cables is also dependent on the following factors:
 
1) The volume of the flammable materials in the flame produced due to exposing in fire or burning of cables themselves.
2) The geometry and surrounded medium of the cables.
3) The ignition temperature of various gases released out from the cables.
4) The amount of the gases released out from the cables under he given temperature rising.
5) The volume of the air that passes though the cable set-up.
6) the cable structure, for example, armored or non-armored.
 
The flammability test of cables under fire conditions is independent to on the other performance of the cables.
 
The flame temperature in the two vertical burning test methods for single insulated cable can be obtained directly through measuring the inner flame temperature using thermocouple (t760),or you may use a pure copper wire with a diameter of 1 mm that is clamped at a support, then adjust the height of the inner flame up to 50-60 mm contacting directly to the copper wire and burning, if the copper wire is burnt out in 1 min, it will indicate that the inner flame temperature t760. The difference between GB/T18380.1-2001 and GB/T18380.2-2001 standards is the difference of flame temperature, the former has definite requirement while the latter no this requirement. In addition there is difference in fire time, the former specifies different fire time according to the diameter of the cables, at least t60 s, while the latter only specifies the fire time as 20 s. Other operations and criteria are basically same.
 
For the vertical flammability test method of single wire and cable various countries in the world developed various testing methods based on their own specific situations and referring to IEC or UL specifications. However the basic testing set-ups, testing methods, and the criteria are similar: the difference among them is that some methods add in some auxiliary criteria, for example, the stipulation about the self extinguishing time of the sample after the fire source removed; the stipulation about the flaming droplet burning the cotton paper paving underneath the sample; and the stipulation about flame propagation to indication flag, etc. the purpose of these stipulations is only to evaluate the capability that the non-metallic materials such as the insulation and jacket materials of cables self extinguish in a period of time after the fire source removed or retard the flame propagation. 
 
GB/T18380.3-2001, the flammability test method for bundled cables is equivalent to IEC60332-31992. It is a type approval test for wires and cables developed starting from the safety in the practical installation places of the wires and cables. The method definitely specified the burning kinds, number of sample pieces, mounting mode, fire time, binding way, judging results, etc.:
 
1) The burning kinds include A, B, and C classes.
 
2) The amount of the sample can be obtained through calculation.
For example: Cut 0.5 m of a PE insulated wire with a diameter of 2.5 mm, strip out the PE, know it is 0.1 kg through weighting, since the density of PE is 0.96g/cm3 , V n=0.1/(0.96×0.5)=0.208L/m.
According to the burning kind that the trust requires: A class 7L/0.208L=34pieces, the tested quantity is 119 m.
 
3) In most cases the mounting mode employs binding in single row.
The classification of binding modes: dense arrangement and spaced arrangement.
For outer diameter D35mm, and AF/R testing type the binding may adopts two rows (divided into front one and back one)
Standard ladder: sample density300mmspace between cables20mmThe distance from cable to the ladder boarder50mm.
Wide ladder: sample density600mmspace between cables20mmThe distance from cable to the ladder boarder50mm.
 
4) Fire time: for class A and B being 40 min; for class C, 20 min. 
 
5) The binding mode is dependent on the cross section areas of the conductors: for d≥35mm spaced arrangement is applied; for d≤35mm, dense arrangement is applied.
 
6) The judgment results, after the fire stopped and the self-burning extinguished (if the burning duration is more than 1 h, forcibly extinguish the burning) , measure the distance from the starting point of char of the burnt sample to the bottom side of the burner: if h≤
2.5mm, passed; and h≥2.5mm, failed.
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