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Measurement of Smoke Density of Cables Burning under Defined Conditions
As is well known that the insulation and jacket materials (including part of shield materials) of most of optical & electric cables are organic polymers, such as PVC, PE, PU. In the places where these cables are used, such as in cabins, aircrafts, vehicles and all kinds of buildings, people usually design and install bundled wires and cables that are used in various applications into very crowded routing troughs as possible, so as to In order to keep visual enjoyment. Once combustion occurs under the action of fire source these organic polymer materials will release massive amount of toxic smokes in limited space. According to the incomplete statistics of National Fire Control Headquarters, most of the death in tens of thousands of fire accidents each year the people died caused by flame directly account for extreme minority, and most of the people died in the fire accidents are killed by the toxic smokes which are released from the burning materials.
Therefore, under the specified space and burnt conditions, the light transmittance of the smoke released out in the burning of the insulation and jacket of optical and electric cables can be used as a means of evaluating different cables or judging if they complying with the specific specifications. And the light transmittance of the smoke can be measured using a fixed light source. The purposes of the test is to prompt the producer choosing quality LSZH ( low smoke zero halogen) materials, so as to expedite the better development of China’s plastics and cable industries.
The smoke density is defined as the density of the smoke emitted from the objects burning caused by fire source and under defined space, in g/cm3. The standard for the measurement of smoke density of electric or optical cables burning under defined conditions is GB/T17651.1-17651.2-1998, which is equivalent to IEC 610341.1-61034.2.1997.
The measurement of smoke density is specified to be carried out in a cubic testing chamber that has 3000±
20 mm3 inner space. Also the standard specifies in detail the mounting position of the light sources, the placing angle of the draught screens, the placing height of the samples and fire sources, the direction of air flow, the adjustment of the light sources and powers, the spectrum response of the light receivers and the adjustment of receiving sensitivity, as well as the configuration of the standard fire sources, the necessity of the blank test, and the test procedures for samples.
1. The test sample shall consist of one or more test sections of cable, each 1, 00 m ± 0, 05 m long which shall be carefully straightened and then conditioned for at least 16 h at 23 ± 5 °C.
2. the number of test pieces required to make up the test sample shall be in accordance with the overall diameter of the cable as followed
D40 mmn=1
D40 mmn=1
20D40 mmn=2
20D40 mmn=2
10D20 mmn=3
10D20 mmn=3
5D10 mmn=45/D
5D10 mmn=45/D
2D5 mm n=45/3D (The number of test pieces, n, shall be rounded downwards to the integer )
3. For each bundle, the test pieces shall be twisted together, and bound with two turns of approximately 0, 5 mm diameter metal wire in the centre and at every 100 mm each side from the centre, meanwhile test pieces shall be bound together at the ends, and at 300 mm from each end, at which place they shall be clamped to the support by means of wire binders. The center of the test pieces axis shall be keeping in a horizontal position with the centre of the rectangular burning tray.
4. Place the draught screen and the flow fan in the position according the dimension defined by the standard.
5. On the basis of light source stabilized, adjust the optical transmitter and receiver, making the light transmittance when receiving to be the critical point of 100 % transmission. Fully cover the light receiving window and adjust the light transmittance displayed in the light receiver to the critical point of 0.00%.
6. Carefully pour mixed standard fire source—alcohol into the burning tray, with the test sample supported above the tray, start the small fan making air circulation and ignite the alcohol. Make sure that all persons leave the cube immediately, and that the door is closedthen observe the change of light transmittance.
7. The test is considered as ended when there is no decrease in light transmittance for 5 min after the fire source has extinguished or when the test duration reaches 40 min.
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