In our previous Keystone Academy blog, &#;What is the Difference Between FRT and FR Cable?&#;, we shared that Fire Resistant (FR) cables are fire safety products which maintain circuit integrity in the presence of fire, while Flame Retardant (FRT) cables reduce the spread of fire.
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Keystone low-smoke, zero-halogen (LSZH) flame retardant (FRT) cables comply with IEC , IEC , and IEC , which ensure that the flame retardant cables reduce flame propagation, prevent the release of toxic gases, and control smoke emission under fire conditions. This article breaks down the standard LSZH FRT test methods in more detail.
Flame Propagation Tests: IEC-1-2, IEC-3
Flame retardant cables prevent flame propagation during a fire emergency. The cable's protective material includes additives such as aluminium hydroxide or magnesium hydroxide. When the material comes into contact with fire, the byproduct from the endothermic reaction is gaseous water which will help envelop the flame and thereby exclude oxygen from the fire.
IEC -1-2 is the test for vertical flame propagation for a single insulated wire or cable.
During the test, a single-core cable with a length of approx. 0.6m is mounted vertically using two clamps, and a flame is applied to the bottom end for 60 seconds (or 120 seconds if the cable&#;s overall diameter is >25mm).
Passing Criteria: After removing the flame, the burning cable extinguishes itself, and the fire damage is at least 50mm below the upper mounting clamp.
IEC -3 tests vertical flame spread of vertically-mounted bunched wires or cables.
This test is conducted as it cannot be assumed that bunched cables will behave the same way in the fire as single cables. This is because flame propagation along a vertical bunch of cables depends on other factors, such as the volume of combustible material exposed and the geometrical configuration of the cables.
Passing Criteria: After the burning has ceased, the charred portion does not exceed a height of 2.5 meters.
Acid Gas Emission Tests: IEC
When fire comes into contact with polyvinyl chloride (PVC) or other chlorine-containing materials, hydrogen chloride gas is released. Hydrogen chloride gas forms corrosive hydrochloric acid (HCl) on contact with water found in body tissues. This irritates the eyes, mouth, throat, nose, and lungs, thus making escape more difficult.
At Keystone Cable, all our fire-resistant and flame-retardant cables use Low Smoke Zero Halogen (LSZH) compounds to prevent the formation of HCl gases from burning cables.
International standard IEC specifies tests for determining the degree of acidity of gases generated during the combustion of materials from electric cables by measuring the pH and conductivity.
Passing Criteria: the weighted pH value is not less than 4.3 when related to 1 litre of water, and the weighted value of conductivity is not more than 10μS/mm when related to 1 litre of water.
Smoke Emission Tests: IEC
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This test measures the smoke density of electric cables burning under defined conditions.
The &#;3-meter cube test&#; measures the amount of smoke generated by cables in the event of a fire. The cables are placed in a 3m3 enclosure. A tray containing alcohol is supported above the ground surface to permit air circulation around and beneath the tray. The test pieces (cables or bundles) touched horizontally and centred above the tray. Air circulation will begin, and the alcohol (1 litre) will be ignited.
A beam of light is transmitted from one window of the chamber to the opposite window. The light intensity is measured between the light source and the photocell. The test is considered done when there is no decrease in light transmittance for 5 minutes after the fire source has been extinguished or when the test duration reaches 40 minutes.
Passing Criteria: The recorded light transmittance is at a minimum 60%, which means the smoke density has a maximum value of 40%.
Flame-retardant cable refer to a cable whose sample is burned under specified experimental conditions and the flame spreads only within a limited range after the removal of the test flame source. Also, they are the cables whose residual flame or residual burn can be extinguished by itself within a limited time.
According to different cable flame-retardant materials, flame-retardant cables are divided into two categories: halogen-containing flame-retardant cable and low smoke zero halogen flame-retardant cable.
Among them, the insulation layer, sheath, outer sheath and auxiliary materials (belting and filling) of the halogen-containing flame-retardant cables ( that are ordinary cables) are all or partly made of halogen-containing polyethylene (PVC) flame-retardant materials, so they have good flame retardant properties. However, when the cables burn, a large amount of thick smoke and halogen acid gas will be released. The halogen acid gas is corrosive to the surrounding electrical equipment. Therefore, rescuers need to wear gas masks to approach the scene to extinguish the fire. When the cables burn, it will cause harm to the surrounding electrical equipment and rescuers, which is not conducive to the fire fighting and rescue work, so that results in serious "secondary hazards".
While, the insulation layer, sheath, outer sheath and auxiliary materials (tape and filling) of low-smoke zero halogen flame-retardant cables are all or partly made of halogen-free cross-linked polyethylene (XLPE) flame retardant materials, which not only have better flame-retardant properties, but also no halogen acid gas is released when the cables burn. What&#;s more, the amount of smoke generated by the cables is also small, which is close to the recognized "low smoke" level.
The following differences between the two should be paid attention to in the selection:
1. Since halogen-containing flame-retardant cables (including flame-retardant cables, low-halogen and low-smoke flame-retardant cables) will release corrosive halogen acid gas when burning, which greatly hinders fire fighting work, so that delays fire fighting time and aggravates the spread of fire. Therefore, public places with high population density should try to design and use low smoke zero halogen cables.
2. Any kind of flame-retardant cable can be used in the operation area with low population density.
3. Compared with halogen-containing flame-retardant cables, low smoke zero halogen cables have the advantages of low corrosion and low smoke, but their electrical and mechanical properties are significantly reduced. Therefore, when laying cables, low smoke zero halogen cables should have a larger bending radius than halogen-containing flame-retardant cables.
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