LSZH Cable | Low Smoke Zero Halogen Cables

The "Low Smoke" element of LSZH cables

According to Part B of The Building Regulations, a leading planning and building regulations resource for England and Wales:

"The primary danger associated with fire in its early stages is not flame but the smoke and noxious gases produced by the fire. They cause most of the casualties and may also obscure the way to escape routes and exits. Measures designed to provide safe means of escape must therefore provide appropriate arrangements to limit the rapid spread of smoke and fumes."

Burning PVC has been found in tests to reduce visibility in the surrounding area by as much as 50% within 10 minutes. After 30 minutes, visibility can be reduced by as 90% making it unnecessarily challenging to escape the area.

By comparison, BS EN specifies a smoke density test, performed inside a chamber 3m x 3m x 3m. The test is often referred to as the 3 metre cube test. By monitoring the transmittance reduction of a white light beam running from one side of the chamber to the other at a set height, the build-up of smoke inside the chamber can be measured. For a cable to be classified as low smoke it will often have a minimum light transmittance of 60%. 

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The "Zero Halogen" element of LSZH cables

Burning PVC and other cable compounds produce a number of chemicals, including hydrogen chloride (HCl), which is highly toxic. Furthermore, when mixed with water, HCl forms hydrochloric acid; a substance which is both toxic and corrosive. In the case of PVC, as much as 30% of the emissions can be HCl.

BS EN stipulates that emissions of HCl by LSZH cables must not exceed 0.5%.

LSZH vs LSF cables

Whilst there is a common misconception that low smoke and fume (LSF) cables and LSZH cables are the same, this is not the case and any confusion between the two could be life-threatening in the event of a fire.

Unlike LSZH cables, LSF cables are manufactured using PVC compounds. If they are labelled as low smoke and fume cables it is due to the manufacturer modifying the compound with various additive in order to reduce (not eliminate) smoke and HCl emissions. However, where low smoke zero halogen cables are subject to strict standards, there are no standards governing LSF cables. As a result, LSF cables have been found to emit HCl levels in excess of 20% when burning.

As a leading rail & metro cable supplier, Eland Cables is well-positioned to understand the implications involved when selecting an LSZH cable for use in any other industry. Please contact our technical team to discuss your specific requirements and our comprehensive range of LSZH cables for all applications, including building & construction, oil & gas, and marine & offshore.

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Low Smoke
In a building fire the thick black smoke given off by conventional PVC cables can cause asphyxiation, obscure escape routes and make it difficult for fire fighters trying to enter or search the building.

Prysmian has tested some cables claiming to be low smoke that within three minutes caused light transmittance (and hence visibility) to drop to less than 5% within three minutes. If this performance is mirrored in buildings where the cable is installed it would be very difficult for people to find their way out of the building and is likely to greatly increase the stress and sense of panic as they try to do so. 

A cable officially described as &#;Low Smoke&#; should have been tested in accordance with the standard BS EN -2.

The standard uses a test which measures the amount of light transmitted through the smoke produced by burning 1 metre-long samples of cable suspended above a 1litre bath of alcohol in a 3m3 sealed cube. Because the alcohol produces negligible amounts of smoke, any smoke produced is from cable combustion. For a cable to be classed as low smoke, a minimum of 80% of the light must pass through the smoke during the test.

The practical effect of passing the test is that the cable produces very little smoke in a fire: visibility should not be reduced by more than 20%, leaving at least 80% of the light getting through and allowing much greater visibility in the building during the critical period when occupants are trying to escape.

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