Jun. 05, 2025
Low smoke zero halogen (hereafter referred to as LSZH but also called LSOH, LS0H, LSFH or OHLS) is a material classification that is common to cable and wire casings.
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When exposed to flames and high levels of heat, LSZH materials produce very little visible smoke (compared to PVC and other similar materials) and no halogens, which can be harmful to humans.
There are several similar classifications such as low smoke and fume (LSF) and low smoke varieties of PVC and cross-linked polyethylene (XLPE). Materials with these classifications have similar properties, but often fail to meet prescribed specifications.
This article deals with:
The halogens are a group of elements that can be found in the second column from the right in the periodic table.
Chemically, they are one electron short of having a full shell. This makes the non-metal elements highly reactive. They constantly try and gain an extra electron by reacting with other things – including organic matter.
The halogens, displayed in yellow on the below periodic table, can be harmful or lethal to humans.
The main cause of death in a building fire is smoke inhalation, not the flames themselves. Toxic smoke containing halogens is particularly dangerous.
PVC is often advertised as being fire resistant. This means that a high temperature is required for it to start burning. But the problem with PVC is that as it is heated, it gives off more than half of its weight as fumes.
Cable jacketing made of PVC can be dangerous because, when burned, it releases a toxic hydrogen chloride gas – chemically like mustard gas. Not only is this harmful to humans but it is also corrosive, and can damage electrical equipment.
Other products of PVC burning, such as dioxin, can cause more ‘slow burning’ health problems, with the effects staying with victims a lot longer.
Aside from the health effects, the thick acrid smoke that is released by burning PVC can reduce visibility in emergency situations. This can be devastating if, for example, the smoke obscures a pilot’s vision during an emergency landing.
PVC cabling can also help fire spread more easily. If lots of PVC wires are bundled together, for example in a group of network cables, travelling between rooms this can help spread flames easily throughout a building and accelerating the destruction.
Regulations have governed the use of PVC in wire and cabling for some time. In certain places, it is ill-advised to use PVC, but it is still common in certain safer settings.
See the difference in combustibility between an LSZH cable and standard cable in the video below.
https://www.youtube.com/watch?v=BBt_P0jZoFE
The first commercial thermoplastic LSZH material for cables was patented by Raychem in . Initially, the material was slow to catch on. But some specifiers saw its value sooner than others – notably those in military and transport sectors.
When HMS Sheffield was hit by an Argentine Exocet missile during the Falklands conflict in , the projectile failed to explode when it hit a control room, but a ferocious fire ripped through the ship, killing 20.
The thick poisonous smoke that emitted from electronics on the ship proved far deadlier than the flames.
A fire at King’s Cross Underground station in killed 31 when a stray match started a fire underneath a wooden escalator. Again, the thick smoke made it harder for passengers to evacuate and made life more difficult for the emergency services.
These tragedies helped crystallise the need for non-smoking, less toxic wiring in the minds of specifiers. Especially in enclosed spaces such as on war ships and in rail stations, but also more widely across society.
In the UK, low fire hazard cables are used widely, even though there are few statutory requirements for their use.
One important regulation comes from the Construction Products Regulation, introduced last year. From the 1st of July , all cables that are intended for permanent installation in buildings must be accompanied by a Declaration of Performance (relating to the cable’s fire resistance) and have a CE marking.
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These factors must be considered by building specifiers, who will factor the cable's properties into any risk assessments they carry out. This forms the basis of most regulations in the UK.
More prescriptive regulations govern the use of LSZH cable jacketing in specific industries. Fire resistant cables are widely used in aircraft, trains, ships, submarines, tunnel systems, nuclear power stations and much more. EU regulations require their use in a wide variety of network cables.
Advancements in thermoplastic LSZH made it available to create other accessories for cable management, including LSZH heat shrink tubing, labelling and fixtures.
This meant that more extensive cable management systems could be made from safer materials. Many LSZH cables are ineffective, because although the cables are made from low-smoking, non-toxic materials, other materials used in a cable management system are harmful. This includes things like heat shrink tubing, labels and cable trunking.
Where these cable management systems travel through a whole building it can enable the much faster travelling of fire.
As with LSZH cables, LSZH heat shrink is low smoking and has no harmful halogens. It is perfect for insulating and protecting sensitive cables, as well as for lightweight harnessing and electrical components in enclosed spaces such as marine, mass transit systems and other sensitive environments.
LSZH or LSOH Cable
It is probably safe to say that the entire construction industry is particularly focused on fire safety right now, and electricians installing cable have a very important role to play.
The need to be vigilant with your product choice has never been higher then, especially when it comes to installing Low Smoke Zero Halogen cables. Use standard PVC cable by mistake, for example, and it will release thick black smoke and toxic gases when reacting to fire – a potentially life-threatening error.
Smoke and Fumes Hazard
Smoke and fumes can be more of hazard than flame in the early stages of a fire when occupants are trying to escape – particularly in a public building, such as an airport, train station or hospital, where people may not be familiar with the layout of the building or the position of exits.
Low fire-hazard cables have a wide range of common names and rely on a variety of standards to describe their performance during a fire. Prysmian’s LSX cable has been specifically designed for use in buildings and can easily replace many standard PVC cables. It is described as a low smoke zero halogen (LSOH) screened wiring cable. But what do these terms actually mean?
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.
Zero Halogen
When burnt, PVC emits acidic, almost invisible hydrogen chloride gas (HCI). This horrible gas typically accounts for over 20% of the emissions produced by burning PVC and, when it mixes with water, it turns into equally dangerous Hydrochloric acid.
In any form, HCI is not something you want to deal with. If inhaled, the gas can irritate the throat lining to such an extent that it becomes very difficult to breathe. Contact with the eyes or skin can cause debilitating burns, further incapacitating those trying to escape a burning building. HCI can also damage sensitive electrical equipment, even if fire is contained and extinguished.
Afumex LSX cable produces zero halogen, which in industry terms is defined as less than 0.5%. The test for this characteristic is BS EN -2-1.
Beware of Fake Cable
Not all low smoke zero halogen cables are the same. Terms such as LSOH and OHLS are specific to individual manufacturers so it is worth checking all of the test standards to which the individual cable complies and be sure that they match the specification required for the building. Also, don’t assume cables described as LSF (Low Smoke and Fume) will pass ‘zero halogen’ tests and have flame retardant characteristics – check because some may not, often they just a modified PVC.
The low smoke zero halogen (LSOH) properties of Prysmian’s LSX cables makes it ideal for general wiring applications such as lighting and power – particularly in public buildings such as multi-storey dwellings, shops, offices and transport hubs such as train stations and airport terminals.
It is important to remember that while Afumex LSX’s low fire hazard LSOH properties make it ideal for non-essential services, such as general lighting and power, installers should continue to use Prysmian FP200 Gold & FP Plus cables for emergency circuits, such as fire alarms and emergency lighting.
For more information see our Low Smoke cables and Fire Resistant cable pages or contact us.
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