With the increasing demand for ‘safe’ buildings, more attention is being paid to the types of cables being installed, and the potential risks in the event of fire. It is now recognised that smoke and poisonous fumes are often a far greater risk to life and property than the fire itself.
The main danger is often the smoke that is produced by combustible material - the fumes pose a huge threat to both people and property. The Kings Cross fire of 1987 still acts as a sobering reminder of the dangers of fire breaking out in enclosed spaces. At the inquest into the Kings Cross fire, many people were found to have been overcome by toxic smoke rather than injured by the flames.
London Underground has since banned halogenated cables from its underground stations following concerns about a repeat of the tragedy. Kings Cross station is now used by over 100,000 passengers every day and in the event of fire safe and fast evacuation is a challenge.
The main problem facing specifiers is the confusion over terminology and standards. Is LSHF better than LSOH? Is LSF the same as LSZH or RP? These terms are all widely used within the cable industry.
PVC: Problems and Perils
Standard PVC is made up of a significant number of halogens which are normally very stable but, when burnt, separate and give of toxic gases in particular hydrogen chloride (HCl). Hydrogen chloride is extremely dangerous and corrosive, and is in fact similar to mustard gas used as a chemical weapon.
These halogenated gases form highly corrosive acid when they come in contact with water – either moisture in the air, fire sprinkler systems or even moisture in the eyes or lungs of people. It is clear to see the devastation that can be caused by burning PVC in the event of fire.
The hazards of hydrochloric acid depend on concentration. Hydrochloric acid in high concentrations forms acid mists. Both the mists and the solution have a corrosive effect on human tissue, potentially damaging respiratory organs, eyes, skin and intestines.
Additionally, the hydrogen chloride given off also reacts with the many other additives present in PVC creating even greater volumes of toxic fumes.
The damage caused by burning PVC is two-fold; firstly, dense smoke will obscure exit routes with fumes choking people. The second and less recognised problem is that the acid gas produced in the fire permeates electronic equipment, settling on and corroding printed circuit boards and over a period of time can cause random, unpredictable failure.
This won’t just affect computers, it will also damage security/access control equipment, building management systems, lifts and just about anything else with a circuit board. The fire may have been extinguished within minutes with no great risk to life but the damage to equipment may be colossal.
IBT (Intelligent Building Technologies) such as PA systems, evacuation systems and CCTV networks are becoming more and more common in airports, train stations, hospitals and other places that can be difficult to evacuate, as are systems for handling data for diagnostics.
As heightened security becomes an issue in public places, particularly airports and train stations, there is a need for critical systems to continue functioning in the event of fire.
In the event of a fire, the high performance cable connecting these networks needs not only to be LSHF, but also carry on functioning to allow the safe monitoring of evacuation routes without putting additional lives at risk.
LSHF cables in themselves do not offer circuit integrity, and this is an important distinction. Cables specifically designed to survive in the event of fire need to be correctly specified. Fire resistant cabling for safety dependent or critical systems is another topic entirely and worthy of its own article.
Reducing the Risk – the Options
There has been a shift in recent years to using newly developed compounds that emit less of the harmful gases, particularly halogens, but still perform well in other respects.
The cheapest alternatives are modified PVC’s - these are termed RP (Reduced Propagation) or in some cases LSF (Low Smoke and Fume). The difficulty for the cable buyer is that there are no specific standards for LSF cables. Ordinary PVC emits approximately 28% HCl, whilst modified PVC could give off a massive 22% HCl and still be sold as LSF.
If you want to be absolutely certain of what you are installing you should insist on a cable that uses insulation and sheathing materials that do not emit any Halogens and have reduced smoke emission properties. These are termed LSHF (Low Smoke Halogen Free), LS0H, LSZH (Low Smoke Zero Halogen) or sometimes OHLS (Zero Halogen Low Smoke).
These products must emit no more than 0.5% HCl. This is achieved by using materials such as polypropylene which don’t produce the same gas or acid emissions when burnt.
Also, don’t accept standard PVC cables over-sheathed with an LSHF jacket or cables with PVC insulation. When the jacket burns through, the PVC inner sheath or insulation will give off poisonous gases in just the same way as if the LSHF jacket wasn’t there! These cables are potential death traps.
Another common misunderstanding is that LSF or LSHF cable is also flame retardant. This is not necessarily true. The cables may spread the fire even though minimal fumes are being emitted. In mainland Europe, polyurethane is popular as it emits very low levels of smoke and fumes. Unfortunately in its standard form it burns ferociously and can drip burning material onto anything below with the potential to rapidly spread the fire. There has been a recent move towards flame retardant varieties known as FRNC (Flame Retardant Non-Corrosive).
Counting the Costs
As with most developing products, the durability of new safer compounds is improving all the time with cost penalties reducing as the market grows. LSHF compounds are approximately 2 - 3 times more expensive than PVC and many are considerably slower to extrude, with resulting production costs being substantially greater. Combine this with the much smaller demand for LSHF cable and you can begin to appreciate why they cost more.
However as the market grows the prices will fall. Standard products such as conduit wiring 6491 and booklet-armoured cables are now more readily available in Low Smoke Halogen Free versions than PVC in some areas.
Why Confusion Occurs
To add to the confusion, some power cables, in particular BS6724, are LSHF. However, some manufacturers class and even print them as LSF. This also extends to some BS5308 cables.
Great caution is needed when buying or specifying data cables particularly American or European. Belden style data cables are now widely used in buildings for security, access control and building management systems.
Specifying – the Guidelines
First and foremost be sure to get written confirmation that the cable is halogen free, which means both insulation and sheath. Don’t accept terms like LSF as they can be meaningless. Also confirm the availability of the product and take into account the manufacturing times and minimum production quantities should it be a non-stock item.
Contractors are increasingly being asked to complete jobs within a month, when the production time to make the cable may be 6 – 8 weeks or longer!
Appropriately specified, most of the popular types are stocked in Low Smoke Halogen Free alternatives, manufactured to extremely high standards but with none of the risks associated with standard PVC cables. A specialist distributor should be able to provide the product with the appropriate certificate of conformity.
The price of LSHF cables is higher than standard PVC, but as more people are specifying PVC free alternatives the price difference is shrinking.
The ultimate LSHF non-combustible cable with a tough but highly flexible jacket with good ageing characteristics and resistance to water, oil and solvents is still to be developed. However with careful selection the most important factors can normally be catered for.