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PVC: Problems and perils

 

When standard PVC burns it gives off a number of poisonous gases, in particular hydrogen chloride (HCI), which when combined with moisture forms hydrochloric acid.  The damage caused by burning PVC is two fold; firstly dense smoke will obscure exit routes with the fumes chocking people.  The second and less recognised problem, is that the acid gas produced permeates electronic equipment setting on printed circuit boards and over a period of days, weeks months or even years, eats through circuits causing random, unpredictable failure.  

 

This won't just affect computers, it will also damage security/access control, building management systems, lifts and just about anything else with a printed circuit board.  The fire may even have been extinguised within munites with no great risk to personnel, but the damage to equipment may be colossal.  

 

Reducing the risk - the options

 

There has been a shift in recent years to using newly developed compund 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 Propogation) or in some cases LSF (Low Smoke and Fume).

 

The difficulty for the cable buyer is that there is no specific standard for LSF cables. Ordinary PVC emits approximately 28% HCI, whilst a modified PVC could give off a massive 22% HCI and still be sold as LSF.  

 

If you want to be absolutely certain of what you are installing insist on a cable that uses less insulation and sheathing materialsthat do not emit any significant amounts of halogens and have reduced smoke emission properties.   These are termed LSHF (Low Smoke Halogen Free) LSOH, or LSZH (Low Smoke Zero Halogen).  

 

These products must emit no more than 0.5% HCI.  Also don't accept standard PVC cables over-sheathed with a LSHF jacket or cables with PVC insulation.  When the jacket burns through, the PVC inner sheath or insulation will give of 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 emitted.  In mainland Europe polyurethene is popular as it emits bery low levels of smoke and fumes.  Unfortunately in it's standard form itt burns ferociously and drips burning material onto anything below with the potential to rapidly spread fire.  There has been a recent move to flame retardant types known as FRNC (Flame Retardant Non Corrosive).  

 

Counting the costs

 

As with most developing products the durability of the new safer compunds is improving all the time with the cost penalties reducing as the market grows.  LSHF compunds are significantly more expensive than PVC and are considerably slower to extrude, with resulting production costs being substantially greater.  Combine this with smaller demand for LSHF cable and you begin to appreciate why they cost more.  

 

As the market grows the prices will fall.  Standard products are now more readily available in Low Smoke Zero Halogen Free versions than PVC.  

 

Why confusion occours

 

To add to the confusion some power cables in particular BS6724 are Low Smoke and Halogen Free, however some manufacturers class and even print them as LSF.  This also extends to some BS5308 cables.  

 

The greatest caution is needed when buying or specifying data cables.  In particular American or European data cables.  Belden styule data cablesare now widely used in buildings for security, access control and building management systems.  

 

Specifying - the guideleins

 

Fist and most improtant get written confirmation the cable is halogen free, that means both insulation and sheath, don't accept terms like LSF as they can be meaningless.  Also confirm the availability of the product and talke into account the manufacturing times and minimum production quanities 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.

 

Appropriateley specified, most of the popular types of data & control cables are stocked in Low Smoke Halogen Free alternatives.  

 

The ultimate LSHF non-combustable 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 importantfactors can normally be catered for.  

 

We always offer our clients the choice of Low Smoke Cabling and often incorporate it into our proposals as standard.    We work with FS Cables for supply and technical advice.  Most cables in their range have an LSHF option.  Feel free to check them out at www.fscables.com

 

 

 

With the increasing demand for 'safe' building, 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 far greater risk to life and property than fire istelf.  

 

The main problem facing specifiers & installers is the confusion over terminology and standards.  Is LSHF better than LSOH? Or is LSF the same as LSZH or RP? And how does FRNC compare with OHLS?  These are all terms widely used within the cable industry.