Fire Performance of Electric Cables

Often the most effective flame retardant cables are halogenated as a end result of both the insulation and outer Jacket are flame retardant but after we need Halogen Free cables we find it is usually only the outer jacket which is flame retardant and the inner insulation is not.
This has significance as a end result of whereas cables with a flame retardant outer jacket will usually pass flame retardance exams with exterior flame, the same cables when subjected to excessive overload or prolonged short circuits have proved in university tests to be extremely flammable and can even start a fire. This effect is understood and revealed (8th International Conference on Insulated Power Cables (Jicable’11 – June 2011) held in Versailles, France) so it is maybe stunning that there aren’t any frequent check protocols for this seemingly widespread occasion and one cited by each authorities and media as explanation for building fires.
Further, in Flame Retardant take a look at strategies such as IEC60332 components 1 & three which make use of an external flame supply, the cable samples usually are not pre-conditioned to normal working temperature however examined at room temperature. This oversight is essential especially for energy circuits as a result of the temperature index of the cable (the temperature at which the cable materials will self-support combustion in regular air) will be significantly affected by its beginning temperature i.e.: The hotter the cable is, the more easily it will propagate fire.
It would seem that a need exists to re-evaluate present cable flame retardance check strategies as these are generally understood by consultants and customers alike to supply a dependable indication of a cables capacity to retard the propagation of fireplace.
If we can’t trust the Standards what can we do?
In the USA many constructing standards do not require halogen free cables. Certainly this is not because Americans are not wisely knowledgeable of the risks; quite the approach taken is that: “It is better to have highly flame retardant cables which don’t propagate fire than minimally flame retardant cables which can spread a fire” – (a small hearth with some halogen may be higher than a big hearth without halogens). One of one of the best ways to make a cable insulation and cable jacket highly flame retardant is through the use of halogens.
Europe and lots of countries around the world undertake a different mentality: Halogen Free and Flame Retardant. Whilst that is an admirable mandate the fact is quite different: Flame propagation checks for cables as adopted in UK and Europe can arguably be mentioned to be much less stringent than a few of the flame propagation exams for cables in USA resulting in the conclusion that frequent checks in UK and Europe may merely be exams the cables can pass somewhat than checks the cables should cross.
Conclusion
For most versatile polymeric cables the selection stays right now between excessive flame propagation performance with halogens or decreased flame propagation performance with out halogens.
Enclosing cables in steel conduit will reduce propagation at the point of fireplace but hydrocarbon based mostly combustion gasses from decomposing polymers are likely propagate by way of the conduits to switchboards, distribution boards and junction bins in other parts of the building. Any spark such because the opening or closing of circuit breakers, or contactors is more probably to ignite the combustible gasses resulting in explosion and spreading the hearth to a different location.
While MICC (Mineral Insulated Metal Sheathed) cables would supply a solution, there is often no singe excellent answer for every installation so designers need to judge the required efficiency on a “project-by-project” basis to determine which know-how is perfect.
The major importance of fire load
Inside all buildings and projects electric cables provide the connectivity which keeps lights on, air-conditioning working and the lifts working. It powers computers, office tools and offers the connection for our phone and computer systems. Even our cell phones need to attach with wi-fi or GSM antennas that are connected to the telecom community by fiber optic or copper cables. Cables guarantee our safety by connecting
fireplace alarms, emergency voice communication, CCTV, smoke shutters, air pressurization fans, emergency lighting, fire sprinkler pumps, smoke and warmth detectors, and so many other features of a modern Building Management System.
Where public safety is important we regularly request cables to have added security options corresponding to flame retardance to make sure the cables do not simply spread hearth, circuit integrity throughout fire in order that important fire-fighting and life safety gear maintain working. Sometimes we could recognize that the combustion of electrical cables produces smoke and this could be poisonous so we name for cables to be Low Smoke and Halogen Free. Logically and intuitively we predict that by requesting these particular properties the cables we purchase and set up shall be safer
Because cables are installed by many different trades for various functions and are principally hidden or embedded in our constructions, what is usually not realized is that the various miles of cables and tons of plastic polymers which make up the cables can represent one of many largest fireplace loads within the constructing. This level is actually value considering more about.
PVC, XLPE, EPR, CSP, LSOH (Low Smoke Zero Halogen) and even HFFR (Halogen Free Flame Retardant) cable supplies are largely based on hydrocarbon polymers. These base materials usually are not typically flame retardant and naturally have a high fire load. Cable producers make them flame retardant by including compounds and chemical substances. Certainly this improves the volatility of burning but the gasoline content material of the bottom polymers stays.
Tables 1 and a couple of above examine the fire load in MJ/Kg for common cable insulating supplies in opposition to some widespread fuels. The Heat Release Rate and volatility in air for these materials will differ however the gas added to a hearth per kilogram and the consequential volume of warmth generated and oxygen consumed is relative.
The volume in kilometers and tons of cables installed in our buildings and the associated fire load of the insulations is appreciable. This is particularly important in initiatives with long egress occasions like high rise, public buildings, tunnels and underground environments, airports, hospitals etc.
When contemplating hearth safety we must first perceive an important factors. Fire consultants tell us most fireplace related deaths in buildings are caused by smoke inhalation, temperature rise and oxygen depletion or by trauma caused by jumping in trying to escape these effects.
Smoke
The first and most important aspect of smoke is how a lot smoke? Typically the bigger the fire the extra smoke is generated so something we can do to reduce the unfold of fireside may even correspondingly scale back the amount of smoke.
Smoke will comprise particulates of carbon, ash and other solids, liquids and gasses, many are toxic and flamable. In particular, fires in confined areas like buildings, tunnels and underground environments cause oxygen levels to drop, this contributes to incomplete burning and smoldering which produces elevated quantities of smoke and poisonous byproducts including CO and CO2. Presence of halogenated materials will launch poisonous Halides like Hydrogen Chloride together with many other poisonous and flammable gasses within the smoke.
For this reason widespread smoke exams performed on cable insulation materials in massive 3 meter3 chambers with plenty of air can provide misleading smoke figures because complete burning will typically launch considerably much less smoke than partial incomplete burning which is likely in apply. Simply specifying IEC 61034 with an outlined obscuration value then thinking this will present a low smoke surroundings throughout fire might sadly be little of assist for the individuals truly concerned.
Halogens, Toxicity, Fuel Element, Oxygen Depletion and Temperature Rise
It is regarding that Europe and other nations undertake the concept of halogen free materials without properly addressing the subject of toxicity. Halogens launched throughout combustion are extremely toxic however so too is carbon monoxide and this is not a halogen gasoline. It is common to name for halogen free cables and then allow the use of Polyethylene as a result of it is halogen free. Burning Polyethylene (which can be seen from the table above has the very best MJ gas load per Kg of all insulations) will generate virtually three occasions extra heat than an equal PVC cable. This means is that burning polyethylene will not solely generate nearly three instances extra heat but in addition devour nearly three times more oxygen and produce considerably more carbon monoxide. Given carbon monoxide is responsible for most toxicity deaths in fires this situation is at greatest alarming!
The fuel elements proven in the desk above indicate the amount of heat which shall be generated by burning 1kg of the common cable insulations tabled. Certainly this warmth will speed up the burning of different adjacent materials and will assist spread the hearth in a building however importantly, in order to generate the warmth energy, oxygen needs to be consumed. The higher the warmth of combustion the extra oxygen is needed, so by choosing insulations with excessive gasoline components is adding significantly to a minimal of 4 of the primary risks of fires: Temperature Rise, Oxygen Depletion, Flame Spread and Carbon Monoxide Release.
Perhaps it is best to install polymeric cables inside metal conduits. This will certainly help flame unfold and minimize smoke because contained in the conduit oxygen is proscribed; nevertheless this is not a solution. As mentioned previously, many of the gasses from the decomposing polymeric insulations inside the conduits are highly flammable and toxic. These gases will migrate along the conduits to junction packing containers, swap panels, distribution boards, motor control centers, lamps, switches, and so forth. On entering the gases can ignite or explode with any arcing such as the make/break of a circuit breaker, contactor, swap or relay inflicting the fireplace to unfold to another location.
Conclusion
The recognition of “Halogen Free” whereas ignoring the opposite poisonous components of fireside is a transparent admission we don’t understand the subject well nor can we easily define the hazards of combined toxic elements or human physiological response to them. เกจวัดแรง is important nevertheless, that we don’t continue to design with only half an understanding of the problem. While no good answer exists for natural based cables, we can actually reduce these critically necessary results of fireside threat:
One option maybe to choose on cable insulations and jacket materials that are halogen free and have a low gasoline element, then set up them in metal conduit or perhaps the American approach is best: to use extremely halogenated insulations so that in case of fireside any flame spread is minimized.
For most energy, management, communication and information circuits there is one full solution obtainable for all the problems raised in this paper. It is a solution which has been used reliably for over eighty years. MICC cables can present a complete and complete answer to all the problems related to the fire security of natural polymer cables.
The copper jacket, magnesium oxide insulation and copper conductors of MICC make sure the cable is successfully fire proof. MICC cables haven’t any natural content material so simply can not propagate flame or generate any smoke. The zero gas load ensures no warmth is added and no oxygen is consumed.
Being inorganic MICC cables cannot generate any halogen or toxic gasses at all including CO.
Unfortunately many common cable fireplace check strategies used today could inadvertently mislead folks into believing the polymeric flexible cable products they buy and use will carry out as anticipated in all fireplace situations. As outlined on this paper, sadly this is most likely not right.
For extra info, go to www.temperature-house.com
Share

Leave a Comment