Fire Performance of Electric Cables

Often the most effective flame retardant cables are halogenated because both the insulation and outer Jacket are flame retardant however once we need Halogen Free cables we discover it’s typically only the outer jacket which is flame retardant and the internal insulation just isn’t.
This has significance as a end result of whereas cables with a flame retardant outer jacket will typically move flame retardance checks with exterior flame, the same cables when subjected to high overload or extended quick circuits have proved in university tests to be highly flammable and might even begin a hearth. This impact is thought and published (8th International Conference on Insulated Power Cables (Jicable’11 – June 2011) held in Versailles, France) so it is perhaps stunning that there are no widespread test protocols for this seemingly common occasion and one cited by both authorities and media as cause of constructing fires.
Further, in Flame Retardant test strategies corresponding to IEC60332 parts 1 & 3 which employ an external flame supply, the cable samples usually are not pre-conditioned to normal working temperature but examined at room temperature. This oversight is important especially for energy circuits because the temperature index of the cable (the temperature at which the cable material will self-support combustion in regular air) shall be considerably affected by its beginning temperature i.e.: The hotter the cable is, the more easily it’s going to propagate fire.
It would appear that a want exists to re-evaluate current cable flame retardance take a look at methods as these are generally understood by consultants and consumers alike to offer a reliable indication of a cables capacity to retard the propagation of fire.
If we can’t trust the Standards what can we do?
In the USA many constructing standards don’t require halogen free cables. Certainly this is not as a result of Americans are not correctly knowledgeable of the risks; rather the method taken is that: “It is best to have highly flame retardant cables which don’t propagate hearth than minimally flame retardant cables which may unfold a fire” – (a small fire with some halogen may be better than a large fireplace with out halogens). One of the best methods to make a cable insulation and cable jacket highly flame retardant is by utilizing halogens.
Europe and many nations around the world adopt a different mentality: Halogen Free and Flame Retardant. Whilst that is an admirable mandate the reality is somewhat totally different: Flame propagation exams for cables as adopted in UK and Europe can arguably be stated to be much less stringent than a few of the flame propagation exams for cables in USA leading to the conclusion that frequent tests in UK and Europe might merely be exams the cables can move quite than exams the cables ought to cross.
Conclusion
For most flexible polymeric cables the choice remains today between excessive flame propagation performance with halogens or decreased flame propagation efficiency without halogens.
Enclosing cables in metal conduit will cut back propagation at the point of fireside however hydrocarbon primarily based combustion gasses from decomposing polymers are doubtless propagate through the conduits to switchboards, distribution boards and junction bins in different elements of the constructing. Any spark such because the opening or closing of circuit breakers, or contactors is prone to ignite the flamable gasses resulting in explosion and spreading the hearth to a different location.
While MICC (Mineral Insulated Metal Sheathed) cables would provide an answer, there is often no singe excellent reply for each installation so designers want to evaluate the required efficiency on a “project-by-project” foundation to determine which technology is perfect.
The primary importance of fireside load
Inside all buildings and tasks electric cables provide the connectivity which retains lights on, air-conditioning working and the lifts running. It powers computer systems, workplace gear and provides the connection for our telephone and computer systems. Even our cellphones want to connect with wi-fi or GSM antennas that are linked to the telecom network by fiber optic or copper cables. Cables ensure our security by connecting
fire alarms, emergency voice communication, CCTV, smoke shutters, air pressurization fans, emergency lighting, fireplace sprinkler pumps, smoke and warmth detectors, and so many different options of a contemporary Building Management System.
Where public security is necessary we frequently request cables to have added security features such as flame retardance to make sure the cables do not simply unfold hearth, circuit integrity throughout fire in order that important fire-fighting and life safety equipment hold working. Sometimes we could recognize that the combustion of electric cables produces smoke and this could be poisonous so we name for cables to be Low Smoke and Halogen Free. Logically and intuitively we think that by requesting these special properties the cables we purchase and set up shall be safer
Because cables are installed by many different trades for various purposes and are largely hidden or embedded in our constructions, what is often not realized is that the numerous miles of cables and tons of plastic polymers which make up the cables can characterize one of many greatest fire loads within the constructing. This level is actually price thinking more about.
PVC, XLPE, EPR, CSP, LSOH (Low Smoke Zero Halogen) and even HFFR (Halogen Free Flame Retardant) cable supplies are mostly based mostly on hydrocarbon polymers. These base materials usually are not usually flame retardant and naturally have a high fire load. Cable producers make them flame retardant by adding compounds and chemicals. Certainly this improves the volatility of burning however the gas content of the base polymers remains.
Tables 1 and a couple of above compare the fireplace load in MJ/Kg for common cable insulating materials in opposition to some common fuels. The Heat Release Rate and volatility in air for these materials will differ but the fuel 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 related fire load of the insulations is appreciable. This is especially essential in initiatives with long egress times like excessive rise, public buildings, tunnels and underground environments, airports, hospitals and so forth.
When considering fireplace safety we should first perceive crucial elements. Fire consultants inform us most fireplace associated deaths in buildings are caused by smoke inhalation, temperature rise and oxygen depletion or by trauma caused by leaping in attempting to escape these effects.
Smoke
The first and most necessary facet of smoke is how a lot smoke? Typically the larger the fire the more smoke is generated so anything we can do to cut back the unfold of fireplace may even correspondingly reduce the quantity of smoke.
Smoke will comprise particulates of carbon, ash and different solids, liquids and gasses, many are toxic and flamable. In explicit, fires in confined areas like buildings, tunnels and underground environments cause oxygen levels to drop, this contributes to incomplete burning and smoldering which produces increased quantities of smoke and toxic byproducts together with CO and CO2. Presence of halogenated supplies will launch toxic Halides like Hydrogen Chloride together with many other poisonous and flammable gasses in the smoke.
For this cause frequent smoke tests performed on cable insulation supplies in giant 3 meter3 chambers with plenty of air can provide misleading smoke figures as a end result of complete burning will usually launch significantly less smoke than partial incomplete burning which is likely in practice. Simply specifying IEC 61034 with an outlined obscuration worth then considering this will provide a low smoke setting throughout hearth may sadly be little of help for the individuals truly concerned.
Halogens, Toxicity, Fuel Element, Oxygen Depletion and Temperature Rise
It is regarding that Europe and other nations adopt the idea of halogen free supplies with out properly addressing the subject of toxicity. Halogens launched throughout combustion are extraordinarily toxic but so too is carbon monoxide and this isn’t a halogen gas. It is common to name for halogen free cables and then permit using Polyethylene because it is halogen free. Burning Polyethylene (which may be seen from the table above has the very best MJ fuel load per Kg of all insulations) will generate almost three occasions more heat than an equal PVC cable. เครื่องมือที่ใช้ในการวัดความดัน is that burning polyethylene won’t only generate nearly 3 occasions more warmth but in addition consume virtually three occasions extra oxygen and produce significantly more carbon monoxide. Given carbon monoxide is responsible for most toxicity deaths in fires this case is at finest alarming!
The gas parts shown within the desk above point out the amount of heat which will be generated by burning 1kg of the widespread cable insulations tabled. Certainly this warmth will speed up the burning of different adjoining materials and may assist spread the fire in a building however importantly, so as to generate the heat power, oxygen must be consumed. The larger the heat of combustion the extra oxygen is required, so by choosing insulations with high fuel parts is including significantly to a minimal of 4 of the first dangers of fires: Temperature Rise, Oxygen Depletion, Flame Spread and Carbon Monoxide Release.
Perhaps it’s best to put in polymeric cables inside steel conduits. This will definitely assist flame spread and minimize smoke as a result of inside the conduit oxygen is restricted; nonetheless this isn’t a solution. As stated beforehand, many of the gasses from the decomposing polymeric insulations inside the conduits are highly flammable and poisonous. These gases will migrate alongside the conduits to junction bins, swap panels, distribution boards, motor management facilities, lamps, switches, and so forth. On getting into the gases can ignite or explode with any arcing such as the make/break of a circuit breaker, contactor, swap or relay causing the hearth to spread to a different location.
Conclusion
The reputation of “Halogen Free” whereas ignoring the other toxic parts of fireside is a clear admission we don’t perceive the topic well nor can we simply outline the dangers of combined poisonous components or human physiological response to them. It is necessary nonetheless, that we do not proceed to design with only half an understanding of the issue. While no excellent solution exists for natural based mostly cables, we can certainly decrease these critically essential effects of fireplace risk:
One option perhaps to choose on cable insulations and jacket supplies that are halogen free and have a low fuel factor, then set up them in steel conduit or maybe the American approach is better: to make use of extremely halogenated insulations so that in case of fireside any flame spread is minimized.
For most energy, management, communication and knowledge circuits there may be one full solution available for all the issues raised in this paper. It is a solution which has been used reliably for over 80 years. MICC cables can provide a complete and complete answer to all the issues related to the fire safety of natural polymer cables.
The copper jacket, magnesium oxide insulation and copper conductors of MICC ensure the cable is effectively hearth proof. MICC cables haven’t any natural content so simply cannot propagate flame or generate any smoke. The zero fuel load ensures no warmth is added and no oxygen is consumed.
Being inorganic MICC cables can’t generate any halogen or toxic gasses at all including CO.
Unfortunately many common cable hearth take a look at strategies used today might inadvertently mislead individuals into believing the polymeric flexible cable products they buy and use will carry out as expected in all fire situations. As outlined in this paper, sadly this may not be correct.
For extra info, go to www.temperature-house.com
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