Fire Performance of Electric Cables

Often the most effective flame retardant cables are halogenated as a outcome of both the insulation and outer Jacket are flame retardant but when we need Halogen Free cables we find it’s usually solely the outer jacket which is flame retardant and the inside insulation is not.
This has significance because whereas cables with a flame retardant outer jacket will typically cross flame retardance tests with exterior flame, the same cables when subjected to excessive overload or extended brief circuits have proved in college exams to be extremely flammable and can even start a fire. This effect is thought and published (8th International Conference on Insulated Power Cables (Jicable’11 – June 2011) held in Versailles, France) so it’s perhaps shocking that there are no frequent check protocols for this seemingly widespread occasion and one cited by both authorities and media as reason for constructing fires.
Further, in Flame Retardant test strategies such as IEC60332 parts 1 & three which make use of an external flame source, the cable samples are not pre-conditioned to normal working temperature however tested at room temperature. This oversight is necessary particularly for energy circuits as a end result of the temperature index of the cable (the temperature at which the cable materials will self-support combustion in normal air) might be significantly affected by its starting temperature i.e.: The hotter the cable is, the extra easily it’ll propagate fireplace.
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 customers alike to supply a dependable indication of a cables ability to retard the propagation of fire.
If we can’t belief the Standards what do we do?
In the USA many building standards do not require halogen free cables. Certainly this isn’t as a result of Americans usually are not wisely knowledgeable of the hazards; quite the method taken is that: “It is better to have extremely flame retardant cables which don’t propagate fireplace than minimally flame retardant cables which can unfold a fire” – (a small fireplace with some halogen could also be higher than a big fireplace without halogens). One of the most effective ways to make a cable insulation and cable jacket extremely flame retardant is by utilizing halogens.
Europe and tons of international locations all over the world adopt a different mentality: Halogen Free and Flame Retardant. เกจวัดแรงดันแก๊สco2 that is an admirable mandate the fact is quite totally different: Flame propagation tests for cables as adopted in UK and Europe can arguably be said to be less stringent than some of the flame propagation tests for cables in USA resulting in the conclusion that frequent exams in UK and Europe may simply be tests the cables can move rather than exams the cables should move.
Conclusion
For most flexible polymeric cables the choice remains at present between high flame propagation performance with halogens or reduced flame propagation efficiency with out halogens.
Enclosing cables in steel conduit will reduce propagation on the level of fireplace but hydrocarbon primarily based combustion gasses from decomposing polymers are doubtless propagate by way of the conduits to switchboards, distribution boards and junction bins in different elements of the building. Any spark such because the opening or closing of circuit breakers, or contactors is likely to ignite the flamable gasses leading to explosion and spreading the fire to a different location.
While MICC (Mineral Insulated Metal Sheathed) cables would provide an answer, there might be often no singe perfect reply for every installation so designers need to judge the required efficiency on a “project-by-project” basis to determine which expertise is optimal.
The primary significance of fire load
Inside all buildings and tasks electrical cables provide the connectivity which keeps lights on, air-conditioning working and the lifts operating. It powers computers, office tools and offers the connection for our phone and computers. Even our cellphones need to attach with wireless or GSM antennas that are related to the telecom community by fiber optic or copper cables. Cables guarantee our security by connecting
fireplace alarms, emergency voice communication, CCTV, smoke shutters, air pressurization followers, emergency lighting, fireplace sprinkler pumps, smoke and heat detectors, and so many other options of a modern Building Management System.
Where public security is important we often request cables to have added safety options such as flame retardance to ensure the cables don’t simply spread fire, circuit integrity during hearth so that essential fire-fighting and life security gear hold working. Sometimes we could recognize that the combustion of electrical cables produces smoke and this might be toxic so we name for cables to be Low Smoke and Halogen Free. Logically and intuitively we think that by requesting these particular properties the cables we buy and install shall be safer
Because cables are installed by many various trades for various functions and are largely hidden or embedded in our constructions, what is often not realized is that the many miles of cables and tons of plastic polymers which make up the cables can represent one of many biggest hearth masses within the constructing. This point is definitely worth thinking 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 supplies aren’t typically flame retardant and naturally have a excessive fireplace load. Cable producers make them flame retardant by including compounds and chemicals. Certainly this improves the volatility of burning but the gasoline content of the base polymers stays.
Tables 1 and a pair of above evaluate the fire load in MJ/Kg for widespread cable insulating materials towards some widespread fuels. The Heat Release Rate and volatility in air for these materials will differ however the gas added to a fireplace per kilogram and the consequential volume of heat generated and oxygen consumed is relative.
The quantity in kilometers and tons of cables installed in our buildings and the related fireplace load of the insulations is considerable. This is particularly necessary in tasks with long egress occasions like excessive rise, public buildings, tunnels and underground environments, airports, hospitals and so on.
When contemplating fireplace safety we should first understand the most important factors. Fire specialists tell us most fireplace associated deaths in buildings are attributable to smoke inhalation, temperature rise and oxygen depletion or by trauma attributable to jumping in making an attempt to flee these results.
Smoke
The first and most important aspect of smoke is how a lot smoke? Typically the larger the fire the more smoke is generated so anything we will do to reduce the spread of fireside will also correspondingly reduce the quantity of smoke.
Smoke will comprise particulates of carbon, ash and different solids, liquids and gasses, many are poisonous and combustible. In explicit, fires in confined areas like buildings, tunnels and underground environments cause oxygen ranges 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 release toxic Halides like Hydrogen Chloride together with many different poisonous and flammable gasses within the smoke.
For this purpose common smoke exams carried out on cable insulation supplies in large 3 meter3 chambers with loads of air can present misleading smoke figures as a end result of full burning will typically launch considerably much less smoke than partial incomplete burning which is likely in practice. Simply specifying IEC 61034 with an outlined obscuration worth then considering this can present a low smoke environment throughout hearth may unfortunately be little of assist for the people actually concerned.
Halogens, Toxicity, Fuel Element, Oxygen Depletion and Temperature Rise
It is concerning that Europe and other international locations undertake the concept of halogen free supplies without correctly addressing the subject of toxicity. Halogens released during combustion are extraordinarily poisonous however so too is carbon monoxide and this is not a halogen fuel. It is common to call for halogen free cables and then enable the use of Polyethylene as a end result of it is halogen free. Burning Polyethylene (which can be seen from the desk above has the highest MJ gas load per Kg of all insulations) will generate nearly 3 occasions more heat than an equal PVC cable. This means is that burning polyethylene will not only generate almost 3 instances extra heat but additionally consume almost three instances more oxygen and produce significantly extra carbon monoxide. Given carbon monoxide is answerable for most toxicity deaths in fires this situation is at best alarming!
The gasoline components shown within the desk above indicate the amount of warmth which might be generated by burning 1kg of the frequent cable insulations tabled. Certainly this warmth will accelerate the burning of different adjacent supplies and should help unfold the hearth in a constructing however importantly, in order to generate the warmth energy, oxygen needs to be consumed. The higher the warmth of combustion the more oxygen is required, so by selecting insulations with excessive gas components is adding considerably to no much less than 4 of the primary risks 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 reduce smoke because inside the conduit oxygen is proscribed; nevertheless this isn’t an answer. As said previously, many of the gasses from the decomposing polymeric insulations inside the conduits are highly flammable and poisonous. These gases will migrate along the conduits to junction boxes, switch panels, distribution boards, motor control facilities, lamps, switches, and so forth. On getting into the gases can ignite or explode with any arcing such because the make/break of a circuit breaker, contactor, swap or relay inflicting the fireplace to spread to a different location.
Conclusion
The recognition of “Halogen Free” whereas ignoring the other poisonous parts of fire is a transparent admission we don’t understand the topic nicely nor can we simply define the dangers of combined toxic elements or human physiological response to them. It is important nevertheless, that we don’t continue to design with solely half an understanding of the problem. While no perfect solution exists for natural based mostly cables, we will certainly decrease these critically necessary results of fire risk:
One possibility possibly to choose cable insulations and jacket supplies that are halogen free and have a low gasoline element, then set up them in metal conduit or possibly the American method is better: to make use of extremely halogenated insulations in order that in case of fireplace any flame unfold is minimized.
For most power, management, communication and information circuits there’s one full answer available for all the issues raised in this paper. It is a solution which has been used reliably for over eighty years. MICC cables can provide a complete and full reply to all the issues related to the hearth security of natural polymer cables.
The copper jacket, magnesium oxide insulation and copper conductors of MICC make certain the cable is effectively hearth proof. MICC cables don’t have any organic content material so merely cannot propagate flame or generate any smoke. The zero gas load ensures no warmth is added and no oxygen is consumed.
Being inorganic MICC cables can not generate any halogen or toxic gasses in any respect including CO.
Unfortunately many widespread cable fireplace take a look at methods used at present may inadvertently mislead people into believing the polymeric flexible cable products they purchase and use will carry out as anticipated in all fire conditions. As outlined on this paper, sadly this is most likely not right.
For extra information, go to www.temperature-house.com
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