Fire Performance of Electric Cables

Often the best flame retardant cables are halogenated as a end result of both the insulation and outer Jacket are flame retardant but after we want Halogen Free cables we find it is usually solely the outer jacket which is flame retardant and the internal insulation is not.
This has significance because whereas cables with a flame retardant outer jacket will typically cross flame retardance tests with external flame, the same cables when subjected to excessive overload or prolonged quick circuits have proved in college exams to be extremely flammable and can even begin a hearth. This effect is understood and revealed (8th International Conference on Insulated Power Cables (Jicable’11 – June 2011) held in Versailles, France) so it’s maybe stunning that there are no frequent test protocols for this seemingly frequent event and one cited by each authorities and media as explanation for building fires.
Further, in Flame Retardant take a look at strategies corresponding to IEC60332 components 1 & three which make use of an external flame supply, the cable samples usually are not pre-conditioned to regular operating temperature however examined at room temperature. This oversight is essential particularly for energy circuits as a end result of the temperature index of the cable (the temperature at which the cable material will self-support combustion in regular air) will be considerably affected by its beginning temperature i.e.: The hotter the cable is, the more simply it will propagate fireplace.
It would seem that a want exists to re-evaluate current cable flame retardance take a look at methods as these are commonly understood by consultants and consumers alike to offer a dependable indication of a cables capacity to retard the propagation of fire.
If we can’t trust the Standards what do we do?
In the USA many building standards don’t require halogen free cables. Certainly this is not as a end result of Americans are not correctly knowledgeable of the dangers; rather the approach taken is that: “It is best to have extremely flame retardant cables which do not propagate fire than minimally flame retardant cables which may spread a fire” – (a small fire with some halogen could additionally be higher than a large hearth with out halogens). One of the most effective methods to make a cable insulation and cable jacket extremely flame retardant is by using halogens.
Europe and many international locations around the globe undertake a special mentality: Halogen Free and Flame Retardant. Whilst this is an admirable mandate the fact is rather totally different: Flame propagation checks for cables as adopted in UK and Europe can arguably be stated to be much less stringent than some of the flame propagation tests for cables in USA leading to the conclusion that frequent exams in UK and Europe may simply be exams the cables can pass quite than exams the cables ought to move.
For most flexible polymeric cables the choice remains today between excessive flame propagation efficiency with halogens or lowered flame propagation efficiency without halogens.
Enclosing cables in metal conduit will cut back propagation at the level of fire however hydrocarbon primarily based combustion gasses from decomposing polymers are doubtless propagate via the conduits to switchboards, distribution boards and junction packing containers 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 resulting in explosion and spreading the fireplace to another location.
While MICC (Mineral Insulated Metal Sheathed) cables would supply an answer, there could be usually no singe excellent reply for each installation so designers need to gauge the required performance on a “project-by-project” basis to resolve which know-how is perfect.
The primary importance of fire load
Inside all buildings and initiatives 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 mobile phones want to attach 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
hearth alarms, emergency voice communication, CCTV, smoke shutters, air pressurization fans, emergency lighting, fireplace sprinkler pumps, smoke and warmth detectors, and so many other features of a modern Building Management System.
Where public safety is essential we frequently request cables to have added safety features similar to flame retardance to ensure the cables don’t simply spread fireplace, circuit integrity during fire so that important fire-fighting and life safety gear keep working. Sometimes we may recognize that the combustion of electrical cables produces smoke and this can be poisonous so we name for cables to be Low Smoke and Halogen Free. Logically and intuitively we expect that by requesting these special properties the cables we buy and install will be safer
Because cables are installed by many various 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 represent one of the greatest hearth hundreds in the constructing. This point is certainly value pondering extra about.
PVC, XLPE, EPR, CSP, LSOH (Low Smoke Zero Halogen) and even HFFR (Halogen Free Flame Retardant) cable supplies are mostly primarily based on hydrocarbon polymers. These base supplies are not generally flame retardant and naturally have a high fire load. Cable manufacturers make them flame retardant by adding compounds and chemical substances. Certainly เกจแรงดัน improves the volatility of burning however the gas content material of the base polymers stays.
Tables 1 and 2 above examine the hearth load in MJ/Kg for frequent cable insulating supplies against some common fuels. The Heat Release Rate and volatility in air for these materials will differ but the fuel added to a fireplace per kilogram and the consequential volume of warmth generated and oxygen consumed is relative.
The volume in kilometers and tons of cables put in in our buildings and the related fire load of the insulations is appreciable. This is particularly necessary in initiatives with long egress occasions like excessive rise, public buildings, tunnels and underground environments, airports, hospitals and so on.
When contemplating fire security we should first perceive an important elements. Fire consultants tell us most fire associated deaths in buildings are brought on by smoke inhalation, temperature rise and oxygen depletion or by trauma caused by leaping in making an attempt to flee these effects.
The first and most important aspect of smoke is how much smoke? Typically the bigger the hearth the extra smoke is generated so anything we are ready to do to reduce the spread of fireside will also correspondingly reduce the quantity of smoke.
Smoke will contain particulates of carbon, ash and other solids, liquids and gasses, many are poisonous 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 amounts of smoke and toxic byproducts including CO and CO2. Presence of halogenated supplies will release toxic Halides like Hydrogen Chloride together with many other poisonous and flammable gasses within the smoke.
For this purpose widespread smoke exams performed on cable insulation materials in giant 3 meter3 chambers with loads of air can present misleading smoke figures as a result of full burning will typically launch significantly much less smoke than partial incomplete burning which is likely in apply. Simply specifying IEC 61034 with a defined obscuration worth then thinking this will provide a low smoke surroundings throughout hearth might unfortunately be little of help for the people actually involved.
Halogens, Toxicity, Fuel Element, Oxygen Depletion and Temperature Rise
It is concerning that Europe and different countries adopt the concept of halogen free materials without properly addressing the topic of toxicity. Halogens released throughout combustion are extremely toxic but so too is carbon monoxide and this isn’t a halogen gas. It is common to name for halogen free cables after which allow using Polyethylene because 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 almost three times extra warmth than an equal PVC cable. This means is that burning polyethylene will not solely generate nearly three occasions more heat but also consume almost 3 instances extra oxygen and produce significantly more carbon monoxide. Given carbon monoxide is liable for most toxicity deaths in fires this case is at best alarming!
The fuel parts proven in the table above point out the quantity of warmth which will be generated by burning 1kg of the common cable insulations tabled. Certainly this heat will speed up the burning of other adjacent materials and should assist unfold the fire in a constructing but importantly, in order to generate the heat power, oxygen needs to be consumed. The larger the heat of combustion the extra oxygen is required, so by selecting insulations with excessive gas elements is including considerably to no less than 4 of the first dangers 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 spread and reduce smoke as a result of contained in the conduit oxygen is limited; however this isn’t a solution. As stated beforehand, pressure gauge แบบ น้ำมัน of the gasses from the decomposing polymeric insulations contained in the conduits are highly flammable and toxic. These gases will migrate along the conduits to junction boxes, switch panels, distribution boards, motor management centers, lamps, switches, and so forth. On coming into the gases can ignite or explode with any arcing such because the make/break of a circuit breaker, contactor, switch or relay inflicting the fireplace to spread to another location.
The recognition of “Halogen Free” while ignoring the opposite toxic components of fireplace is a transparent admission we don’t perceive the topic well nor can we simply outline the risks of mixed poisonous elements or human physiological response to them. It is necessary nonetheless, that we do not continue to design with solely half an understanding of the problem. While no excellent answer exists for organic based cables, we are ready to actually reduce these critically necessary results of fireplace threat:
One choice maybe to decide on cable insulations and jacket supplies which are halogen free and have a low gas element, then set up them in steel conduit or possibly the American method is healthier: to use extremely halogenated insulations so that in case of fire any flame spread is minimized.
For most energy, management, communication and data circuits there’s one full resolution out there for all the issues raised on this paper. It is a solution which has been used reliably for over eighty years. MICC cables can provide a total and complete answer to all the issues associated with the fireplace safety of organic polymer cables.
The copper jacket, magnesium oxide insulation and copper conductors of MICC ensure the cable is successfully fireplace proof. MICC cables haven’t any organic content material so simply can’t 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’t generate any halogen or poisonous gasses at all including CO.
Unfortunately many frequent cable fire check strategies used at present could inadvertently mislead folks into believing the polymeric flexible cable merchandise they purchase and use will carry out as anticipated in all fire conditions. As outlined in this paper, sadly this may not be right.
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