Fire Performance of Electric Cables

Often one of the best flame retardant cables are halogenated because each the insulation and outer Jacket are flame retardant but when we want Halogen Free cables we discover it’s often only the outer jacket which is flame retardant and the inner insulation isn’t.
This has significance because while cables with a flame retardant outer jacket will usually move flame retardance exams with external flame, the identical cables when subjected to excessive overload or extended quick circuits have proved in college exams to be extremely flammable and may even start a fire. This effect is known and revealed (8th International Conference on Insulated Power Cables (Jicable’11 – June 2011) held in Versailles, France) so it’s perhaps stunning that there are no frequent check protocols for this seemingly frequent occasion and one cited by each authorities and media as reason for building fires.
Further, in Flame Retardant check strategies similar to IEC60332 parts 1 & 3 which make use of an exterior flame supply, the cable samples are not pre-conditioned to normal operating temperature but examined at room temperature. This oversight is important especially for power circuits as a outcome of the temperature index of the cable (the temperature at which the cable materials will self-support combustion in regular air) shall be considerably affected by its beginning temperature i.e.: The hotter the cable is, the extra easily it’s going to propagate fire.
It would appear that a need exists to re-evaluate present cable flame retardance take a look at methods as these are generally understood by consultants and customers alike to offer a reliable indication of a cables capacity to retard the propagation of fireside.
If we can’t belief the Standards what do we do?
In the USA many constructing requirements don’t require halogen free cables. Certainly this is not as a result of Americans aren’t wisely knowledgeable of the hazards; somewhat the strategy taken is that: “It is better to have extremely flame retardant cables which do not propagate fireplace than minimally flame retardant cables which can spread a fire” – (a small fire with some halogen could also be better than a big fire without halogens). One of the best methods to make a cable insulation and cable jacket highly flame retardant is by using halogens.
Europe and lots of countries around the globe adopt a special 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 stated to be less stringent than some of the flame propagation checks for cables in USA resulting in the conclusion that common checks in UK and Europe could simply be exams the cables can cross somewhat than checks the cables should move.
For most versatile polymeric cables the choice remains today between excessive flame propagation efficiency with halogens or decreased flame propagation efficiency with out halogens.
Enclosing cables in steel conduit will cut back propagation at the point of fire however hydrocarbon based mostly combustion gasses from decomposing polymers are likely propagate through the conduits to switchboards, distribution boards and junction bins in different parts of the constructing. 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 provide an answer, there’s usually no singe perfect answer for every installation so designers need to judge the required efficiency on a “project-by-project” basis to resolve which expertise is perfect.
The major importance of fireside load
Inside all buildings and projects electric cables provide the connectivity which keeps lights on, air-conditioning working and the lifts operating. It powers computer systems, workplace gear and offers the connection for our telephone and computer systems. Even our cell phones want to attach with wi-fi or GSM antennas which are connected to the telecom network by fiber optic or copper cables. Cables guarantee our safety by connecting
fireplace alarms, emergency voice communication, CCTV, smoke shutters, air pressurization followers, emergency lighting, hearth sprinkler pumps, smoke and heat detectors, and so many different features of a modern Building Management System.
Where public safety is essential we often request cables to have added security features similar to flame retardance to ensure the cables don’t simply spread fireplace, circuit integrity throughout fireplace so that important fire-fighting and life safety equipment maintain working. Sometimes we might recognize that the combustion of electric cables produces smoke and this can be toxic 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 buy and install might be safer
Because cables are installed by many different trades for various applications and are mostly 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 symbolize one of many biggest hearth hundreds within the constructing. This point is definitely price thinking extra about.
PVC, XLPE, EPR, CSP, LSOH (Low Smoke Zero Halogen) and even HFFR (Halogen Free Flame Retardant) cable materials are largely based on hydrocarbon polymers. These base materials are not generally flame retardant and naturally have a excessive fire load. Cable producers make them flame retardant by including compounds and chemicals. Certainly this improves the volatility of burning but the gas content material of the base polymers remains.
Tables 1 and a pair of above compare the fireplace load in MJ/Kg for frequent cable insulating supplies towards some widespread fuels. The Heat Release Rate and volatility in air for these materials will differ but the gasoline added to a hearth per kilogram and the consequential volume of heat generated and oxygen consumed is relative.
The volume in kilometers and tons of cables installed in our buildings and the associated fireplace load of the insulations is considerable. This is especially essential in tasks with lengthy egress occasions like excessive rise, public buildings, tunnels and underground environments, airports, hospitals etc.
When considering fireplace security we should first perceive an important elements. Fire specialists tell us most fire related deaths in buildings are caused by smoke inhalation, temperature rise and oxygen depletion or by trauma caused by leaping in trying to flee these effects.
The first and most necessary facet of smoke is how a lot smoke? Typically the bigger the fire the extra smoke is generated so anything we will do to scale back the spread 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 specific, fires in confined areas like buildings, tunnels and underground environments cause oxygen ranges to drop, this contributes to incomplete burning and smoldering which produces elevated quantities of smoke and poisonous byproducts together with CO and CO2. Presence of halogenated supplies will launch poisonous Halides like Hydrogen Chloride together with many different poisonous and flammable gasses in the smoke.
For this reason frequent smoke tests performed on cable insulation supplies in large 3 meter3 chambers with loads of air can present misleading smoke figures because complete burning will usually launch significantly less smoke than partial incomplete burning which is most likely going in follow. Simply specifying pressure gauge 4 นิ้ว with a defined obscuration value then thinking this can provide a low smoke environment throughout hearth may unfortunately be little of help for the folks actually concerned.
Halogens, Toxicity, Fuel Element, Oxygen Depletion and Temperature Rise
It is concerning that Europe and other countries adopt the concept of halogen free supplies with out correctly addressing the subject of toxicity. Halogens released throughout combustion are extraordinarily toxic but so too is carbon monoxide and this is not a halogen gasoline. It is widespread to call for halogen free cables after which permit the use of Polyethylene because it is halogen free. Burning Polyethylene (which can be seen from the desk above has the best MJ gasoline load per Kg of all insulations) will generate nearly 3 times extra heat than an equal PVC cable. This means is that burning polyethylene is not going to only generate nearly 3 occasions more warmth but also consume nearly 3 instances more oxygen and produce significantly more carbon monoxide. Given carbon monoxide is responsible for most toxicity deaths in fires this example is at finest alarming!
The gas parts proven in the desk above indicate the amount of warmth which will be generated by burning 1kg of the common cable insulations tabled. Certainly this warmth will speed up the burning of different adjacent supplies and should help spread the hearth in a building but importantly, to find a way to generate the warmth power, oxygen must be consumed. The higher the heat of combustion the extra oxygen is required, so by choosing insulations with high gasoline elements is adding considerably to no much less than 4 of the first dangers of fires: Temperature Rise, Oxygen Depletion, Flame Spread and Carbon Monoxide Release.
Perhaps it’s best to install polymeric cables inside metal conduits. This will definitely help flame spread and minimize smoke as a result of inside the conduit oxygen is proscribed; nevertheless this is not an answer. As stated previously, lots 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 facilities, lamps, switches, and so on. On entering the gases can ignite or explode with any arcing such because the make/break of a circuit breaker, contactor, swap or relay causing the fireplace to unfold to a different location.
The recognition of “Halogen Free” while ignoring the other toxic components of fireplace is a clear admission we don’t understand the subject well nor can we easily define the hazards of combined poisonous elements or human physiological response to them. It is necessary however, that we do not continue to design with solely half an understanding of the issue. While no good solution exists for organic based mostly cables, we will actually reduce these critically important results of fireplace risk:
One option maybe to choose cable insulations and jacket materials which are halogen free and have a low gasoline component, then set up them in metal conduit or maybe the American strategy is healthier: to use extremely halogenated insulations in order that in case of fireplace any flame spread is minimized.
For most power, management, communication and knowledge circuits there’s one full solution obtainable for all the problems raised on this paper. It is an answer which has been used reliably for over 80 years. MICC cables can provide a total and full reply to all the problems related to the fireplace safety of organic polymer cables.
The copper jacket, magnesium oxide insulation and copper conductors of MICC ensure the cable is successfully fire proof. MICC cables haven’t any organic content so simply cannot propagate flame or generate any smoke. The zero fuel load ensures no heat is added and no oxygen is consumed.
Being inorganic MICC cables cannot generate any halogen or toxic gasses at all together with CO.
Unfortunately many frequent cable fire test methods used today might inadvertently mislead individuals into believing the polymeric flexible cable merchandise they purchase and use will perform as expected in all fire conditions. As outlined on this paper, sadly this is in all probability not appropriate.
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