Fire Performance of Electric Cables

Often one of the best flame retardant cables are halogenated as a end result of both the insulation and outer Jacket are flame retardant but once we want Halogen Free cables we discover it’s often only the outer jacket which is flame retardant and the inside insulation is not.
This has significance as a outcome of while cables with a flame retardant outer jacket will typically move flame retardance tests with exterior flame, the identical cables when subjected to excessive overload or prolonged quick circuits have proved in university exams to be extremely flammable and can even start a fire. This impact is known and revealed (8th International Conference on Insulated Power Cables (Jicable’11 – June 2011) held in Versailles, France) so it’s perhaps surprising that there are no widespread test protocols for this seemingly widespread occasion and one cited by both authorities and media as reason for building fires.
Further, in Flame Retardant take a look at strategies similar to IEC60332 parts 1 & three which employ an exterior flame source, the cable samples aren’t pre-conditioned to normal operating temperature but tested at room temperature. This oversight is necessary especially for power circuits because 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 extra easily it’ll propagate fire.
It would seem that a need exists to re-evaluate present cable flame retardance test methods as these are commonly understood by consultants and shoppers alike to offer a dependable indication of a cables capacity to retard the propagation of fire.
If we can’t belief the Standards what do we do?
In the USA many constructing requirements do not require halogen free cables. Certainly this is not as a result of Americans usually are not properly informed of the risks; somewhat the strategy taken is that: “It is best to have highly flame retardant cables which don’t propagate hearth than minimally flame retardant cables which may spread a fire” – (a small hearth with some halogen may be better than a large fire without halogens). Interesting of the most effective ways to make a cable insulation and cable jacket highly flame retardant is by using halogens.
Europe and lots of countries all over the world adopt a unique mentality: Halogen Free and Flame Retardant. Whilst this is an admirable mandate the fact is quite different: Flame propagation exams for cables as adopted in UK and Europe can arguably be stated to be less stringent than a few of the flame propagation checks for cables in USA resulting in the conclusion that common tests in UK and Europe might simply be tests the cables can pass somewhat than checks the cables ought to move.
For most flexible polymeric cables the choice remains at present between excessive flame propagation performance with halogens or reduced flame propagation efficiency without halogens.
Enclosing cables in metal conduit will cut back propagation on the point of fireplace but hydrocarbon based combustion gasses from decomposing polymers are probably propagate through the conduits to switchboards, distribution boards and junction bins in other components of the constructing. Any spark such as the opening or closing of circuit breakers, or contactors is more doubtless to ignite the flamable gasses leading to explosion and spreading the fireplace to another location.
While MICC (Mineral Insulated Metal Sheathed) cables would supply a solution, there’s often no singe excellent reply for each installation so designers need to judge the required performance on a “project-by-project” foundation to resolve which know-how is optimal.
The major importance of fire load
Inside all buildings and tasks electric cables provide the connectivity which retains lights on, air-conditioning working and the lifts working. It powers computers, workplace equipment and supplies the connection for our phone and computer systems. Even our mobile phones need to connect with wireless or GSM antennas which are connected to the telecom community by fiber optic or copper cables. Cables guarantee our security by connecting
fire alarms, emergency voice communication, CCTV, smoke shutters, air pressurization fans, emergency lighting, fireplace sprinkler pumps, smoke and heat detectors, and so many other options of a contemporary Building Management System.
Where public security is important we often request cables to have added safety features corresponding to flame retardance to make sure the cables don’t easily spread fire, circuit integrity during fire in order that important fire-fighting and life security tools keep working. Sometimes we might recognize that the combustion of electric cables produces smoke and this could be toxic so we call for cables to be Low Smoke and Halogen Free. Logically and intuitively we expect that by requesting these particular properties the cables we buy and set up will be safer
Because cables are installed by many different trades for various purposes and are largely hidden or embedded in our constructions, what is usually not realized is that the many miles of cables and tons of plastic polymers which make up the cables can represent one of the biggest fireplace masses within the constructing. This level is actually worth pondering more about.
PVC, XLPE, EPR, CSP, LSOH (Low Smoke Zero Halogen) and even HFFR (Halogen Free Flame Retardant) cable supplies are largely primarily based on hydrocarbon polymers. These base supplies aren’t typically flame retardant and naturally have a high hearth load. Cable producers make them flame retardant by including compounds and chemical compounds. Certainly this improves the volatility of burning but the fuel content material of the base polymers stays.
Tables 1 and 2 above evaluate the hearth load in MJ/Kg for widespread cable insulating materials against some common fuels. The Heat Release Rate and volatility in air for these supplies will differ however the gasoline added to a fireplace per kilogram and the consequential quantity of heat generated and oxygen consumed is relative.
The quantity in kilometers and tons of cables put in in our buildings and the associated fireplace 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 forth.
When considering fireplace security we should first perceive an important components. Fire specialists tell us most fireplace related deaths in buildings are caused by smoke inhalation, temperature rise and oxygen depletion or by trauma brought on by leaping in trying to escape these results.
The first and most essential side of smoke is how much smoke? Typically the larger the fire the extra smoke is generated so something we will do to scale back the unfold of fireside may even correspondingly reduce the amount of smoke.
Smoke will comprise 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 elevated amounts of smoke and poisonous byproducts including CO and CO2. Presence of halogenated supplies will launch toxic Halides like Hydrogen Chloride along with many different poisonous and flammable gasses within the smoke.
For this cause frequent smoke tests carried out on cable insulation materials in massive three meter3 chambers with plenty of air can provide deceptive smoke figures as a result of complete burning will often launch considerably less smoke than partial incomplete burning which is likely in apply. Simply specifying IEC 61034 with a defined obscuration value then pondering it will provide a low smoke environment during fireplace may sadly be little of assist for the individuals actually involved.
Halogens, Toxicity, Fuel Element, Oxygen Depletion and Temperature Rise
It is concerning that Europe and different nations adopt the concept of halogen free supplies with out correctly addressing the subject of toxicity. Halogens released throughout combustion are extremely toxic but so too is carbon monoxide and this is not a halogen fuel. It is common to name for halogen free cables after which permit the use of Polyethylene as a end 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 instances more heat than an equal PVC cable. This means is that burning polyethylene is not going to only generate nearly 3 instances more heat but additionally eat nearly three instances extra oxygen and produce significantly more carbon monoxide. Given carbon monoxide is responsible for most toxicity deaths in fires this example is at best alarming!
The gasoline components shown within the table above point out the amount of warmth which might be generated by burning 1kg of the frequent cable insulations tabled. Certainly this heat will accelerate the burning of other adjoining supplies and will help unfold the fireplace in a building however importantly, in order to generate the heat energy, oxygen must be consumed. The larger the warmth of combustion the extra oxygen is needed, so by choosing insulations with high fuel components is adding significantly to a minimal of 4 of the primary dangers of fires: Temperature Rise, Oxygen Depletion, Flame Spread and Carbon Monoxide Release.
Perhaps it is best to put in polymeric cables inside steel conduits. This will definitely help flame spread and decrease smoke because inside the conduit oxygen is proscribed; nonetheless this isn’t an answer. As stated beforehand, many of the gasses from the decomposing polymeric insulations inside the conduits are extremely flammable and toxic. These gases will migrate alongside the conduits to junction bins, swap panels, distribution boards, motor management centers, lamps, switches, and so on. On coming into the gases can ignite or explode with any arcing such because the make/break of a circuit breaker, contactor, change or relay inflicting the fireplace to unfold to another location.
The recognition of “Halogen Free” whereas ignoring the opposite poisonous components of fireplace is a transparent admission we do not perceive the topic nicely nor can we easily outline the dangers of mixed poisonous components or human physiological response to them. It is important nonetheless, that we do not proceed to design with only half an understanding of the problem. While no perfect resolution exists for natural based mostly cables, we will actually reduce these critically important results of fireplace risk:
One choice maybe to choose cable insulations and jacket materials which are halogen free and have a low gas element, then install them in steel conduit or possibly the American approach is best: to make use of extremely halogenated insulations in order that in case of fire any flame spread is minimized.
For most power, control, communication and information circuits there might be one full solution out there for all the issues raised in this paper. It is an answer which has been used reliably for over eighty years. MICC cables can provide a complete and complete reply to all the issues related to the fireplace security of natural polymer cables.
The copper jacket, magnesium oxide insulation and copper conductors of MICC ensure the cable is successfully hearth proof. MICC cables haven’t any organic content material so simply cannot propagate flame or generate any smoke. The zero gasoline 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 common cable fire check methods used today could inadvertently mislead folks into believing the polymeric flexible cable merchandise they purchase and use will carry out as anticipated in all fireplace situations. As outlined on this paper, sadly this may not be right.
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