Fire Performance of Electric Cables

Often the best flame retardant cables are halogenated because both the insulation and outer Jacket are flame retardant but when we want Halogen Free cables we discover it’s usually solely the outer jacket which is flame retardant and the inner insulation just isn’t.
This has significance as a end result of while cables with a flame retardant outer jacket will typically pass flame retardance tests with external flame, the same cables when subjected to excessive overload or extended short circuits have proved in college checks to be extremely flammable and may even begin a fire. This impact is known and printed (8th International Conference on Insulated Power Cables (Jicable’11 – June 2011) held in Versailles, France) so it is perhaps shocking that there are not any frequent take a look at protocols for this seemingly common occasion and one cited by both authorities and media as reason for building fires.
Further, in Flame Retardant take a look at strategies such as IEC60332 components 1 & three which make use of an exterior flame source, the cable samples usually are not pre-conditioned to normal operating temperature but tested at room temperature. This oversight is essential particularly for power circuits as a outcome of the temperature index of the cable (the temperature at which the cable material will self-support combustion in normal 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 appear that a need exists to re-evaluate present cable flame retardance take a look at methods as these are commonly understood by consultants and consumers alike to provide a reliable indication of a cables capacity to retard the propagation of fireplace.
If we can’t trust the Standards what will we do?
In the USA many building standards don’t require halogen free cables. Certainly this is not as a end result of Americans aren’t properly knowledgeable of the hazards; somewhat the method taken is that: “It is healthier to have extremely flame retardant cables which don’t propagate fireplace than minimally flame retardant cables which can spread a fire” – (a small hearth with some halogen could also be better than a large fire with out halogens). One of one of the best methods to make a cable insulation and cable jacket highly flame retardant is through the use of halogens.
Europe and many countries all over the world undertake a special mentality: Halogen Free and Flame Retardant. Whilst that is an admirable mandate the fact is rather 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 leading to the conclusion that widespread exams in UK and Europe might merely be tests the cables can pass quite than tests the cables ought to pass.
For most flexible polymeric cables the choice stays today between high flame propagation performance with halogens or lowered flame propagation efficiency without halogens.
Enclosing cables in steel conduit will scale back propagation on the point of fire but hydrocarbon based mostly combustion gasses from decomposing polymers are doubtless propagate by way of the conduits to switchboards, distribution boards and junction bins in other parts of the building. 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 fire to another location.
While MICC (Mineral Insulated Metal Sheathed) cables would offer a solution, there’s usually no singe excellent reply for every installation so designers need to gauge the required efficiency on a “project-by-project” foundation to determine which know-how is optimal.
The main significance of fireplace load
Inside all buildings and tasks electric cables provide the connectivity which keeps lights on, air-conditioning working and the lifts working. It powers computers, office tools and supplies the connection for our phone and computers. Even our cell phones want to connect with wireless or GSM antennas which are connected to the telecom network by fiber optic or copper cables. Cables guarantee our safety by connecting
fire alarms, emergency voice communication, CCTV, smoke shutters, air pressurization followers, emergency lighting, fire sprinkler pumps, smoke and heat detectors, and so many other features of a contemporary Building Management System.
Where public safety is necessary we regularly request cables to have added safety options corresponding to flame retardance to ensure the cables do not simply unfold fireplace, circuit integrity during fire so that important fire-fighting and life safety equipment maintain working. Sometimes we might recognize that the combustion of electrical cables produces smoke and this may 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 purchase and set up will be safer
Because cables are put in by many different trades for different functions and are mostly hidden or embedded in our constructions, what is usually not realized is that the numerous miles of cables and tons of plastic polymers which make up the cables can represent one of the largest fireplace masses in the building. This level is actually price considering more about.
PVC, XLPE, EPR, CSP, LSOH (Low Smoke Zero Halogen) and even HFFR (Halogen Free Flame Retardant) cable supplies are principally based mostly on hydrocarbon polymers. These base materials usually are not typically flame retardant and naturally have a excessive fire load. Cable manufacturers make them flame retardant by including compounds and chemicals. Certainly this improves the volatility of burning but the gas content of the base polymers remains.
Tables 1 and a pair of above examine the hearth load in MJ/Kg for frequent cable insulating supplies towards some common fuels. The Heat Release Rate and volatility in air for these supplies will differ but the fuel added to a hearth per kilogram and the consequential quantity of warmth 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 considerable. This is particularly essential in initiatives with lengthy egress instances like excessive rise, public buildings, tunnels and underground environments, airports, hospitals and so forth.
When contemplating hearth safety we should first perceive the most important components. Fire consultants tell us most fire related deaths in buildings are brought on by smoke inhalation, temperature rise and oxygen depletion or by trauma attributable to leaping in trying to escape these results.
The first and most necessary side of smoke is how a lot smoke? Typically the larger the fireplace the more smoke is generated so anything we are in a position to do to scale back the spread of fireplace will also correspondingly scale back the quantity of smoke.
Smoke will include particulates of carbon, ash and other solids, liquids and gasses, many are poisonous and combustible. In particular, 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 amounts of smoke and toxic byproducts together with CO and CO2. Presence of halogenated supplies will launch poisonous Halides like Hydrogen Chloride along with many other toxic and flammable gasses within the smoke.
For this purpose frequent smoke exams performed on cable insulation supplies in giant 3 meter3 chambers with plenty of air can provide misleading smoke figures because full burning will usually launch considerably less smoke than partial incomplete burning which is most likely going in apply. Simply specifying IEC 61034 with an outlined obscuration worth then thinking it will provide a low smoke setting throughout fire may sadly be little of help for the folks really concerned.
Halogens, Toxicity, Fuel Element, Oxygen Depletion and Temperature Rise
It is concerning that Europe and different countries adopt the idea of halogen free materials without correctly addressing the topic of toxicity. Halogens released throughout combustion are extremely toxic however so too is carbon monoxide and this isn’t a halogen fuel. It is common to name for halogen free cables after which permit the utilization of Polyethylene as a result of it is halogen free. Burning Polyethylene (which can be seen from the table above has the highest MJ gas load per Kg of all insulations) will generate virtually 3 instances more warmth than an equal PVC cable. เครื่องมือที่ใช้วัดความดัน is that burning polyethylene will not solely generate almost three instances extra heat but in addition devour virtually three instances more oxygen and produce significantly extra carbon monoxide. Given carbon monoxide is answerable for most toxicity deaths in fires this case is at best alarming!
The gasoline elements proven within the desk above point out the amount of warmth which might be generated by burning 1kg of the widespread cable insulations tabled. Certainly this heat will accelerate the burning of different adjoining supplies and should help spread the fireplace in a constructing however importantly, to have the ability to generate the warmth energy, oxygen needs to be consumed. The larger the heat of combustion the extra oxygen is needed, so by selecting insulations with excessive gas parts is adding considerably to at least 4 of the first risks of fires: Temperature Rise, Oxygen Depletion, Flame Spread and Carbon Monoxide Release.
Perhaps it is best to put in polymeric cables inside metallic conduits. This will definitely assist flame spread and minimize smoke as a result of contained in the conduit oxygen is limited; nevertheless this isn’t a solution. As stated beforehand, most of the gasses from the decomposing polymeric insulations contained in the conduits are extremely flammable and toxic. These gases will migrate alongside the conduits to junction boxes, change panels, distribution boards, motor management facilities, lamps, switches, etc. On entering the gases can ignite or explode with any arcing such as the make/break of a circuit breaker, contactor, change or relay causing the fire to unfold to another location.
The popularity of “Halogen Free” whereas ignoring the opposite poisonous parts of fire is a clear admission we don’t understand the topic nicely nor can we simply outline the dangers of mixed toxic components or human physiological response to them. It is important nonetheless, that we do not continue to design with solely half an understanding of the issue. While no perfect solution exists for organic primarily based cables, we can actually decrease these critically necessary results of fireside risk:
One option perhaps to choose cable insulations and jacket materials which are halogen free and have a low gas element, then set up them in metal conduit or perhaps the American method is best: to make use of highly halogenated insulations so that in case of fire any flame unfold is minimized.
For most power, management, communication and data circuits there is one complete solution 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 present a total and full answer to all the problems related to the hearth safety of natural polymer cables.
The copper jacket, magnesium oxide insulation and copper conductors of MICC make positive the cable is effectively fireplace proof. MICC cables have no natural content so simply can not propagate flame or generate any smoke. The zero fuel load ensures no heat is added and no oxygen is consumed.
Being inorganic MICC cables can’t generate any halogen or poisonous gasses in any respect including CO.
Unfortunately many widespread cable fireplace test strategies used today could inadvertently mislead people into believing the polymeric flexible cable merchandise they purchase and use will perform as expected in all hearth conditions. As outlined in this paper, sadly this may not be appropriate.
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