Fire Performance of Electric Cables

Often one of the best flame retardant cables are halogenated as a end result of each the insulation and outer Jacket are flame retardant but when we need Halogen Free cables we find it is often only the outer jacket which is flame retardant and the inside insulation just isn’t.
This has significance as a outcome of while cables with a flame retardant outer jacket will typically cross flame retardance checks with exterior flame, the identical cables when subjected to high overload or prolonged short circuits have proved in university tests to be extremely flammable and may even begin a fire. This impact is thought and printed (8th International Conference on Insulated Power Cables (Jicable’11 – June 2011) held in Versailles, France) so it’s maybe stunning that there are no common take a look at protocols for this seemingly widespread event and one cited by both authorities and media as explanation for building fires.
Further, in Flame Retardant take a look at methods similar to IEC60332 parts 1 & 3 which make use of an external flame supply, the cable samples are not pre-conditioned to normal operating temperature but examined at room temperature. This oversight is necessary especially for energy circuits because the temperature index of the cable (the temperature at which the cable material will self-support combustion in regular air) might be significantly affected by its starting temperature i.e.: The hotter the cable is, the extra simply it’s going to propagate hearth.
It would appear that a need exists to re-evaluate current cable flame retardance check strategies as these are commonly understood by consultants and consumers alike to provide a dependable indication of a cables ability to retard the propagation of fire.
If we can’t trust the Standards what will we do?
In the USA many building requirements do not require halogen free cables. Certainly this is not as a end result of Americans aren’t correctly knowledgeable of the risks; quite the method taken is that: “It is better to have highly flame retardant cables which don’t propagate fireplace than minimally flame retardant cables which can spread a fire” – (a small fire with some halogen may be better than a big hearth without halogens). Under wraps of the best methods to make a cable insulation and cable jacket highly flame retardant is through the use of halogens.
Europe and tons of countries around the world undertake a special mentality: Halogen Free and Flame Retardant. Whilst this is an admirable mandate the reality is somewhat totally 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 frequent tests in UK and Europe could simply be tests the cables can move quite than tests the cables ought to cross.
For most versatile polymeric cables the selection stays at present between high flame propagation efficiency with halogens or decreased flame propagation performance without halogens.
Enclosing cables in steel conduit will cut back propagation at the level of fireside however hydrocarbon primarily based combustion gasses from decomposing polymers are likely propagate by way of the conduits to switchboards, distribution boards and junction packing containers in other elements of the building. Any spark such as the opening or closing of circuit breakers, or contactors is more doubtless to ignite the flamable gasses resulting in explosion and spreading the fireplace to a different location.
While MICC (Mineral Insulated Metal Sheathed) cables would provide an answer, there’s typically no singe perfect reply for each set up so designers need to evaluate the required performance on a “project-by-project” basis to determine which expertise is perfect.
The main importance of fire load
Inside all buildings and projects electric cables provide the connectivity which retains lights on, air-conditioning working and the lifts operating. It powers computer systems, office gear and offers the connection for our telephone and computers. Even our mobile phones need to attach with wireless or GSM antennas that are related to the telecom community 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, fireplace sprinkler pumps, smoke and heat detectors, and so many different features of a contemporary Building Management System.
Where public safety is necessary we often request cables to have added security options similar to flame retardance to make sure the cables do not simply spread fire, circuit integrity throughout fire in order that essential fire-fighting and life security gear hold 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 think that by requesting these special properties the cables we purchase and install shall be safer
Because cables are put in by many various trades for various applications and are principally 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 characterize one of the greatest hearth hundreds within the building. This point is certainly worth pondering more about.
PVC, XLPE, EPR, CSP, LSOH (Low Smoke Zero Halogen) and even HFFR (Halogen Free Flame Retardant) cable materials are principally based mostly on hydrocarbon polymers. These base materials usually are not usually flame retardant and naturally have a high hearth load. Cable manufacturers make them flame retardant by adding compounds and chemicals. Certainly this improves the volatility of burning but the gas content of the base polymers stays.
Tables 1 and a couple of above evaluate the fireplace load in MJ/Kg for common cable insulating materials 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 volume in kilometers and tons of cables put in in our buildings and the associated fire load of the insulations is appreciable. This is particularly essential in tasks with long egress times like high rise, public buildings, tunnels and underground environments, airports, hospitals and so forth.
When contemplating fireplace security we should first perceive crucial components. Fire experts tell us most hearth related deaths in buildings are brought on by smoke inhalation, temperature rise and oxygen depletion or by trauma caused by jumping in attempting to flee these effects.
The first and most necessary facet 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 cut back the unfold 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 toxic and flamable. In specific, 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 quantities of smoke and poisonous byproducts together with CO and CO2. Presence of halogenated supplies will launch poisonous Halides like Hydrogen Chloride along with many different toxic and flammable gasses in the smoke.
For this cause common smoke exams conducted on cable insulation materials in massive 3 meter3 chambers with plenty of air can present misleading smoke figures as a end result of full burning will usually release considerably much less smoke than partial incomplete burning which is likely in follow. Simply specifying IEC 61034 with a defined obscuration worth then pondering it will present a low smoke setting during fireplace could unfortunately be little of assist for the people truly concerned.
Halogens, Toxicity, Fuel Element, Oxygen Depletion and Temperature Rise
It is regarding that Europe and other nations adopt the idea of halogen free supplies with out properly addressing the subject of toxicity. Halogens released throughout combustion are extremely poisonous however so too is carbon monoxide and this is not a halogen gasoline. Deadline is frequent to call for halogen free cables and then enable the utilization of Polyethylene because it’s halogen free. Burning Polyethylene (which can be seen from the table above has the highest MJ gasoline load per Kg of all insulations) will generate nearly three occasions extra warmth than an equal PVC cable. This means is that burning polyethylene is not going to solely generate virtually three instances more heat but also eat nearly three instances more oxygen and produce considerably extra carbon monoxide. Given carbon monoxide is responsible for most toxicity deaths in fires this example is at greatest alarming!
The gasoline parts shown in the desk above point out the amount of warmth which will be generated by burning 1kg of the frequent cable insulations tabled. Certainly this heat will accelerate the burning of other adjacent materials and will assist unfold the fireplace in a constructing however importantly, so as to generate the heat energy, oxygen must be consumed. The greater the warmth of combustion the more oxygen is required, so by selecting insulations with high fuel components is adding significantly to a minimal of four 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 metal conduits. This will definitely assist flame unfold and minimize smoke as a outcome of contained in the conduit oxygen is proscribed; however this is not an answer. As stated previously, most of the gasses from the decomposing polymeric insulations contained in the conduits are extremely flammable and toxic. These gases will migrate along the conduits to junction boxes, change 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, change or relay causing the hearth to spread to another location.
The recognition of “Halogen Free” whereas ignoring the opposite poisonous parts of fire is a transparent admission we do not understand the topic well nor can we easily define the hazards of mixed poisonous parts or human physiological response to them. It is necessary nonetheless, that we don’t continue to design with only half an understanding of the problem. While no perfect solution exists for natural primarily based cables, we are able to certainly minimize these critically essential results of fire threat:
One possibility possibly to choose cable insulations and jacket materials which are halogen free and have a low gas component, then set up them in metal conduit or perhaps the American approach is healthier: to make use of extremely halogenated insulations so that in case of fireside any flame spread is minimized.
For most energy, management, communication and data circuits there could be one full answer out there for all the issues raised in this paper. It is a solution which has been used reliably for over 80 years. MICC cables can provide a total and complete answer to all the issues associated with the fire security of natural polymer cables.
The copper jacket, magnesium oxide insulation and copper conductors of MICC ensure the cable is successfully fire proof. MICC cables don’t have any organic content material so simply cannot propagate flame or generate any smoke. The zero fuel load ensures no warmth is added and no oxygen is consumed.
Being inorganic MICC cables can not generate any halogen or poisonous gasses at all together with CO.
Unfortunately many common cable fireplace test strategies used today might inadvertently mislead individuals into believing the polymeric flexible cable merchandise they buy and use will carry out as expected in all fire conditions. As outlined in this paper, sadly this will not be right.
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