Fire Performance of Electric Cables

Often the most effective flame retardant cables are halogenated as a result of each the insulation and outer Jacket are flame retardant but when we need Halogen Free cables we find it is usually only the outer jacket which is flame retardant and the inside insulation isn’t.
This has significance because whereas cables with a flame retardant outer jacket will often move flame retardance checks with external flame, the identical cables when subjected to high overload or prolonged quick circuits have proved in university exams to be highly flammable and can even begin a hearth. This effect is known and published (8th International Conference on Insulated Power Cables (Jicable’11 – June 2011) held in Versailles, France) so it is maybe stunning that there are not any widespread check protocols for this seemingly widespread event and one cited by both authorities and media as explanation for building fires.
Further, in Flame Retardant test strategies such as IEC60332 elements 1 & 3 which make use of an exterior flame supply, the cable samples are not pre-conditioned to regular working temperature but tested 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 material will self-support combustion in normal air) shall be significantly affected by its starting temperature i.e.: The hotter the cable is, the more simply it will propagate fire.
It would appear that a want exists to re-evaluate present cable flame retardance check methods as these are commonly understood by consultants and customers alike to supply a reliable indication of a cables capability to retard the propagation of fireplace.
If we can’t belief the Standards what do we do?
In the USA many constructing standards do not require halogen free cables. Certainly this isn’t because Americans usually are not correctly informed of the risks; quite the approach taken is that: “It is best to have highly flame retardant cables which do not propagate hearth than minimally flame retardant cables which can unfold a fire” – (a small hearth with some halogen could also be better than a large hearth without halogens). One of one of the best ways to make a cable insulation and cable jacket highly flame retardant is by using halogens.
Europe and many nations around the globe adopt a different mentality: Halogen Free and Flame Retardant. Whilst that is an admirable mandate the fact is rather different: Flame propagation exams for cables as adopted in UK and Europe can arguably be mentioned to be less stringent than a few of the flame propagation tests for cables in USA leading to the conclusion that common tests in UK and Europe could simply be tests the cables can cross rather than exams the cables should move.
Conclusion
For most flexible polymeric cables the choice stays at present between excessive flame propagation performance with halogens or reduced flame propagation efficiency without halogens.
Enclosing cables in metal conduit will scale back propagation at the point of fireside but hydrocarbon based mostly combustion gasses from decomposing polymers are likely propagate by way of the conduits to switchboards, distribution boards and junction packing containers in different elements of the constructing. Any spark such as the opening or closing of circuit breakers, or contactors is likely to ignite the combustible gasses resulting in explosion and spreading the fireplace to another location.
While MICC (Mineral Insulated Metal Sheathed) cables would supply an answer, there might be typically no singe perfect reply for every installation so designers need to evaluate the required performance on a “project-by-project” foundation to determine which technology is optimal.
The main importance of fireplace load
Inside all buildings and initiatives electric cables provide the connectivity which keeps lights on, air-conditioning working and the lifts running. It powers computers, office gear and supplies the connection for our telephone and computers. Even our cellphones want to attach with wireless or GSM antennas that are linked to the telecom community by fiber optic or copper cables. Cables guarantee our safety by connecting
hearth alarms, emergency voice communication, CCTV, smoke shutters, air pressurization followers, emergency lighting, hearth sprinkler pumps, smoke and warmth detectors, and so many different features of a contemporary Building Management System.
Where public safety is important we regularly request cables to have added security options similar to flame retardance to make sure the cables do not easily spread fireplace, circuit integrity during hearth in order that essential fire-fighting and life safety tools hold working. Sometimes we may recognize that the combustion of electrical 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 particular properties the cables we buy and install shall be safer
Because cables are installed by many alternative trades for various functions and are mostly 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 fire loads in the building. This point is certainly worth thinking more about.
PVC, XLPE, EPR, CSP, LSOH (Low Smoke Zero Halogen) and even HFFR (Halogen Free Flame Retardant) cable materials are principally primarily based on hydrocarbon polymers. These base supplies are not generally flame retardant and naturally have a excessive fireplace load. Cable producers make them flame retardant by including compounds and chemical substances. Certainly this improves the volatility of burning but the fuel content material of the bottom polymers stays.
Tables 1 and 2 above evaluate the fire load in MJ/Kg for frequent cable insulating supplies against some common fuels. The Heat Release Rate and volatility in air for these supplies will differ but the gasoline added to a fire per kilogram and the consequential quantity of warmth generated and oxygen consumed is relative.
The volume in kilometers and tons of cables installed in our buildings and the associated hearth load of the insulations is appreciable. This is especially essential in tasks with long egress times like excessive rise, public buildings, tunnels and underground environments, airports, hospitals etc.
When considering hearth safety we must first perceive the most important elements. Fire specialists inform us most fire related deaths in buildings are attributable to smoke inhalation, temperature rise and oxygen depletion or by trauma caused by jumping in attempting to escape these effects.
Smoke
The first and most essential facet of smoke is how a lot smoke? Typically the larger the fire the extra smoke is generated so something we can do to reduce the spread of fireside may also correspondingly scale back the amount of smoke.
Smoke will comprise particulates of carbon, ash and different solids, liquids and gasses, many are poisonous and combustible. In explicit, fires in confined areas like buildings, tunnels and underground environments trigger oxygen levels to drop, this contributes to incomplete burning and smoldering which produces increased quantities of smoke and toxic byproducts including 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 cause widespread smoke tests performed on cable insulation materials in giant 3 meter3 chambers with plenty of air can present misleading smoke figures because complete burning will often release considerably less smoke than partial incomplete burning which is likely in apply. Simply specifying IEC 61034 with an outlined obscuration value then considering this can provide a low smoke setting during fireplace might sadly be little of assist for the individuals really concerned.
Halogens, Toxicity, Fuel Element, Oxygen Depletion and Temperature Rise
It is concerning that Europe and other nations adopt the concept of halogen free materials without correctly addressing the topic of toxicity. Halogens released during combustion are extraordinarily poisonous 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 use of Polyethylene as a result of it’s halogen free. Burning Polyethylene (which may be seen from the table above has the best MJ fuel load per Kg of all insulations) will generate almost three instances more heat than an equal PVC cable. This means is that burning polyethylene won’t solely generate nearly 3 instances more warmth but in addition consume nearly three occasions extra oxygen and produce significantly more carbon monoxide. Given carbon monoxide is answerable for most toxicity deaths in fires this case is at greatest alarming!
The gas elements proven within the desk above indicate the quantity of warmth which will be generated by burning 1kg of the frequent cable insulations tabled. Certainly this warmth will accelerate the burning of different adjoining materials and may assist unfold the hearth in a building however importantly, to have the ability to generate the heat energy, oxygen needs to be consumed. The higher the warmth of combustion the extra oxygen is required, so by choosing insulations with excessive gasoline components is including considerably to no less than 4 of the primary risks 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 reduce smoke as a end result of contained in the conduit oxygen is limited; nevertheless this is not a solution. As stated previously, many of the gasses from the decomposing polymeric insulations inside the conduits are highly flammable and toxic. These gases will migrate alongside the conduits to junction packing containers, change panels, distribution boards, motor management facilities, lamps, switches, and so forth. On entering the gases can ignite or explode with any arcing such as the make/break of a circuit breaker, contactor, switch or relay inflicting the hearth to spread to another location.
Conclusion
The reputation of “Halogen Free” whereas ignoring the other toxic elements of fireplace is a clear admission we do not understand the subject nicely nor can we simply outline the hazards of combined poisonous components or human physiological response to them. It is essential nevertheless, that we do not continue to design with only half an understanding of the problem. While no perfect resolution exists for organic primarily based cables, we are able to actually minimize these critically essential results of fireplace danger:
One choice perhaps to choose cable insulations and jacket supplies that are halogen free and have a low gas element, then install them in steel conduit or maybe the American method is healthier: to make use of highly halogenated insulations in order that in case of fireplace any flame spread is minimized.
For most energy, control, communication and data circuits there is one complete answer out there for all the issues raised on this paper. It is an answer which has been used reliably for over 80 years. MICC cables can provide a complete and complete answer to all the problems associated with the fire safety of natural polymer cables.
The copper jacket, magnesium oxide insulation and copper conductors of MICC make sure the cable is successfully fireplace proof. MICC cables don’t have any organic content material so simply cannot propagate flame or generate any smoke. เกจวัดแรงดันเบนซิน ensures no heat is added and no oxygen is consumed.
Being inorganic MICC cables can not generate any halogen or poisonous gasses at all including CO.
Unfortunately many frequent cable hearth take a look at strategies used right now might inadvertently mislead people into believing the polymeric flexible cable merchandise they buy and use will carry out as anticipated in all fireplace situations. As outlined on this paper, sadly this will not be appropriate.
For extra info, go to www.temperature-house.com
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