Fire Performance of Electric Cables

Often the most effective flame retardant cables are halogenated as a end result of each the insulation and outer Jacket are flame retardant however after we need Halogen Free cables we discover it is often solely the outer jacket which is flame retardant and the inside insulation isn’t.
This has significance as a end result of while cables with a flame retardant outer jacket will typically pass flame retardance exams with external flame, the same cables when subjected to high overload or extended short circuits have proved in college exams to be extremely flammable and might even start a hearth. This effect is understood and published (8th International Conference on Insulated Power Cables (Jicable’11 – June 2011) held in Versailles, France) so it’s perhaps surprising that there are no common test protocols for this seemingly common event and one cited by each authorities and media as cause of constructing fires.
Further, in Flame Retardant check methods corresponding to IEC60332 parts 1 & three which make use of an exterior flame supply, the cable samples aren’t pre-conditioned to normal working temperature but tested at room temperature. This oversight is necessary 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 regular air) will be significantly affected by its beginning temperature i.e.: The hotter the cable is, the more easily it’s going to propagate fire.
It would seem that a need exists to re-evaluate current cable flame retardance take a look at strategies as these are generally understood by consultants and consumers alike to supply a dependable indication of a cables capability to retard the propagation of fireside.
If we can’t trust the Standards what can we do?
In the USA many constructing requirements don’t require halogen free cables. Certainly this is not because Americans aren’t properly knowledgeable of the dangers; somewhat the method taken is that: “It is better to have highly flame retardant cables which do not propagate fireplace than minimally flame retardant cables which may spread a fire” – (a small fireplace with some halogen could additionally be higher than a big fireplace without halogens). One of one of the best methods to make a cable insulation and cable jacket highly flame retardant is by utilizing halogens.
Europe and tons of international locations all over the world adopt a unique mentality: Halogen Free and Flame Retardant. Whilst this is an admirable mandate the fact is rather totally different: Flame propagation exams for cables as adopted in UK and Europe can arguably be mentioned to be much less stringent than some of the flame propagation checks for cables in USA leading to the conclusion that widespread checks in UK and Europe could merely be tests the cables can cross rather than tests the cables ought to cross.
For most flexible polymeric cables the selection remains at present between high flame propagation efficiency with halogens or lowered flame propagation performance without halogens.
Enclosing cables in steel conduit will cut back propagation at the level of fireside but hydrocarbon based combustion gasses from decomposing polymers are likely propagate via the conduits to switchboards, distribution boards and junction packing containers in different components of the building. Any spark such as the opening or closing of circuit breakers, or contactors is likely to ignite the flamable 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 often no singe perfect answer for each set up so designers want to judge the required efficiency on a “project-by-project” foundation to determine which technology is perfect.
The primary significance of fire load
Inside all buildings and tasks electrical cables provide the connectivity which keeps lights on, air-conditioning working and the lifts working. It powers computers, office equipment and provides the connection for our phone and computers. Even our mobile phones want to attach with wireless or GSM antennas which are linked 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, fireplace sprinkler pumps, smoke and warmth detectors, and so many other options of a modern Building Management System.
Where public safety is necessary we regularly request cables to have added safety options such as flame retardance to ensure the cables don’t easily unfold hearth, circuit integrity during fire in order that essential fire-fighting and life security equipment maintain working. Sometimes we may acknowledge that the combustion of electrical cables produces smoke and this could be poisonous so we call for cables to be Low Smoke and Halogen Free. Logically and intuitively we expect that by requesting these special properties the cables we purchase and set up will be safer
Because cables are put in by many various trades for different 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 greatest fire hundreds in the building. This point is definitely value thinking extra about.
PVC, XLPE, EPR, CSP, LSOH (Low Smoke Zero Halogen) and even HFFR (Halogen Free Flame Retardant) cable supplies are largely based mostly on hydrocarbon polymers. These base materials usually are not generally flame retardant and naturally have a excessive hearth load. Cable producers make them flame retardant by adding compounds and chemical compounds. Certainly this improves the volatility of burning however the gasoline content material of the base polymers remains.
Tables 1 and a pair of above evaluate the hearth load in MJ/Kg for widespread cable insulating materials towards some frequent 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 quantity of warmth generated and oxygen consumed is relative.
The quantity in kilometers and tons of cables put in in our buildings and the related hearth load of the insulations is appreciable. This is particularly important in projects with lengthy egress occasions like high rise, public buildings, tunnels and underground environments, airports, hospitals and so on.
When considering hearth safety we should first understand the most 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 attributable to leaping in trying to escape these results.
The first and most important facet of smoke is how much smoke? Typically the bigger the fire the more smoke is generated so anything we can do to reduce the unfold of fireplace may also correspondingly reduce the quantity of smoke.
Smoke will contain particulates of carbon, ash and other solids, liquids and gasses, many are poisonous and flamable. In particular, 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 poisonous byproducts together with CO and CO2. Presence of halogenated supplies will launch poisonous Halides like Hydrogen Chloride together with many other poisonous and flammable gasses in the smoke.
For this cause frequent smoke tests performed on cable insulation supplies in large three meter3 chambers with loads of air can provide misleading smoke figures as a outcome of full 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 will present a low smoke environment throughout fireplace might unfortunately be little of help for the people actually involved.
Halogens, Toxicity, Fuel Element, Oxygen Depletion and Temperature Rise
It is concerning that Europe and other countries adopt the concept of halogen free materials without correctly addressing the subject of toxicity. Halogens released during combustion are extraordinarily poisonous but so too is carbon monoxide and this isn’t a halogen gasoline. It is common to call for halogen free cables and then enable using Polyethylene as a end result of it’s halogen free. Burning Polyethylene (which can be seen from the table above has the highest MJ fuel load per Kg of all insulations) will generate nearly 3 occasions extra warmth than an equal PVC cable. This means is that burning polyethylene won’t solely generate virtually three occasions more heat but additionally consume nearly 3 times more oxygen and produce considerably extra carbon monoxide. Given carbon monoxide is responsible for most toxicity deaths in fires this situation is at greatest alarming!
The gas components proven in the desk above indicate the quantity of warmth which shall be generated by burning 1kg of the frequent cable insulations tabled. Certainly this warmth will speed up the burning of other adjacent supplies and should assist unfold the hearth in a constructing but importantly, in order to generate the warmth vitality, oxygen needs to be consumed. The higher the heat of combustion the more oxygen is needed, so by selecting insulations with high gas parts is including considerably to no less than four of the primary risks of fires: Temperature Rise, Oxygen Depletion, Flame Spread and Carbon Monoxide Release.
Perhaps it’s best to put in polymeric cables inside steel conduits. This will certainly help flame unfold and decrease smoke as a outcome of inside the conduit oxygen is proscribed; however this isn’t an answer. As stated previously, most of the gasses from the decomposing polymeric insulations contained in the conduits are highly flammable and poisonous. These gases will migrate alongside the conduits to junction packing containers, swap panels, distribution boards, motor control centers, lamps, switches, etc. On getting into 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 unfold to another location.
The popularity of “Halogen Free” whereas ignoring the opposite toxic elements of fire is a transparent admission we don’t perceive the subject well nor can we simply define the dangers of combined toxic parts or human physiological response to them. It is necessary nevertheless, that we don’t continue to design with solely half an understanding of the problem. While no excellent answer exists for organic primarily based cables, we can actually minimize these critically necessary effects of fire danger:
One choice perhaps to choose cable insulations and jacket supplies that are halogen free and have a low fuel element, then install them in metal conduit or maybe the American approach is better: to use extremely halogenated insulations in order that in case of fire any flame spread is minimized.
For most energy, control, communication and knowledge circuits there is one complete resolution out there for all the problems raised in this paper. It is a solution which has been used reliably for over eighty years. MICC cables can provide a total and complete answer to all the issues related to the hearth security of organic polymer cables.
The copper jacket, magnesium oxide insulation and copper conductors of MICC ensure the cable is effectively fireplace proof. MICC cables don’t have any organic content material so simply cannot propagate flame or generate any smoke. The zero gas load ensures no heat 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 fire take a look at methods used today may inadvertently mislead folks into believing the polymeric versatile cable merchandise they purchase and use will carry out as anticipated in all fire situations. As outlined on this paper, sadly this will not be correct.
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