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 once we want Halogen Free cables we discover it’s often solely 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 pass flame retardance exams with external flame, the same cables when subjected to excessive overload or extended short circuits have proved in university tests to be extremely flammable and may even begin a fire. This effect is known and published (8th International Conference on Insulated Power Cables (Jicable’11 – June 2011) held in Versailles, France) so it’s maybe surprising that there aren’t any common take a look at protocols for this seemingly widespread occasion and one cited by each authorities and media as explanation for constructing fires.
Further, in Flame Retardant take a look at strategies similar to IEC60332 parts 1 & three which employ an external flame source, the cable samples aren’t pre-conditioned to normal working temperature but tested at room temperature. This oversight is important especially for power circuits as a end result of the temperature index of the cable (the temperature at which the cable materials will self-support combustion in normal air) might be considerably affected by its starting temperature i.e.: The hotter the cable is, the more simply it’ll propagate hearth.
It would appear that a need exists to re-evaluate present cable flame retardance take a look at methods as these are generally understood by consultants and shoppers alike to supply a reliable indication of a cables capability to retard the propagation of fireside.
If we can’t belief the Standards what will we do?
In the USA many constructing standards do not require halogen free cables. Certainly this isn’t because Americans aren’t properly informed of the risks; rather the strategy taken is that: “It is healthier to have highly flame retardant cables which don’t propagate hearth than minimally flame retardant cables which can unfold a fire” – (a small fire with some halogen may be higher than a large hearth with out halogens). One of the most effective ways to make a cable insulation and cable jacket highly flame retardant is through the use of halogens.
Europe and lots of countries around the world undertake a different mentality: Halogen Free and Flame Retardant. Whilst this is an admirable mandate the truth is somewhat completely different: Flame propagation exams for cables as adopted in UK and Europe can arguably be stated to be much less stringent than some of the flame propagation exams for cables in USA resulting in the conclusion that common exams in UK and Europe might merely be exams the cables can pass quite than exams the cables ought to cross.
For most flexible polymeric cables the choice stays right now between high flame propagation efficiency with halogens or lowered flame propagation efficiency with out halogens.
Enclosing cables in metal conduit will scale back propagation on the level of fire but hydrocarbon based combustion gasses from decomposing polymers are likely propagate by way of the conduits to switchboards, distribution boards and junction packing containers in different parts of the building. 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 fire to another location.
While MICC (Mineral Insulated Metal Sheathed) cables would provide a solution, there’s usually no singe excellent answer for every set up so designers want to judge the required efficiency on a “project-by-project” foundation to determine which technology is perfect.
The major importance of fireside load
Inside all buildings and tasks electric cables present the connectivity which retains lights on, air-conditioning working and the lifts operating. It powers computer systems, office gear and provides the connection for our phone and computers. Even our mobile phones need to attach with wireless or GSM antennas which are related 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, hearth sprinkler pumps, smoke and heat detectors, and so many other features of a contemporary Building Management System.
Where public security is necessary we often request cables to have added security features such as flame retardance to make sure the cables don’t simply spread hearth, circuit integrity during fire in order that essential fire-fighting and life safety tools maintain working. Sometimes we may acknowledge that the combustion of electric cables produces smoke and this can be poisonous so we name 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 install shall be safer
Because cables are put in by many different trades for different functions and are principally hidden or embedded in our constructions, what is commonly not realized is that the various miles of cables and tons of plastic polymers which make up the cables can represent one of the largest hearth loads within the building. This level is actually value considering extra about.
PVC, XLPE, EPR, CSP, LSOH (Low Smoke Zero Halogen) and even HFFR (Halogen Free Flame Retardant) cable supplies are mostly based on hydrocarbon polymers. These base supplies usually are not generally flame retardant and naturally have a excessive fire load. Cable manufacturers make them flame retardant by adding compounds and chemicals. Certainly this improves the volatility of burning but the gas content material of the bottom polymers remains.
Tables 1 and 2 above evaluate the fire load in MJ/Kg for frequent cable insulating supplies in opposition to some frequent fuels. The Heat Release Rate and volatility in air for these materials will differ but the fuel added to a fireplace per kilogram and the consequential volume of heat generated and oxygen consumed is relative.
The volume in kilometers and tons of cables installed in our buildings and the related fireplace load of the insulations is appreciable. This is especially essential in initiatives with lengthy egress occasions like high rise, public buildings, tunnels and underground environments, airports, hospitals and so on.
When contemplating fireplace safety we must first understand an important components. Fire specialists inform us most fire related deaths in buildings are attributable to smoke inhalation, temperature rise and oxygen depletion or by trauma brought on by leaping in making an attempt to flee these results.
The first and most important facet of smoke is how much smoke? Typically the bigger the fire the extra smoke is generated so something we are able to do to cut back the unfold of fireside may even correspondingly reduce the quantity of smoke.
Smoke will contain particulates of carbon, ash and different 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 materials will release toxic Halides like Hydrogen Chloride along with many different toxic and flammable gasses within the smoke.
For this purpose common smoke exams conducted on cable insulation supplies in large 3 meter3 chambers with loads of air can provide deceptive smoke figures as a outcome of complete burning will often release considerably much less smoke than partial incomplete burning which is in all probability going in practice. Simply specifying IEC 61034 with an outlined obscuration worth then considering this will provide a low smoke environment throughout fire might sadly be little of assist for the people actually concerned.
Halogens, Toxicity, Fuel Element, Oxygen Depletion and Temperature Rise
It is regarding that Europe and different international locations adopt the concept of halogen free materials without correctly addressing the topic of toxicity. Halogens released during combustion are extremely poisonous however so too is carbon monoxide and this isn’t a halogen gasoline. It is widespread to call for halogen free cables after which permit the use of Polyethylene as a result of it is halogen free. Burning Polyethylene (which may be seen from the desk above has the very best MJ gasoline load per Kg of all insulations) will generate nearly three occasions more heat than an equivalent PVC cable. This means is that burning polyethylene will not only generate nearly three occasions more warmth but also devour nearly 3 times more oxygen and produce significantly extra carbon monoxide. Given carbon monoxide is responsible for most toxicity deaths in fires this situation is at greatest alarming!
The fuel components proven in the desk above indicate the amount of warmth which might be generated by burning 1kg of the common cable insulations tabled. Certainly this heat will accelerate the burning of other adjoining supplies and should help spread the fireplace in a building however importantly, in order to generate the warmth vitality, oxygen must be consumed. The higher the heat of combustion the more oxygen is required, so by choosing insulations with excessive gasoline parts is including considerably to a minimal of four of the first 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 certainly assist flame spread and decrease smoke as a outcome of contained in the conduit oxygen is limited; nevertheless this isn’t an answer. As said previously, lots 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, switch panels, distribution boards, motor management centers, lamps, switches, etc. On entering the gases can ignite or explode with any arcing such as the make/break of a circuit breaker, contactor, switch or relay causing the fire to spread to a different location.
The recognition of “Halogen Free” whereas ignoring the opposite toxic components of fire is a clear admission we don’t understand the topic well nor can we easily outline the risks of combined toxic parts or human physiological response to them. It is necessary nonetheless, that we don’t continue to design with solely half an understanding of the issue. While no perfect solution exists for organic based mostly cables, we will certainly reduce these critically important results of fire risk:
One possibility maybe to determine on cable insulations and jacket materials which are halogen free and have a low fuel element, then install them in metal conduit or possibly the American strategy is healthier: to use extremely halogenated insulations in order that in case of fireplace any flame unfold is minimized.
For most power, control, communication and knowledge circuits there is one complete solution out there for all the problems raised on this paper. It is an answer which has been used reliably for over eighty years. เกจวัดแรงดันเครื่องกรองน้ำ can present a complete and complete reply to all the issues related to the fire safety of organic polymer cables.
The copper jacket, magnesium oxide insulation and copper conductors of MICC make sure the cable is successfully fireplace proof. MICC cables have no natural 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’t generate any halogen or poisonous gasses at all together with CO.
Unfortunately many widespread cable hearth take a look at methods used at present may inadvertently mislead individuals into believing the polymeric flexible cable products they buy and use will perform as anticipated in all fire conditions. As outlined in this paper, sadly this will not be correct.
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