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 however after we want Halogen Free cables we find it is usually only the outer jacket which is flame retardant and the internal insulation isn’t.
This has significance as a end result of whereas cables with a flame retardant outer jacket will typically pass flame retardance checks with external flame, the same cables when subjected to high overload or prolonged short circuits have proved in university exams to be extremely flammable and might even begin a hearth. This impact is understood and printed (8th International Conference on Insulated Power Cables (Jicable’11 – June 2011) held in Versailles, France) so it’s perhaps shocking that there aren’t any widespread take a look at protocols for this seemingly frequent occasion and one cited by each authorities and media as cause of constructing fires.
Further, in Flame Retardant take a look at strategies corresponding to IEC60332 components 1 & three which make use of an external flame supply, the cable samples are not pre-conditioned to normal operating temperature however tested at room temperature. This oversight is essential particularly for energy circuits because 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 starting temperature i.e.: The hotter the cable is, the extra simply it’ll propagate fire.
It would appear that a need exists to re-evaluate present cable flame retardance take a look at strategies as these are generally understood by consultants and consumers alike to offer a dependable 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 as a outcome of Americans are not wisely knowledgeable of the dangers; rather the method taken is that: “It is better to have extremely flame retardant cables which don’t propagate fireplace than minimally flame retardant cables which may spread a fire” – (a small fire with some halogen could additionally be higher than a big hearth with out halogens). One of the most effective ways to make a cable insulation and cable jacket extremely flame retardant is through the use of halogens.
Europe and many countries all over the world adopt a different mentality: Halogen Free and Flame Retardant. Whilst this is an admirable mandate the reality is quite different: Flame propagation tests for cables as adopted in UK and Europe can arguably be said to be much less stringent than some of the flame propagation exams for cables in USA resulting in the conclusion that widespread exams in UK and Europe could merely be tests the cables can cross rather than exams the cables ought to move.
For most versatile polymeric cables the selection remains right now between excessive flame propagation efficiency with halogens or reduced flame propagation performance without halogens.
Enclosing cables in metal conduit will reduce propagation on the level of fireplace however hydrocarbon based mostly combustion gasses from decomposing polymers are doubtless propagate through the conduits to switchboards, distribution boards and junction boxes in other elements 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 hearth to another location.
While MICC (Mineral Insulated Metal Sheathed) cables would supply a solution, there is often no singe good reply for each set up so designers need to gauge the required performance on a “project-by-project” foundation to decide which technology is optimal.
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 working. It powers computers, workplace equipment and supplies the connection for our phone and computers. Even our cell phones want to connect with wi-fi or GSM antennas that are linked to the telecom network by fiber optic or copper cables. Cables guarantee our security by connecting
fire alarms, emergency voice communication, CCTV, smoke shutters, air pressurization fans, emergency lighting, fireplace sprinkler pumps, smoke and warmth detectors, and so many different features of a contemporary Building Management System.
Where public safety is important we often request cables to have added safety features similar to flame retardance to make sure the cables don’t easily unfold fire, circuit integrity throughout fireplace in order that important fire-fighting and life security equipment hold working. Sometimes we may acknowledge that the combustion of electrical cables produces smoke and this might be poisonous so we name for cables to be Low Smoke and Halogen Free. Logically and intuitively we expect that by requesting these particular properties the cables we buy and set up will be safer
Because cables are put in by many alternative trades for different purposes and are principally 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 represent one of many greatest fireplace masses within the constructing. This level is certainly value pondering more about.
PVC, XLPE, EPR, CSP, LSOH (Low Smoke Zero Halogen) and even HFFR (Halogen Free Flame Retardant) cable supplies are largely based on hydrocarbon polymers. These base materials aren’t typically flame retardant and naturally have a high hearth load. Cable manufacturers make them flame retardant by adding compounds and chemical substances. Certainly this improves the volatility of burning but the gasoline content material of the bottom polymers stays.
Tables 1 and a pair of above compare the fireplace load in MJ/Kg for widespread cable insulating materials in opposition to some common fuels. The Heat Release Rate and volatility in air for these materials will differ however the gas added to a fire per kilogram and the consequential volume of heat generated and oxygen consumed is relative.
The quantity in kilometers and tons of cables put in in our buildings and the related fire load of the insulations is considerable. This is particularly necessary in projects with lengthy egress times like excessive rise, public buildings, tunnels and underground environments, airports, hospitals and so on.
When considering hearth security we should first understand crucial factors. Fire specialists inform us most fire associated deaths in buildings are caused by smoke inhalation, temperature rise and oxygen depletion or by trauma brought on by jumping in making an attempt to flee these effects.
The first and most necessary side of smoke is how a lot smoke? Typically the bigger the fireplace the more smoke is generated so something we are able to do to reduce the spread of fire may also correspondingly cut back the amount of smoke.
Smoke will comprise particulates of carbon, ash and other solids, liquids and gasses, many are poisonous and combustible. In specific, fires in confined areas like buildings, tunnels and underground environments trigger oxygen ranges to drop, this contributes to incomplete burning and smoldering which produces elevated amounts of smoke and toxic byproducts including CO and CO2. Presence of halogenated materials will launch poisonous Halides like Hydrogen Chloride along with many different poisonous and flammable gasses in the smoke.
For this reason frequent smoke tests performed on cable insulation materials in giant three meter3 chambers with plenty of air can present misleading smoke figures because complete burning will often release significantly much less smoke than partial incomplete burning which is likely in follow. Simply specifying IEC 61034 with an outlined obscuration value then thinking this will provide a low smoke setting during hearth may unfortunately be little of help for the people actually concerned.
Halogens, Toxicity, Fuel Element, Oxygen Depletion and Temperature Rise
It is regarding that Europe and different nations undertake the concept of halogen free materials with out correctly addressing the topic of toxicity. Halogens launched during combustion are extremely toxic however so too is carbon monoxide and this isn’t a halogen gas. It is common to name for halogen free cables and then permit the utilization of Polyethylene as a end result of it’s halogen free. Burning Polyethylene (which can be seen from the desk above has the highest MJ gasoline load per Kg of all insulations) will generate almost three occasions extra heat than an equivalent PVC cable. This means is that burning polyethylene won’t only generate nearly three times more heat but additionally devour nearly three occasions more oxygen and produce considerably extra carbon monoxide. Given carbon monoxide is liable for most toxicity deaths in fires this example is at greatest alarming!
The gasoline elements shown within the table above point out the quantity of heat which will be generated by burning 1kg of the frequent cable insulations tabled. Certainly this heat will accelerate the burning of different adjoining materials and may help spread the hearth in a building but importantly, in order to generate the warmth energy, oxygen needs to be consumed. The greater the warmth of combustion the extra oxygen is required, so by selecting insulations with high gas parts is adding considerably to at least four of the primary risks of fires: Temperature Rise, Oxygen Depletion, Flame Spread and Carbon Monoxide Release.
Perhaps it is best to install polymeric cables inside metal conduits. This will certainly assist flame spread and minimize smoke as a result of contained in the conduit oxygen is limited; nevertheless this isn’t a solution. As mentioned beforehand, No obligation 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 packing containers, swap panels, distribution boards, motor control facilities, lamps, switches, and so on. On entering the gases can ignite or explode with any arcing such as the make/break of a circuit breaker, contactor, swap or relay inflicting the fireplace to unfold to another location.
The recognition of “Halogen Free” whereas ignoring the opposite poisonous components of fireplace is a clear admission we don’t perceive the subject well nor can we easily define the hazards of mixed poisonous components or human physiological response to them. It is necessary nonetheless, that we don’t proceed to design with solely half an understanding of the issue. While no perfect resolution exists for natural primarily based cables, we will actually minimize these critically necessary effects of fireplace danger:
One option maybe to determine on cable insulations and jacket materials that are halogen free and have a low gas factor, then install them in metal conduit or possibly the American strategy is best: to use extremely halogenated insulations so that in case of fireplace any flame unfold is minimized.
For most energy, control, communication and knowledge circuits there might be one complete answer available for all the issues raised on this paper. It is a solution which has been used reliably for over eighty years. MICC cables can provide a total and full answer to all the problems related to the fire safety of natural polymer cables.
The copper jacket, magnesium oxide insulation and copper conductors of MICC ensure the cable is successfully hearth proof. MICC cables haven’t any organic content material so merely can’t 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 at all including CO.
Unfortunately many widespread cable fireplace test strategies used at present might inadvertently mislead folks into believing the polymeric flexible cable products they purchase and use will carry out as anticipated in all hearth conditions. As outlined in this paper, sadly this is probably not right.
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