Fire Performance of Electric Cables

Often one of the best flame retardant cables are halogenated as a result of each the insulation and outer Jacket are flame retardant but once we want Halogen Free cables we find it’s typically solely the outer jacket which is flame retardant and the inside insulation just isn’t.
This has significance as a result of while cables with a flame retardant outer jacket will typically cross flame retardance checks with exterior flame, the identical cables when subjected to excessive overload or extended brief circuits have proved in university checks to be extremely flammable and might even start a fireplace. This impact is understood and printed (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 frequent check protocols for this seemingly common occasion and one cited by both authorities and media as cause of constructing fires.
Further, in Flame Retardant check methods corresponding to IEC60332 components 1 & three which employ an external flame source, the cable samples aren’t pre-conditioned to normal working temperature however tested at room temperature. This oversight is necessary especially for energy 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 beginning temperature i.e.: The hotter the cable is, the more simply it will propagate hearth.
It would seem that a need exists to re-evaluate current cable flame retardance take a look at strategies as these are commonly understood by consultants and shoppers alike to offer a dependable indication of a cables capacity to retard the propagation of fire.
If we can’t belief the Standards what can we do?
In the USA many building standards don’t require halogen free cables. Certainly this isn’t as a outcome of Americans usually are not correctly informed of the hazards; somewhat the approach taken is that: “It is healthier to have extremely flame retardant cables which don’t propagate fire than minimally flame retardant cables which can unfold a fire” – (a small fireplace with some halogen could also be higher than a large fire without halogens). One of the most effective methods to make a cable insulation and cable jacket highly flame retardant is through the use of halogens.
Europe and a lot of countries around the globe adopt a different mentality: Halogen Free and Flame Retardant. Whilst that is an admirable mandate the fact is quite different: Flame propagation tests for cables as adopted in UK and Europe can arguably be stated to be less stringent than some of the flame propagation checks for cables in USA leading to the conclusion that widespread checks in UK and Europe might merely be checks the cables can pass rather than exams the cables should move.
For most flexible polymeric cables the choice remains right now between high flame propagation efficiency with halogens or reduced flame propagation performance with out halogens.
Enclosing cables in metal conduit will cut back propagation at the level 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 other parts of the constructing. Any spark such because the opening or closing of circuit breakers, or contactors is more doubtless to ignite the combustible gasses resulting in explosion and spreading the hearth to a different location.
While MICC (Mineral Insulated Metal Sheathed) cables would offer an answer, there’s typically no singe excellent answer for each set up so designers want to gauge the required performance on a “project-by-project” basis to determine which know-how is perfect.
The main significance of fireside load
Inside all buildings and tasks electric cables provide the connectivity which retains lights on, air-conditioning working and the lifts operating. It powers computer systems, office equipment and supplies the connection for our phone and computer systems. Even our mobile phones want to attach with wireless or GSM antennas that are connected to the telecom network by fiber optic or copper cables. Cables ensure our security by connecting
fireplace alarms, emergency voice communication, CCTV, smoke shutters, air pressurization fans, emergency lighting, fireplace sprinkler pumps, smoke and warmth detectors, and so many other options of a contemporary Building Management System.
Where public safety is important we often request cables to have added security features similar to flame retardance to ensure the cables do not simply spread fireplace, circuit integrity throughout fireplace so that essential fire-fighting and life safety gear keep working. Sometimes we could acknowledge that the combustion of electric cables produces smoke and this might be toxic so we call 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 install will be safer
Because cables are put in by many alternative trades for various applications and are largely 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 largest fireplace loads within the building. This point is actually worth considering more 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 aren’t generally flame retardant and naturally have a high fireplace load. Cable manufacturers make them flame retardant by adding compounds and chemicals. Certainly this improves the volatility of burning however the gasoline content of the bottom polymers stays.
Tables 1 and a pair of above evaluate the fire load in MJ/Kg for common 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 fireplace per kilogram and the consequential quantity of heat generated and oxygen consumed is relative.
The volume in kilometers and tons of cables installed in our buildings and the related fire load of the insulations is appreciable. This is especially essential in initiatives with long egress occasions like excessive rise, public buildings, tunnels and underground environments, airports, hospitals etc.
When considering fire safety we should first perceive an important elements. Fire experts tell us most hearth related deaths in buildings are attributable to 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 bigger the fireplace the more smoke is generated so something we are in a position to do to scale back the unfold of fireside may also correspondingly reduce the amount of smoke.
Smoke will include particulates of carbon, ash and other solids, liquids and gasses, many are poisonous and flamable. In explicit, 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 poisonous byproducts including CO and CO2. Presence of halogenated materials will release toxic Halides like Hydrogen Chloride along with many other toxic and flammable gasses within the smoke.
For this cause widespread smoke tests performed on cable insulation supplies in massive three meter3 chambers with plenty of air can provide misleading smoke figures as a end result of complete burning will typically release considerably less smoke than partial incomplete burning which is probably going in practice. Simply specifying IEC 61034 with an outlined obscuration value then considering this will present a low smoke surroundings throughout fire could sadly be little of help for the people really involved.
Halogens, Toxicity, Fuel Element, Oxygen Depletion and Temperature Rise
It is concerning that Europe and different international locations adopt the concept of halogen free materials without correctly addressing the topic of toxicity. Halogens launched throughout combustion are extremely toxic however so too is carbon monoxide and this is not a halogen fuel. เกจวัดแรงดันต่ำ is common to call for halogen free cables after which allow the use of Polyethylene as a outcome of it is halogen free. Burning Polyethylene (which can be seen from the desk above has the very best MJ gas load per Kg of all insulations) will generate virtually 3 occasions more heat than an equal PVC cable. This means is that burning polyethylene is not going to only generate nearly three times more heat but additionally devour virtually 3 times more oxygen and produce considerably extra carbon monoxide. Given carbon monoxide is liable for most toxicity deaths in fires this case is at finest alarming!
The gas elements proven in the table above indicate the quantity of warmth which shall be generated by burning 1kg of the frequent cable insulations tabled. Certainly this heat will accelerate the burning of different adjacent supplies and may assist unfold the hearth in a building but importantly, in order to generate the warmth vitality, oxygen needs to be consumed. The greater the warmth of combustion the more oxygen is required, so by choosing insulations with high gasoline parts is adding considerably to a minimal of 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 assist flame spread and minimize smoke as a outcome of inside the conduit oxygen is proscribed; nonetheless this is not an answer. As stated previously, most of the gasses from the decomposing polymeric insulations contained in the conduits are highly flammable and toxic. These gases will migrate along the conduits to junction packing containers, swap panels, distribution boards, motor management facilities, lamps, switches, etc. On getting into the gases can ignite or explode with any arcing such because the make/break of a circuit breaker, contactor, change or relay inflicting the hearth to spread to another location.
The reputation of “Halogen Free” whereas ignoring the other poisonous parts of fireside is a clear admission we don’t understand the topic properly nor can we simply outline the hazards of mixed toxic parts or human physiological response to them. It is important nevertheless, that we do not proceed to design with solely half an understanding of the issue. While no good resolution exists for natural primarily based cables, we are ready to definitely minimize these critically necessary results of fireside threat:
One choice perhaps to choose cable insulations and jacket supplies that are halogen free and have a low gas factor, then install them in metal conduit or possibly the American method is healthier: to use extremely halogenated insulations in order that in case of fireside any flame spread is minimized.
For most energy, management, communication and data circuits there’s one full resolution out there for all the issues 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 associated with the fire security of natural polymer cables.
The copper jacket, magnesium oxide insulation and copper conductors of MICC make sure the cable is effectively hearth proof. MICC cables have no natural content material so merely can not propagate flame or generate any smoke. The zero gas load ensures no warmth is added and no oxygen is consumed.
Being inorganic MICC cables cannot generate any halogen or poisonous gasses at all together with CO.
Unfortunately many common cable fire take a look at methods used today could inadvertently mislead individuals into believing the polymeric versatile cable products they purchase and use will carry out as expected in all fire situations. As outlined on this paper, sadly this is probably not right.
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