Fire Performance of Electric Cables

Often the most effective flame retardant cables are halogenated as a result of both the insulation and outer Jacket are flame retardant but after we want Halogen Free cables we find it is typically only the outer jacket which is flame retardant and the inner insulation just isn’t.
This has significance as a result of whereas cables with a flame retardant outer jacket will often move flame retardance exams with exterior flame, the identical cables when subjected to excessive overload or extended brief circuits have proved in college exams to be extremely flammable and can 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 is perhaps stunning that there aren’t any widespread check protocols for this seemingly widespread occasion and one cited by each authorities and media as cause of building fires.
Further, in Flame Retardant take a look at methods similar to IEC60332 parts 1 & three which make use of an external flame supply, the cable samples are not pre-conditioned to regular operating temperature however tested at room temperature. This oversight is necessary especially for power circuits because the temperature index of the cable (the temperature at which the cable materials will self-support combustion in regular air) might be considerably affected by its beginning temperature i.e.: The hotter the cable is, the extra easily it’s going to propagate fireplace.
It would seem that a want 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 reliable indication of a cables capacity to retard the propagation of fireside.
If we can’t belief the Standards what do we do?
In the USA many building requirements don’t require halogen free cables. Certainly this is not as a result of Americans aren’t correctly knowledgeable of the risks; quite the approach taken is that: “It is healthier to have extremely flame retardant cables which do not propagate fire than minimally flame retardant cables which may unfold a fire” – (a small hearth with some halogen could also be better than a big fireplace without halogens). One of the best methods to make a cable insulation and cable jacket highly flame retardant is by utilizing halogens.
Europe and heaps of nations around the globe undertake a special mentality: Halogen Free and Flame Retardant. Whilst this is an admirable mandate the truth is somewhat different: Flame propagation exams for cables as adopted in UK and Europe can arguably be said to be much less stringent than some of the flame propagation checks for cables in USA leading to the conclusion that widespread tests in UK and Europe may merely be exams the cables can cross somewhat than checks the cables should cross.
For most flexible polymeric cables the choice remains at present between excessive flame propagation efficiency with halogens or lowered flame propagation performance without halogens.
Enclosing cables in metal conduit will reduce propagation at the point of fire however hydrocarbon primarily based combustion gasses from decomposing polymers are probably propagate by way of the conduits to switchboards, distribution boards and junction boxes in different elements of the building. Any spark such because the opening or closing of circuit breakers, or contactors is more likely to ignite the combustible gasses leading to explosion and spreading the fire to another location.
While MICC (Mineral Insulated Metal Sheathed) cables would provide a solution, there could be usually no singe good reply for every set up so designers want to evaluate the required efficiency on a “project-by-project” basis to resolve which technology is optimal.
The major importance of fire load
Inside all buildings and initiatives electrical cables present the connectivity which retains lights on, air-conditioning working and the lifts running. It powers computer systems, office tools and supplies the connection for our telephone and computer systems. Even our mobile phones want to attach with wireless or GSM antennas which are connected to the telecom network by fiber optic or copper cables. Cables guarantee our security by connecting
fireplace alarms, emergency voice communication, CCTV, smoke shutters, air pressurization followers, emergency lighting, hearth sprinkler pumps, smoke and heat detectors, and so many other options of a modern 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 don’t easily unfold fireplace, circuit integrity during hearth so that essential fire-fighting and life safety tools hold working. Sometimes we could recognize 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 predict that by requesting these special properties the cables we purchase and install 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 commonly not realized is that the many miles of cables and tons of plastic polymers which make up the cables can characterize one of many biggest fire hundreds in the building. This level is certainly price pondering 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 supplies usually are not typically flame retardant and naturally have a excessive fireplace load. Cable manufacturers make them flame retardant by adding compounds and chemical substances. Certainly this improves the volatility of burning but the gasoline content of the base polymers remains.
Tables 1 and 2 above examine the fireplace load in MJ/Kg for frequent cable insulating supplies in opposition to some widespread fuels. The Heat Release Rate and volatility in air for these supplies will differ however the gasoline added to a fireplace per kilogram and the consequential quantity of warmth generated and oxygen consumed is relative.
The volume in kilometers and tons of cables put in in our buildings and the associated hearth load of the insulations is considerable. This is especially important in tasks with long egress times like high rise, public buildings, tunnels and underground environments, airports, hospitals and so forth.
When contemplating fireplace security we must first understand the most important factors. Fire consultants inform us most hearth related deaths in buildings are brought on by smoke inhalation, temperature rise and oxygen depletion or by trauma brought on by jumping in trying to escape these effects.
The first and most important aspect of smoke is how much smoke? Typically the bigger the fire the more smoke is generated so something we will do to scale back the unfold of fireside may even correspondingly reduce the quantity of smoke.
Smoke will include particulates of carbon, ash and different 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 increased amounts of smoke and poisonous byproducts together with CO and CO2. Presence of halogenated materials will launch poisonous Halides like Hydrogen Chloride along with many different poisonous and flammable gasses within the smoke.
For this reason widespread smoke checks conducted 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 often release considerably much less smoke than partial incomplete burning which is probably going in apply. Simply specifying IEC 61034 with a defined obscuration worth then considering this will present a low smoke setting throughout fire may unfortunately 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 international locations adopt the idea of halogen free supplies without correctly addressing the topic of toxicity. Halogens launched throughout combustion are extremely toxic however so too is carbon monoxide and this isn’t a halogen gasoline. It is common to call for halogen free cables after which allow the utilization of Polyethylene as a outcome of it’s halogen free. Burning Polyethylene (which could be seen from the desk above has the best MJ fuel load per Kg of all insulations) will generate virtually 3 occasions extra warmth than an equivalent PVC cable. This means is that burning polyethylene will not solely generate nearly three occasions more heat but in addition eat almost 3 instances more oxygen and produce significantly more carbon monoxide. Given carbon monoxide is answerable for most toxicity deaths in fires this situation is at best alarming!
The gas elements proven in the desk above indicate the quantity of warmth which might be generated by burning 1kg of the frequent cable insulations tabled. Certainly this heat will speed up the burning of different adjoining supplies and may assist unfold the fireplace in a building but importantly, so as to generate the heat power, oxygen must be consumed. The higher the heat of combustion the more oxygen is needed, so by selecting insulations with excessive gas parts is including considerably to no much less than four 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 assist flame spread and minimize smoke because contained in the conduit oxygen is restricted; nevertheless this is not an answer. As said previously, most of the gasses from the decomposing polymeric insulations inside the conduits are extremely flammable and toxic. These gases will migrate along the conduits to junction bins, swap panels, distribution boards, motor control centers, lamps, switches, and so forth. On entering the gases can ignite or explode with any arcing such because the make/break of a circuit breaker, contactor, switch or relay causing the fireplace to unfold to a different location.
The reputation of “Halogen Free” while ignoring the opposite poisonous components of fireside is a clear admission we do not understand the topic nicely nor can we simply outline the hazards of combined poisonous components or human physiological response to them. It is necessary nonetheless, that we do not continue to design with only half an understanding of the issue. While no good resolution exists for natural based cables, we can definitely minimize these critically essential results of fire threat:
One option possibly to decide on cable insulations and jacket supplies that are halogen free and have a low gasoline component, then install them in metal conduit or maybe the American approach is better: to make use of extremely halogenated insulations so that in case of fire any flame spread is minimized.
For most power, management, communication and data circuits there’s one complete solution obtainable for all the problems raised on this paper. It is a solution which has been used reliably for over 80 years. MICC cables can provide a complete and complete reply to all the problems related to the fireplace safety of organic polymer cables.
The copper jacket, magnesium oxide insulation and copper conductors of MICC make positive the cable is successfully hearth proof. MICC cables haven’t any organic content so simply can not propagate flame or generate any smoke. The zero gasoline load ensures no heat 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 frequent cable hearth test strategies used at present may inadvertently mislead individuals into believing the polymeric flexible cable merchandise they buy and use will perform as expected in all fire conditions. As outlined on this paper, sadly this will not be appropriate.
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