Fire Performance of Electric Cables

Often one of the best flame retardant cables are halogenated as a result of both the insulation and outer Jacket are flame retardant but once we need Halogen Free cables we discover it is typically solely the outer jacket which is flame retardant and the inner insulation is not.
This has significance as a end result of while cables with a flame retardant outer jacket will usually pass flame retardance tests with exterior flame, the same cables when subjected to high overload or prolonged brief circuits have proved in college exams to be extremely flammable and may even begin a fire. 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 not any widespread take a look at protocols for this seemingly widespread event and one cited by each authorities and media as reason for constructing fires.
Further, in Flame Retardant test methods such as IEC60332 parts 1 & 3 which make use of an exterior flame supply, the cable samples aren’t pre-conditioned to regular working temperature however tested at room temperature. This oversight is necessary particularly for power circuits because the temperature index of the cable (the temperature at which the cable materials 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’ll propagate hearth.
It would seem that a need exists to re-evaluate current cable flame retardance take a look at methods as these are commonly understood by consultants and shoppers alike to provide a dependable indication of a cables capacity to retard the propagation of fireplace.
If we can’t belief the Standards what do we do?
In the USA many building standards do not require halogen free cables. Certainly this is not as a outcome of Americans usually are not properly knowledgeable of the dangers; somewhat the approach taken is that: “It is best to have extremely flame retardant cables which do not propagate hearth than minimally flame retardant cables which may spread a fire” – (a small fireplace with some halogen could additionally be higher than a large fireplace 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 many nations around the world undertake a unique mentality: Halogen Free and Flame Retardant. Whilst this is an admirable mandate the reality is somewhat totally different: Flame propagation exams for cables as adopted in UK and Europe can arguably be mentioned to be less stringent than a few of the flame propagation tests for cables in USA resulting in the conclusion that frequent tests in UK and Europe might merely be checks the cables can cross rather than checks the cables should cross.
For most flexible polymeric cables the choice remains right now between excessive flame propagation efficiency with halogens or lowered flame propagation performance with out halogens.
Enclosing cables in steel conduit will cut back propagation at the point of fire however hydrocarbon based combustion gasses from decomposing polymers are doubtless propagate by way of the conduits to switchboards, distribution boards and junction packing containers in other parts of the building. Any spark such as the opening or closing of circuit breakers, or contactors is prone to ignite the combustible gasses resulting in explosion and spreading the hearth to a different location.
While MICC (Mineral Insulated Metal Sheathed) cables would provide an answer, there’s typically no singe good answer for every set up so designers want to gauge the required performance on a “project-by-project” basis to determine which expertise is optimal.
The major significance of fireside load
Inside all buildings and initiatives electric cables provide the connectivity which keeps lights on, air-conditioning working and the lifts running. It powers computer systems, workplace equipment and supplies the connection for our telephone and computers. Even our mobile phones need to attach with wireless or GSM antennas that are linked to the telecom network by fiber optic or copper cables. Cables guarantee our security by connecting
hearth alarms, emergency voice communication, CCTV, smoke shutters, air pressurization followers, emergency lighting, fire sprinkler pumps, smoke and warmth detectors, and so many different features of a contemporary Building Management System.
Where public security is important we frequently request cables to have added safety features such as flame retardance to ensure the cables do not simply unfold hearth, circuit integrity throughout hearth so that essential fire-fighting and life safety tools maintain working. Sometimes we could acknowledge that the combustion of electrical 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 particular properties the cables we purchase and set up might be safer
Because cables are put in by many different trades for different functions and are largely hidden or embedded in our constructions, what is commonly not realized is that the numerous miles of cables and tons of plastic polymers which make up the cables can symbolize one of many biggest fire masses in the building. This level is actually value thinking more about.
PVC, XLPE, EPR, CSP, LSOH (Low Smoke Zero Halogen) and even HFFR (Halogen Free Flame Retardant) cable supplies are principally primarily based on hydrocarbon polymers. These base materials are not usually flame retardant and naturally have a excessive fire load. Cable producers make them flame retardant by adding compounds and chemical compounds. Certainly this improves the volatility of burning however the fuel content material of the bottom polymers stays.
Tables 1 and 2 above compare the fire load in MJ/Kg for widespread cable insulating supplies towards some widespread fuels. The Heat Release Rate and volatility in air for these materials will differ but the fuel added to a fire per kilogram and the consequential volume of warmth generated and oxygen consumed is relative.
The quantity in kilometers and tons of cables put in in our buildings and the related fireplace load of the insulations is appreciable. This is especially important in tasks with lengthy egress instances like excessive rise, public buildings, tunnels and underground environments, airports, hospitals etc.
When considering fireplace safety we should first perceive the most important elements. Fire experts tell us most fire associated deaths in buildings are attributable to smoke inhalation, temperature rise and oxygen depletion or by trauma attributable to jumping in trying to flee these effects.
The first and most necessary side of smoke is how a lot smoke? Typically the bigger the hearth the more smoke is generated so anything we are able to do to reduce the spread of fireside will also correspondingly reduce the quantity of smoke.
Smoke will comprise particulates of carbon, ash and other solids, liquids and gasses, many are toxic and flamable. In explicit, 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 materials will release toxic Halides like Hydrogen Chloride together with many different toxic and flammable gasses within the smoke.
For Unheard of carried out on cable insulation supplies in large three meter3 chambers with plenty of air can present misleading smoke figures because complete burning will typically launch significantly much less smoke than partial incomplete burning which is most likely going in follow. Simply specifying IEC 61034 with an outlined obscuration value then thinking this will present a low smoke surroundings throughout fireplace may unfortunately be little of help for the individuals truly involved.
Halogens, Toxicity, Fuel Element, Oxygen Depletion and Temperature Rise
It is regarding that Europe and different countries undertake the idea of halogen free supplies with out correctly addressing the topic of toxicity. Halogens launched throughout combustion are extraordinarily poisonous but so too is carbon monoxide and this isn’t a halogen gasoline. It is widespread to name for halogen free cables after which permit the usage of Polyethylene as a outcome of it is halogen free. Burning Polyethylene (which can be seen from the table above has the best MJ gas load per Kg of all insulations) will generate almost 3 instances extra heat than an equal PVC cable. This means is that burning polyethylene will not solely generate almost three occasions more heat but also devour almost 3 times more oxygen and produce considerably extra carbon monoxide. Given carbon monoxide is answerable for most toxicity deaths in fires this situation is at best alarming!
The gas parts shown in the desk above indicate the amount of heat which shall be generated by burning 1kg of the widespread cable insulations tabled. Certainly this warmth will accelerate the burning of other adjacent supplies and may help unfold the fireplace in a building but importantly, in order to generate the warmth power, oxygen must be consumed. The higher the warmth of combustion the extra oxygen is required, so by selecting insulations with high gasoline parts is including considerably to no much less than four of the primary dangers of fires: Temperature Rise, Oxygen Depletion, Flame Spread and Carbon Monoxide Release.
Perhaps it is best to install polymeric cables inside steel conduits. This will definitely assist flame unfold and minimize smoke as a end result of inside the conduit oxygen is proscribed; nonetheless this isn’t an answer. As stated previously, most of the gasses from the decomposing polymeric insulations contained in the conduits are extremely flammable and poisonous. These gases will migrate along the conduits to junction boxes, switch panels, distribution boards, motor management facilities, lamps, switches, and so forth. On entering the gases can ignite or explode with any arcing such as the make/break of a circuit breaker, contactor, change or relay inflicting the fireplace to spread to another location.
The popularity of “Halogen Free” while ignoring the other poisonous elements of fireside is a transparent admission we do not understand the subject nicely nor can we easily outline the hazards of mixed toxic elements or human physiological response to them. It is necessary nonetheless, that we don’t proceed to design with only half an understanding of the problem. While Savings exists for organic based cables, we can definitely reduce these critically necessary effects of fireplace threat:
One choice maybe to decide on cable insulations and jacket materials which are halogen free and have a low gasoline element, then install them in metal conduit or perhaps the American approach is best: to make use of highly halogenated insulations in order that in case of fireplace any flame spread is minimized.
For most power, management, communication and data circuits there is one full answer available for all the issues raised in this paper. It is an answer which has been used reliably for over 80 years. MICC cables can provide a total and complete answer 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 positive the cable is effectively fire proof. MICC cables haven’t any organic content so merely 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 can’t generate any halogen or poisonous gasses at all including CO.
Unfortunately many widespread cable fireplace check methods used today may inadvertently mislead individuals into believing the polymeric versatile cable merchandise they buy and use will perform as expected in all fire situations. As outlined on this paper, sadly this may not be correct.
For extra information, go to

Leave a Comment