Fire Performance of Electric Cables

Often the best flame retardant cables are halogenated as a outcome of each the insulation and outer Jacket are flame retardant however when we need Halogen Free cables we find it’s often only the outer jacket which is flame retardant and the inside insulation just isn’t.
This has significance because while cables with a flame retardant outer jacket will typically pass flame retardance checks with exterior flame, the same cables when subjected to excessive overload or prolonged quick circuits have proved in university tests to be highly flammable and may even start a hearth. Cringeworthy is understood and published (8th International Conference on Insulated Power Cables (Jicable’11 – June 2011) held in Versailles, France) so it is maybe stunning that there are no common check protocols for this seemingly widespread event and one cited by each authorities and media as reason for constructing fires.
Further, in Flame Retardant take a look at methods such as IEC60332 components 1 & 3 which make use of an exterior flame supply, the cable samples aren’t pre-conditioned to normal working temperature but tested at room temperature. This oversight is necessary especially for energy circuits because the temperature index of the cable (the temperature at which the cable materials will self-support combustion in regular air) might be significantly affected by its beginning temperature i.e.: The hotter the cable is, the extra easily it will propagate fire.
It would appear that a want exists to re-evaluate present cable flame retardance take a look at strategies as these are commonly understood by consultants and customers alike to supply a dependable indication of a cables ability to retard the propagation of fireplace.
If we can’t trust the Standards what will we do?
In the USA many building standards do not require halogen free cables. Certainly this is not as a end result of Americans usually are not properly informed of the dangers; quite the method taken is that: “It is better to have extremely flame retardant cables which do not propagate fire than minimally flame retardant cables which may unfold a fire” – (a small fireplace with some halogen may be better than a big fireplace without halogens). One of the most effective ways to make a cable insulation and cable jacket extremely flame retardant is by utilizing halogens.
Europe and many nations around the world adopt a unique mentality: Halogen Free and Flame Retardant. Whilst that is an admirable mandate the fact is quite totally different: Flame propagation checks for cables as adopted in UK and Europe can arguably be said to be much less stringent than a few of the flame propagation checks for cables in USA resulting in the conclusion that widespread tests in UK and Europe may simply be exams the cables can move quite than tests the cables ought to move.
Conclusion
For most versatile polymeric cables the selection stays today between high flame propagation performance with halogens or decreased flame propagation efficiency without halogens.
Enclosing cables in metal conduit will cut back propagation on the point of fireplace however hydrocarbon based combustion gasses from decomposing polymers are likely propagate by way of the conduits to switchboards, distribution boards and junction bins in different elements of the building. Any spark such because the opening or closing of circuit breakers, or contactors is more probably to ignite the flamable gasses resulting in explosion and spreading the fireplace to another location.
While MICC (Mineral Insulated Metal Sheathed) cables would supply an answer, there’s usually no singe excellent reply for each set up so designers want to evaluate the required efficiency on a “project-by-project” basis to decide which know-how is optimal.
The main importance of fire load
Inside all buildings and projects electric cables present the connectivity which retains lights on, air-conditioning working and the lifts working. It powers computer systems, office equipment and provides the connection for our telephone and computers. Even our cell phones need to connect with wi-fi or GSM antennas that are connected to the telecom network by fiber optic or copper cables. Cables ensure our security by connecting
fire alarms, emergency voice communication, CCTV, smoke shutters, air pressurization fans, emergency lighting, fire sprinkler pumps, smoke and warmth detectors, and so many different options of a contemporary Building Management System.
Where public safety is essential we often request cables to have added safety options such as flame retardance to ensure the cables do not easily spread fire, circuit integrity during hearth in order that essential fire-fighting and life safety gear maintain working. Sometimes we may recognize 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 predict that by requesting these particular properties the cables we purchase and set up might be safer
Because cables are put in by many various trades for various purposes and are mostly 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 represent one of the largest fire hundreds in the building. This point is actually value considering 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 usually are not typically flame retardant and naturally have a excessive hearth load. Cable producers make them flame retardant by including compounds and chemical substances. Certainly this improves the volatility of burning but the gasoline content material of the base polymers remains.
Tables 1 and a pair of above examine the fire load in MJ/Kg for common cable insulating materials against 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 quantity in kilometers and tons of cables installed in our buildings and the related fireplace load of the insulations is considerable. This is particularly essential in tasks with long egress instances like excessive rise, public buildings, tunnels and underground environments, airports, hospitals and so on.
When contemplating fireplace safety we should first perceive an important factors. Fire experts tell us most fire associated deaths in buildings are caused by smoke inhalation, temperature rise and oxygen depletion or by trauma caused by leaping in trying to escape these results.
Smoke
The first and most necessary facet of smoke is how a lot smoke? Typically the bigger the hearth the more smoke is generated so anything we can do to scale back the unfold of fire will also correspondingly cut back the quantity of smoke.
Smoke will contain 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 cause oxygen ranges to drop, this contributes to incomplete burning and smoldering which produces elevated quantities of smoke and toxic byproducts together with CO and CO2. Presence of halogenated materials will release poisonous Halides like Hydrogen Chloride along with many different toxic and flammable gasses in the smoke.
For this cause widespread smoke checks conducted on cable insulation supplies in massive 3 meter3 chambers with loads of air can provide misleading smoke figures as a outcome of full burning will usually release considerably less smoke than partial incomplete burning which is most likely going in practice. Simply specifying IEC 61034 with an outlined obscuration value then pondering this can provide a low smoke setting throughout hearth may sadly be little of assist for the people actually involved.
Halogens, Toxicity, Fuel Element, Oxygen Depletion and Temperature Rise
It is concerning that Europe and other countries adopt the idea of halogen free materials with out properly addressing the topic of toxicity. Halogens released throughout combustion are extremely poisonous but so too is carbon monoxide and this is not a halogen gasoline. It is frequent to call for halogen free cables after which enable using Polyethylene as a end result of it is halogen free. Burning Polyethylene (which may be seen from the table above has the highest MJ gasoline load per Kg of all insulations) will generate virtually 3 times extra warmth than an equivalent PVC cable. This means is that burning polyethylene will not solely generate virtually three times extra warmth but also eat almost 3 times extra oxygen and produce significantly extra carbon monoxide. Given carbon monoxide is answerable for most toxicity deaths in fires this situation is at greatest alarming!
The fuel elements proven in the table above indicate the amount of warmth which might be generated by burning 1kg of the common cable insulations tabled. Certainly this heat will speed up the burning of other adjoining supplies and may help spread the hearth in a building but importantly, to have the ability to generate the heat energy, oxygen needs to be consumed. The greater the warmth of combustion the more oxygen is required, so by choosing insulations with excessive gas components is adding considerably to no less than 4 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 metallic conduits. This will definitely help flame spread and decrease smoke as a outcome of contained in the conduit oxygen is restricted; nevertheless this is not an answer. As said previously, lots of the gasses from the decomposing polymeric insulations inside the conduits are highly flammable and toxic. These gases will migrate alongside the conduits to junction boxes, switch panels, distribution boards, motor control centers, 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, swap or relay inflicting the fire to spread to another location.
Conclusion
The recognition of “Halogen Free” whereas ignoring the opposite toxic elements of fireside is a transparent admission we do not perceive the topic well nor can we simply outline the risks of mixed poisonous elements or human physiological response to them. It is important however, that we do not continue to design with only half an understanding of the problem. While no good answer exists for natural based cables, we can actually reduce these critically important results of fire threat:
One choice maybe to choose cable insulations and jacket materials that are halogen free and have a low gas element, then set up them in metal conduit or possibly the American strategy is healthier: to use highly halogenated insulations in order that in case of fireplace any flame spread is minimized.
For most energy, control, communication and knowledge circuits there’s one complete solution out there for all the issues raised on this paper. It is an answer which has been used reliably for over 80 years. MICC cables can provide a total and full reply to all the issues associated with the hearth safety of organic polymer cables.
The copper jacket, magnesium oxide insulation and copper conductors of MICC ensure the cable is successfully fireplace proof. MICC cables have no natural content 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 cannot generate any halogen or toxic gasses at all including CO.
Unfortunately many common cable fireplace test strategies used at present may inadvertently mislead folks into believing the polymeric flexible cable merchandise they purchase and use will perform as anticipated in all fire conditions. As outlined in this paper, sadly this will not be correct.
For more information, go to www.temperature-house.com
Share

Leave a Comment