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 but when we want Halogen Free cables we find it is usually only the outer jacket which is flame retardant and the inside insulation isn’t.
This has significance because whereas cables with a flame retardant outer jacket will usually pass flame retardance exams with exterior flame, the same cables when subjected to excessive overload or prolonged quick circuits have proved in college tests to be highly flammable and might even start a hearth. This impact is thought and published (8th International Conference on Insulated Power Cables (Jicable’11 – June 2011) held in Versailles, France) so it is perhaps surprising that there are no frequent test protocols for this seemingly frequent event and one cited by both authorities and media as explanation for building fires.
Further, in Flame Retardant test strategies corresponding to IEC60332 elements 1 & 3 which employ an external flame source, the cable samples aren’t pre-conditioned to normal operating temperature however examined at room temperature. This oversight is essential especially for energy circuits as a end result of the temperature index of the cable (the temperature at which the cable material will self-support combustion in normal air) shall be considerably affected by its starting temperature i.e.: The hotter the cable is, the extra simply it will propagate fireplace.
It would seem that a need exists to re-evaluate current cable flame retardance check 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 fire.
If we can’t belief the Standards what will we do?
In the USA many building standards don’t require halogen free cables. Certainly this isn’t as a end result of Americans usually are not properly knowledgeable of the risks; somewhat the approach taken is that: “It is healthier to have extremely flame retardant cables which do not propagate fireplace than minimally flame retardant cables which may spread a fire” – (a small hearth with some halogen may be higher than a big fire with out halogens). One of one of the best ways to make a cable insulation and cable jacket highly flame retardant is through the use of halogens.
Europe and lots of countries around the world undertake a different mentality: Halogen Free and Flame Retardant. Whilst this is an admirable mandate the fact is somewhat completely different: Flame propagation checks 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 leading to the conclusion that widespread checks in UK and Europe may merely be exams the cables can cross quite than checks the cables ought to move.
For most flexible polymeric cables the selection remains at present between excessive flame propagation performance with halogens or reduced flame propagation performance without halogens.
Enclosing cables in steel conduit will reduce propagation on the point of fire but hydrocarbon primarily based combustion gasses from decomposing polymers are doubtless propagate through the conduits to switchboards, distribution boards and junction packing containers in different elements of the constructing. Any spark such as the opening or closing of circuit breakers, or contactors is prone to ignite the flamable gasses leading to explosion and spreading the fireplace to another location.
While MICC (Mineral Insulated Metal Sheathed) cables would offer a solution, there’s often no singe perfect answer for every set up so designers need to evaluate the required efficiency on a “project-by-project” basis to determine which technology is optimal.
Covert of fireplace load
Inside all buildings and projects electrical cables present the connectivity which retains lights on, air-conditioning working and the lifts running. It powers computer systems, workplace equipment and supplies the connection for our phone and computers. Even our cell phones want to attach with wireless or GSM antennas that are connected to the telecom community 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 heat detectors, and so many different options of a modern Building Management System.
Where public security is important we regularly request cables to have added security options such as flame retardance to ensure the cables don’t easily spread hearth, circuit integrity throughout hearth in order that essential fire-fighting and life security equipment maintain working. Sometimes we may recognize that the combustion of electric cables produces smoke and this can 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 buy and install might be safer
Because cables are installed by many different trades for various functions and are principally hidden or embedded in our constructions, what is commonly not realized is that the various miles of cables and tons of plastic polymers which make up the cables can represent one of the biggest fireplace loads in the constructing. This level is certainly worth considering extra 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 usually are not typically flame retardant and naturally have a excessive fire load. Cable manufacturers make them flame retardant by including compounds and chemical substances. Certainly this improves the volatility of burning however the gas content material of the base polymers stays.
Tables 1 and a couple of above evaluate the fire load in MJ/Kg for common cable insulating materials against some common fuels. The Heat Release Rate and volatility in air for these materials will differ however the fuel added to a fire per kilogram and the consequential quantity of heat generated and oxygen consumed is relative.
The quantity in kilometers and tons of cables put in in our buildings and the associated hearth load of the insulations is appreciable. This is particularly necessary in tasks with lengthy egress instances like high rise, public buildings, tunnels and underground environments, airports, hospitals and so forth.
When contemplating fire safety we should first perceive an important factors. Fire consultants tell us most hearth associated deaths in buildings are caused by smoke inhalation, temperature rise and oxygen depletion or by trauma attributable to jumping in attempting to escape these effects.
The first and most necessary side of smoke is how a lot smoke? Typically the larger the fire the more smoke is generated so something we can do to reduce the spread of fireplace will also correspondingly cut back the quantity of smoke.
Smoke will comprise particulates of carbon, ash and other solids, liquids and gasses, many are poisonous and flamable. In particular, 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 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 reason common smoke checks conducted on cable insulation supplies in giant 3 meter3 chambers with plenty of air can present deceptive smoke figures as a end result of complete burning will usually release 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 pondering this can present a low smoke setting throughout fireplace could unfortunately be little of help for the individuals really involved.
Halogens, Toxicity, Fuel Element, Oxygen Depletion and Temperature Rise
It is regarding that Europe and different nations adopt the concept of halogen free supplies with out properly addressing the subject of toxicity. Halogens launched during combustion are extraordinarily toxic but so too is carbon monoxide and this is not a halogen fuel. It is frequent to call for halogen free cables and then permit using Polyethylene as a end result of it is halogen free. Burning Polyethylene (which may be seen from the desk above has the best MJ gas load per Kg of all insulations) will generate almost three instances extra warmth than an equal PVC cable. This means is that burning polyethylene will not only generate almost 3 times extra warmth but additionally consume virtually 3 instances extra oxygen and produce considerably extra carbon monoxide. Given carbon monoxide is liable for most toxicity deaths in fires this situation is at best alarming!
The fuel elements shown within the table above point out the amount of warmth which might be generated by burning 1kg of the frequent cable insulations tabled. Certainly this warmth will speed up the burning of other adjoining supplies and may assist spread the hearth in a constructing but importantly, to have the ability to generate the heat energy, oxygen needs to be consumed. The higher the heat of combustion the extra oxygen is needed, so by choosing insulations with high gas elements is including significantly to at least 4 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 steel conduits. This will definitely assist flame spread and reduce smoke because inside the conduit oxygen is restricted; nevertheless this isn’t a solution. As mentioned beforehand, most of the gasses from the decomposing polymeric insulations contained in the conduits are extremely flammable and toxic. These gases will migrate along the conduits to junction boxes, swap panels, distribution boards, motor control facilities, lamps, switches, and so forth. On coming into the gases can ignite or explode with any arcing such because the make/break of a circuit breaker, contactor, switch or relay causing the fire to spread to a different location.
The popularity of “Halogen Free” while ignoring the opposite poisonous parts of fireside is a clear admission we do not perceive the topic properly nor can we easily define the hazards of mixed poisonous 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 problem. While no good resolution exists for natural primarily based cables, we are in a position to definitely minimize these critically necessary results of fireside risk:
One option perhaps to choose cable insulations and jacket supplies which are halogen free and have a low fuel factor, then set up them in metal conduit or perhaps the American strategy is better: to use highly halogenated insulations in order that in case of fire any flame spread is minimized.
For most energy, control, communication and knowledge circuits there is one full solution obtainable 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 complete and complete reply to all the problems associated with the fireplace safety of natural polymer cables.
The copper jacket, magnesium oxide insulation and copper conductors of MICC ensure the cable is effectively fireplace proof. MICC cables haven’t any organic content material so simply can’t propagate flame or generate any smoke. The zero gas load ensures no warmth is added and no oxygen is consumed.
Being inorganic MICC cables can not generate any halogen or poisonous gasses at all including CO.
Unfortunately many frequent cable fireplace take a look at strategies used today could inadvertently mislead people into believing the polymeric versatile cable products they purchase and use will carry out as anticipated in all fire conditions. As outlined in this paper, sadly this is in all probability not right.
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