Fire Performance of Electric Cables

Often the best flame retardant cables are halogenated as a outcome of both the insulation and outer Jacket are flame retardant but once we want Halogen Free cables we discover it’s typically solely the outer jacket which is flame retardant and the internal insulation isn’t.
This has significance as a result of while cables with a flame retardant outer jacket will usually move flame retardance checks with exterior flame, the identical cables when subjected to high overload or extended brief circuits have proved in university tests to be extremely flammable and might even start a fireplace. Cancel anytime is understood and printed (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 check protocols for this seemingly frequent occasion and one cited by each authorities and media as explanation for constructing fires.
Further, in Flame Retardant take a look at methods corresponding to IEC60332 elements 1 & 3 which employ an external flame source, the cable samples aren’t pre-conditioned to normal operating temperature but tested at room temperature. This oversight is important particularly for energy circuits as a result of 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 extra easily it’s going to propagate fireplace.
It would seem that a want exists to re-evaluate present cable flame retardance check methods as these are generally understood by consultants and customers alike to supply a reliable indication of a cables capability to retard the propagation of fireside.
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 because Americans aren’t correctly informed of the hazards; quite the method taken is that: “It is healthier to have highly flame retardant cables which don’t propagate hearth than minimally flame retardant cables which can unfold a fire” – (a small fire with some halogen could additionally be higher than a large hearth 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 international locations all over the world undertake a special mentality: Halogen Free and Flame Retardant. Whilst this is an admirable mandate the reality is somewhat totally different: Flame propagation checks for cables as adopted in UK and Europe can arguably be stated to be much less stringent than a few of the flame propagation exams for cables in USA resulting in the conclusion that widespread tests in UK and Europe might simply be checks the cables can pass rather than checks the cables ought to pass.
For most flexible polymeric cables the selection stays right now between excessive flame propagation performance with halogens or lowered flame propagation performance without halogens.
Enclosing cables in metal conduit will cut back propagation on the level of fireplace but hydrocarbon primarily based combustion gasses from decomposing polymers are probably propagate by way of the conduits to switchboards, distribution boards and junction boxes in other parts of the constructing. Any spark such because the opening or closing of circuit breakers, or contactors is prone to ignite the flamable gasses resulting in explosion and spreading the fireplace to another location.
While MICC (Mineral Insulated Metal Sheathed) cables would supply a solution, there is often no singe excellent reply for each set up so designers want to judge the required efficiency on a “project-by-project” foundation to resolve which know-how is optimal.
The primary importance of fire load
Inside all buildings and initiatives electric cables provide the connectivity which retains lights on, air-conditioning working and the lifts running. It powers computer systems, office tools and offers the connection for our telephone and computer systems. Even our cellphones want to attach with wi-fi or GSM antennas that are related to the telecom community by fiber optic or copper cables. Cables ensure our safety by connecting
fire alarms, emergency voice communication, CCTV, smoke shutters, air pressurization followers, emergency lighting, fireplace 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 safety features such as flame retardance to ensure the cables don’t easily spread fireplace, circuit integrity during hearth in order that important fire-fighting and life security gear keep 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 expect that by requesting these particular properties the cables we buy and set up shall be safer
Because cables are put in by many different trades for different applications and are principally 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 symbolize one of many biggest fireplace loads in the building. This point is definitely worth thinking more about.
PVC, XLPE, EPR, CSP, LSOH (Low Smoke Zero Halogen) and even HFFR (Halogen Free Flame Retardant) cable supplies are principally based on hydrocarbon polymers. These base supplies are not usually flame retardant and naturally have a high fireplace load. Cable producers make them flame retardant by including compounds and chemicals. Certainly this improves the volatility of burning however the fuel content of the bottom polymers remains.
Tables 1 and 2 above examine the fireplace load in MJ/Kg for common cable insulating materials towards some frequent fuels. The Heat Release Rate and volatility in air for these supplies will differ however the gas added to a fireplace per kilogram and the consequential volume of warmth 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 important in initiatives with long egress times like high rise, public buildings, tunnels and underground environments, airports, hospitals etc.
When contemplating hearth safety we must first understand the most important factors. Fire specialists inform us most fire associated deaths in buildings are brought on by smoke inhalation, temperature rise and oxygen depletion or by trauma brought on by leaping in attempting to flee these results.
The first and most essential facet of smoke is how a lot smoke? Typically the larger the hearth the extra smoke is generated so anything we are able to do to scale back the spread of fire may even correspondingly reduce the amount of smoke.
Smoke will include particulates of carbon, ash and other solids, liquids and gasses, many are toxic 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 increased quantities of smoke and toxic byproducts including CO and CO2. Presence of halogenated supplies will release toxic Halides like Hydrogen Chloride along with many different toxic and flammable gasses within the smoke.
For this purpose frequent smoke tests conducted on cable insulation supplies in massive 3 meter3 chambers with loads of air can provide deceptive smoke figures because full burning will typically launch considerably less smoke than partial incomplete burning which is likely in apply. Simply specifying IEC 61034 with an outlined obscuration value then considering it will provide a low smoke setting during fireplace may unfortunately be little of help for the individuals really concerned.
Halogens, Toxicity, Fuel Element, Oxygen Depletion and Temperature Rise
It is regarding that Europe and different countries adopt the concept of halogen free supplies with out correctly addressing the subject of toxicity. Halogens launched during combustion are extremely poisonous however so too is carbon monoxide and this isn’t a halogen fuel. It is widespread to name for halogen free cables after which enable the usage of Polyethylene because it’s halogen free. Burning Polyethylene (which could be seen from the table above has the highest MJ gas load per Kg of all insulations) will generate nearly three times more heat than an equal PVC cable. This means is that burning polyethylene is not going to solely generate virtually 3 occasions extra warmth but additionally eat virtually 3 times more oxygen and produce significantly more carbon monoxide. Given carbon monoxide is answerable for most toxicity deaths in fires this case is at finest alarming!
The fuel components shown in the desk above point out the amount of heat which will be generated by burning 1kg of the common cable insulations tabled. Certainly this warmth will speed up the burning of other adjacent supplies and should assist spread the hearth in a constructing however importantly, so as to generate the heat vitality, oxygen needs to be consumed. The greater the warmth of combustion the extra oxygen is needed, so by choosing insulations with excessive gas 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 unfold and reduce smoke as a result of inside the conduit oxygen is proscribed; however this isn’t an answer. As stated previously, many of the gasses from the decomposing polymeric insulations contained in the conduits are highly flammable and toxic. These gases will migrate alongside the conduits to junction boxes, swap panels, distribution boards, motor control facilities, lamps, switches, and so on. On getting 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 hearth to unfold to another location.
The popularity of “Halogen Free” whereas ignoring the opposite toxic components of fire is a clear admission we do not perceive the topic properly nor can we simply outline the risks of combined poisonous parts or human physiological response to them. It is essential nevertheless, that we do not proceed to design with solely half an understanding of the issue. While no perfect solution exists for organic primarily based cables, we can definitely decrease these critically essential effects of fire danger:
One possibility perhaps to choose cable insulations and jacket materials which are halogen free and have a low gas factor, then install them in steel conduit or maybe the American approach is best: to make use of highly halogenated insulations in order that in case of fire any flame spread is minimized.
For most energy, control, communication and data circuits there might be 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 complete 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 make positive the cable is effectively hearth proof. MICC cables don’t have any organic content so simply cannot propagate flame or generate any smoke. The zero gasoline load ensures no warmth is added and no oxygen is consumed.
Being inorganic MICC cables can’t generate any halogen or toxic gasses at all including CO.
Unfortunately many widespread cable fireplace check methods used today might inadvertently mislead people into believing the polymeric flexible cable products they buy and use will carry out as anticipated in all fireplace conditions. As outlined on this paper, sadly this is in all probability not right.
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