Fire Performance of Electric Cables

Often one of the best flame retardant cables are halogenated as a outcome of both the insulation and outer Jacket are flame retardant however when we need Halogen Free cables we discover it is often only the outer jacket which is flame retardant and the inner insulation isn’t.
This has significance because while cables with a flame retardant outer jacket will typically cross flame retardance checks with external flame, the same cables when subjected to high overload or extended short circuits have proved in university tests to be highly flammable and might even start a fireplace. This effect is thought and revealed (8th International Conference on Insulated Power Cables (Jicable’11 – June 2011) held in Versailles, France) so it’s perhaps shocking that there are no widespread take a look at protocols for this seemingly frequent occasion and one cited by both authorities and media as reason for constructing fires.
Further, in Flame Retardant take a look at strategies corresponding to IEC60332 parts 1 & three which employ an exterior flame supply, the cable samples usually are not pre-conditioned to normal operating temperature however examined at room temperature. This oversight is important especially for power 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 significantly affected by its starting temperature i.e.: The hotter the cable is, the more easily it’ll propagate hearth.
It would appear that a need exists to re-evaluate present cable flame retardance check methods as these are generally understood by consultants and consumers alike to offer a reliable indication of a cables capability to retard the propagation of fireplace.
If we can’t trust the Standards what can we do?
In the USA many constructing standards don’t require halogen free cables. Certainly this is not as a result of Americans usually are not properly informed of the dangers; somewhat the method taken is that: “It is better to have extremely flame retardant cables which don’t propagate fireplace than minimally flame retardant cables which may unfold a fire” – (a small hearth with some halogen may be better than a large hearth without halogens). One of the most effective methods to make a cable insulation and cable jacket extremely flame retardant is by using halogens.
Europe and tons of countries all over the world adopt a different mentality: Halogen Free and Flame Retardant. Whilst this is an admirable mandate the fact is somewhat totally different: Flame propagation tests for cables as adopted in UK and Europe can arguably be stated to be less stringent than a variety of the flame propagation tests for cables in USA leading to the conclusion that widespread checks in UK and Europe might merely be checks the cables can pass rather than checks the cables ought to pass.
For most flexible polymeric cables the selection remains right now between high flame propagation efficiency with halogens or lowered flame propagation efficiency without halogens.
Enclosing cables in steel conduit will cut back propagation on the level of fire however hydrocarbon primarily based combustion gasses from decomposing polymers are likely propagate by way of the conduits to switchboards, distribution boards and junction packing containers in different parts of the constructing. Any spark such because the opening or closing of circuit breakers, or contactors is likely to ignite the flamable 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 typically no singe excellent reply for every set up so designers want to judge the required performance on a “project-by-project” foundation to decide which expertise is optimal.
The primary importance of fireside load
Inside all buildings and projects electric cables provide the connectivity which keeps lights on, air-conditioning working and the lifts working. It powers computers, workplace tools and offers the connection for our phone and computers. Even our mobile phones want to connect with wi-fi or GSM antennas which are related to the telecom community 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, fireplace sprinkler pumps, smoke and heat detectors, and so many other features of a contemporary Building Management System.
Where public safety is essential we frequently request cables to have added security features such as flame retardance to ensure the cables don’t easily spread hearth, circuit integrity throughout fireplace so that important fire-fighting and life security tools keep working. Sometimes we may recognize that the combustion of electric cables produces smoke and this can be poisonous so we call for cables to be Low Smoke and Halogen Free. Logically and intuitively we think that by requesting these particular properties the cables we purchase and install shall be safer
Because cables are installed by many various trades for various applications 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 symbolize one of many greatest hearth hundreds in the constructing. This level is actually price considering more about.
PVC, XLPE, EPR, CSP, LSOH (Low Smoke Zero Halogen) and even HFFR (Halogen Free Flame Retardant) cable materials are principally based on hydrocarbon polymers. These base materials are not typically flame retardant and naturally have a high hearth load. Cable producers make them flame retardant by including compounds and chemical compounds. Certainly this improves the volatility of burning but the fuel content of the bottom polymers stays.
Tables 1 and a couple of above compare the fireplace load in MJ/Kg for widespread cable insulating materials in opposition to some common 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 volume in kilometers and tons of cables put in in our buildings and the associated fire load of the insulations is considerable. This is especially essential in tasks with lengthy egress occasions like high rise, public buildings, tunnels and underground environments, airports, hospitals and so forth.
When considering fireplace security we should first perceive the most important elements. Fire specialists inform us most fireplace associated deaths in buildings are attributable to smoke inhalation, temperature rise and oxygen depletion or by trauma attributable to jumping in attempting to flee these results.
The first and most essential facet of smoke is how much smoke? Typically the larger the fire the extra smoke is generated so anything we can do to reduce the unfold of fireside may also correspondingly cut back the amount of smoke.
Smoke will comprise particulates of carbon, ash and different solids, liquids and gasses, many are toxic and flamable. In particular, 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 toxic byproducts together with CO and CO2. Presence of halogenated materials will release poisonous Halides like Hydrogen Chloride together with many other toxic and flammable gasses in the smoke.
For this cause frequent smoke tests performed on cable insulation supplies in large 3 meter3 chambers with loads of air can present deceptive smoke figures because full burning will typically launch significantly much less smoke than partial incomplete burning which is most likely going in apply. Simply specifying IEC 61034 with an outlined obscuration value then thinking this can present a low smoke surroundings throughout fire could unfortunately be little of assist for the individuals actually involved.
Halogens, Toxicity, Fuel Element, Oxygen Depletion and Temperature Rise
It is regarding that Europe and different countries undertake the concept of halogen free materials with out correctly addressing the subject of toxicity. Halogens released throughout combustion are extremely toxic however so too is carbon monoxide and this is not a halogen gasoline. It is widespread to call for halogen free cables and then permit using Polyethylene as a result of it’s halogen free. Burning Polyethylene (which can be seen from the table above has the very best MJ gas load per Kg of all insulations) will generate nearly 3 occasions more heat than an equal PVC cable. This means is that burning polyethylene will not only generate nearly 3 occasions extra heat but in addition consume almost 3 instances extra oxygen and produce considerably extra carbon monoxide. Given carbon monoxide is answerable for most toxicity deaths in fires this case is at finest alarming!
The fuel components proven within the desk above indicate the amount of heat which might be generated by burning 1kg of the widespread cable insulations tabled. Certainly this warmth will speed up the burning of other adjoining supplies and will help unfold the hearth in a constructing however importantly, in order to generate the warmth vitality, oxygen must be consumed. เกจวัดแก๊ส of combustion the extra oxygen is required, so by choosing insulations with excessive fuel elements is including significantly to at least 4 of the primary dangers of fires: Temperature Rise, Oxygen Depletion, Flame Spread and Carbon Monoxide Release.
Perhaps it’s best to put in polymeric cables inside metallic conduits. This will definitely help flame unfold and reduce smoke because inside the conduit oxygen is proscribed; however this isn’t a solution. As mentioned previously, many 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, change panels, distribution boards, motor control centers, lamps, switches, etc. On coming into the gases can ignite or explode with any arcing such as the make/break of a circuit breaker, contactor, switch or relay inflicting the fireplace to unfold to a different location.
The recognition of “Halogen Free” whereas ignoring the other toxic components of fireside is a clear admission we don’t perceive the topic nicely nor can we easily define the risks of mixed toxic elements or human physiological response to them. It is important nevertheless, that we don’t continue to design with only half an understanding of the issue. While เครื่องมือวัดpressure exists for organic primarily based cables, we can actually minimize these critically necessary effects of fireside threat:
One possibility maybe to choose cable insulations and jacket materials which are halogen free and have a low gasoline component, then set up them in metal conduit or maybe the American approach is healthier: to use extremely halogenated insulations so that in case of fire any flame unfold is minimized.
For most energy, management, communication and data circuits there could be one full answer obtainable for all the issues raised in this paper. It is a solution which has been used reliably for over 80 years. MICC cables can provide a total and complete answer to all the problems related to the fire safety of organic polymer cables.
The copper jacket, magnesium oxide insulation and copper conductors of MICC ensure the cable is effectively fire proof. MICC cables don’t have any organic content material so merely can not propagate flame or generate any smoke. The zero gas load ensures no warmth is added and no oxygen is consumed.
Being inorganic MICC cables cannot generate any halogen or toxic gasses at all including CO.
Unfortunately many frequent cable fireplace test methods used at present could inadvertently mislead people into believing the polymeric versatile cable merchandise they purchase and use will perform as expected in all fireplace situations. As outlined in this paper, sadly this is in all probability not right.
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