Fire Performance of Electric Cables

Often the most effective flame retardant cables are halogenated as a outcome of each the insulation and outer Jacket are flame retardant but once we want Halogen Free cables we discover it is often only the outer jacket which is flame retardant and the inside insulation isn’t.
This has significance as a end result of whereas cables with a flame retardant outer jacket will often move flame retardance checks with exterior flame, the same cables when subjected to high overload or extended quick circuits have proved in university exams to be extremely flammable and may even start a hearth. This impact is known and printed (8th International Conference on Insulated Power Cables (Jicable’11 – June 2011) held in Versailles, France) so it’s perhaps shocking that there are not any common check protocols for this seemingly frequent event and one cited by both authorities and media as reason for building fires.
Further, in Flame Retardant test strategies such as IEC60332 parts 1 & three which employ an external flame source, the cable samples are not pre-conditioned to regular operating temperature but tested at room temperature. This oversight is necessary 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 regular air) will be considerably affected by its starting temperature i.e.: The hotter the cable is, the extra easily it will propagate fire.
It would seem that a need exists to re-evaluate present cable flame retardance test methods as these are commonly understood by consultants and customers alike to supply a reliable indication of a cables ability to retard the propagation of fireside.
If we can’t belief the Standards what do we do?
In the USA many constructing requirements don’t require halogen free cables. Certainly this is not as a result of Americans usually are not correctly informed of the dangers; rather the strategy taken is that: “It is better to have highly flame retardant cables which don’t propagate fireplace than minimally flame retardant cables which can unfold a fire” – (a small hearth with some halogen may 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 plenty of international locations all over the world adopt a unique mentality: Halogen Free and Flame Retardant. Whilst that is an admirable mandate the reality is rather completely different: Flame propagation tests for cables as adopted in UK and Europe can arguably be said to be less stringent than a few of the flame propagation exams for cables in USA resulting in the conclusion that common tests in UK and Europe may simply be tests the cables can move rather than exams the cables should move.
For most versatile polymeric cables the choice stays today between high flame propagation efficiency with halogens or decreased flame propagation performance without halogens.
Enclosing cables in metal conduit will reduce propagation at the level of fire but hydrocarbon based combustion gasses from decomposing polymers are likely propagate through the conduits to switchboards, distribution boards and junction boxes in different components of the building. Any spark such because the opening or closing of circuit breakers, or contactors is likely to ignite the flamable gasses resulting in explosion and spreading the fire to another location.
While MICC (Mineral Insulated Metal Sheathed) cables would supply an answer, there’s often no singe perfect reply for each set up so designers want to judge the required performance on a “project-by-project” basis to resolve which expertise is optimal.
The primary importance of fire load
Inside all buildings and tasks electrical cables provide the connectivity which keeps lights on, air-conditioning working and the lifts operating. It powers computer systems, office equipment and offers the connection for our telephone and computer systems. Even our cell phones need to connect with wi-fi or GSM antennas which are linked to the telecom community by fiber optic or copper cables. Cables guarantee our safety by connecting
fireplace alarms, emergency voice communication, CCTV, smoke shutters, air pressurization followers, emergency lighting, hearth sprinkler pumps, smoke and heat detectors, and so many different options of a modern Building Management System.
Where public security is essential we often request cables to have added safety options similar to flame retardance to make sure the cables do not easily spread fireplace, circuit integrity throughout fireplace so that essential fire-fighting and life security gear hold working. Sometimes we could 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 think that by requesting these particular properties the cables we buy and install shall be safer
Because cables are put in by many alternative trades for different functions and are largely 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 represent one of many greatest fireplace loads in the building. This point is certainly value pondering extra about.
PVC, XLPE, EPR, CSP, LSOH (Low Smoke Zero Halogen) and even HFFR (Halogen Free Flame Retardant) cable materials are mostly based on hydrocarbon polymers. These base supplies usually are not usually flame retardant and naturally have a excessive hearth load. Cable manufacturers make them flame retardant by adding compounds and chemical substances. Certainly this improves the volatility of burning however the gas content of the base polymers stays.
Tables 1 and 2 above examine the hearth load in MJ/Kg for frequent cable insulating supplies towards some common fuels. The Heat Release Rate and volatility in air for these supplies will differ but the gasoline added to a hearth per kilogram and the consequential quantity of warmth generated and oxygen consumed is relative.
The volume in kilometers and tons of cables installed in our buildings and the associated hearth load of the insulations is considerable. This is especially necessary in projects with lengthy egress instances like excessive rise, public buildings, tunnels and underground environments, airports, hospitals and so on.
When contemplating fire security we must first understand crucial components. Fire consultants tell us most hearth associated deaths in buildings are attributable to smoke inhalation, temperature rise and oxygen depletion or by trauma brought on by leaping in making an attempt to flee these effects.
The first and most essential facet of smoke is how much smoke? Typically the bigger the fireplace the extra smoke is generated so something we will do to minimize back the spread 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 toxic and combustible. 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 toxic byproducts including CO and CO2. Presence of halogenated supplies will release poisonous Halides like Hydrogen Chloride together with many other poisonous and flammable gasses within the smoke.
For this reason common smoke tests conducted on cable insulation supplies in massive 3 meter3 chambers with loads of air can present deceptive smoke figures as a end result of full burning will typically release significantly less smoke than partial incomplete burning which is most likely going in apply. Simply specifying IEC 61034 with a defined obscuration worth then thinking this will provide a low smoke surroundings throughout fire may unfortunately be little of help for the folks truly involved.
Halogens, Toxicity, Fuel Element, Oxygen Depletion and Temperature Rise
It is concerning that Europe and different nations undertake the idea of halogen free materials without properly addressing the topic of toxicity. Halogens launched during combustion are extremely poisonous however so too is carbon monoxide and this isn’t a halogen fuel. It is common to call for halogen free cables and then allow the utilization of Polyethylene as a result of it is halogen free. Burning Polyethylene (which may be seen from the table above has the highest MJ gas load per Kg of all insulations) will generate virtually three instances more warmth than an equal PVC cable. This means is that burning polyethylene will not only generate virtually 3 instances more heat but in addition eat almost three instances extra oxygen and produce considerably extra carbon monoxide. Given carbon monoxide is responsible for most toxicity deaths in fires this case is at finest alarming!
The fuel components proven in the desk above indicate the amount of warmth which shall be generated by burning 1kg of the frequent cable insulations tabled. Certainly this warmth will accelerate the burning of different adjacent materials and will assist unfold the fire in a building but importantly, so as to generate the heat energy, oxygen must be consumed. The greater the warmth of combustion the extra oxygen is required, so by choosing insulations with high fuel components is adding considerably to at least 4 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 certainly help flame unfold and reduce smoke as a result of contained in the conduit oxygen is restricted; however this is not an answer. As stated beforehand, many of the gasses from the decomposing polymeric insulations contained in the conduits are highly flammable and toxic. These gases will migrate along the conduits to junction bins, switch panels, distribution boards, motor control centers, 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 causing the fire to unfold to another location.
The popularity of “Halogen Free” while ignoring the opposite poisonous components of fireplace is a transparent admission we do not understand the subject properly nor can we easily define the dangers of mixed poisonous elements or human physiological response to them. It is necessary nevertheless, that we do not proceed to design with only half an understanding of the issue. While New exists for natural based cables, we can definitely decrease these critically important results of fireplace risk:
One choice possibly to choose on cable insulations and jacket materials that are halogen free and have a low gasoline element, then install them in metal conduit or perhaps the American method is best: to make use of extremely halogenated insulations so that in case of fireplace any flame spread is minimized.
For most power, control, communication and information circuits there could be one full answer out there for all the problems raised in this paper. It is an answer which has been used reliably for over 80 years. MICC cables can present a total and complete reply to all the problems associated with the fireplace security of natural polymer cables.
The copper jacket, magnesium oxide insulation and copper conductors of MICC ensure the cable is successfully fire proof. MICC cables don’t have any organic content material 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 poisonous gasses at all including CO.
Unfortunately many frequent cable fire check strategies used right now could inadvertently mislead people into believing the polymeric versatile cable products they purchase and use will perform as anticipated in all fire conditions. As outlined in this paper, sadly this may not be appropriate.
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