Fire Performance of Electric Cables

Often the best flame retardant cables are halogenated as a result of each the insulation and outer Jacket are flame retardant however when we need Halogen Free cables we discover it’s usually only the outer jacket which is flame retardant and the internal insulation isn’t.
This has significance as a end result of whereas cables with a flame retardant outer jacket will usually move flame retardance checks with external flame, the identical cables when subjected to excessive overload or prolonged brief circuits have proved in university checks to be highly flammable and may even begin a fire. This effect is understood and published (8th International Conference on Insulated Power Cables (Jicable’11 – June 2011) held in Versailles, France) so it’s perhaps stunning that there are not any common take a look at protocols for this seemingly widespread event and one cited by each authorities and media as cause of building fires.
Further, in Flame Retardant test methods similar to IEC60332 elements 1 & 3 which make use of an exterior flame source, the cable samples aren’t pre-conditioned to regular working temperature however examined at room temperature. This oversight is important particularly 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 regular air) will be considerably affected by its starting temperature i.e.: The hotter the cable is, the extra simply it will propagate hearth.
It would appear that a want exists to re-evaluate present cable flame retardance check methods as these are generally understood by consultants and consumers alike to supply a reliable indication of a cables capacity to retard the propagation of fireplace.
If we can’t belief 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 are not correctly informed of the dangers; rather the approach taken is that: “It is better to have highly flame retardant cables which do not propagate fire than minimally flame retardant cables which can unfold a fire” – (a small hearth with some halogen could also be better than a large hearth with out halogens). One of the most effective methods to make a cable insulation and cable jacket highly flame retardant is by utilizing halogens.
Europe and lots of countries around the globe adopt a unique mentality: Halogen Free and Flame Retardant. Whilst this is an admirable mandate the reality is rather different: Flame propagation checks for cables as adopted in UK and Europe can arguably be mentioned to be less stringent than some of the flame propagation tests for cables in USA leading to the conclusion that common checks in UK and Europe may simply be checks the cables can move quite than checks the cables should move.
For most flexible polymeric cables the selection remains right now between excessive flame propagation efficiency with halogens or lowered flame propagation performance without halogens.
Enclosing cables in metal conduit will reduce propagation at the point of fire however hydrocarbon primarily based combustion gasses from decomposing polymers are probably propagate by way of 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 leading to explosion and spreading the fire to a different location.
While MICC (Mineral Insulated Metal Sheathed) cables would offer an answer, there may be often no singe excellent answer for every installation so designers want to judge the required performance on a “project-by-project” foundation to determine which expertise is optimal.
The main significance of fire load
Inside all buildings and tasks electric cables present the connectivity which keeps lights on, air-conditioning working and the lifts running. It powers computers, office gear and supplies the connection for our telephone and computer systems. Even our mobile phones need to connect with wireless or GSM antennas that are related to the telecom community by fiber optic or copper cables. Cables ensure our security 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 safety is important we often request cables to have added security features corresponding to flame retardance to make sure the cables don’t easily spread hearth, circuit integrity throughout fire in order that essential fire-fighting and life safety tools maintain working. Sometimes Feast could recognize that the combustion of electrical cables produces smoke and this could be poisonous 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 set up will be safer
Because cables are put in by many different trades for different applications 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 biggest fire hundreds within the constructing. This point is actually price considering extra about.
PVC, XLPE, EPR, CSP, LSOH (Low Smoke Zero Halogen) and even HFFR (Halogen Free Flame Retardant) cable supplies are mostly primarily based on hydrocarbon polymers. These base materials aren’t generally flame retardant and naturally have a excessive hearth load. Cable manufacturers make them flame retardant by including compounds and chemical substances. Certainly this improves the volatility of burning however the gasoline content material of the base polymers remains.
Tables 1 and a pair of above compare the fire load in MJ/Kg for frequent cable insulating supplies in opposition to some frequent fuels. The Heat Release Rate and volatility in air for these materials will differ but the fuel 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 related fireplace load of the insulations is considerable. This is particularly essential in projects with long egress occasions like high rise, public buildings, tunnels and underground environments, airports, hospitals etc.
When considering fire security we should first understand crucial factors. Fire specialists inform us most fireplace associated deaths in buildings are attributable to smoke inhalation, temperature rise and oxygen depletion or by trauma brought on by jumping in trying to escape these results.
The first and most necessary aspect of smoke is how a lot smoke? Typically the larger the fireplace the more smoke is generated so anything we will do to reduce the spread of fire may also correspondingly scale back the amount of smoke.
Smoke will comprise 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 cause oxygen levels to drop, this contributes to incomplete burning and smoldering which produces elevated quantities of smoke and toxic byproducts including CO and CO2. Presence of halogenated materials will release poisonous Halides like Hydrogen Chloride together with many different toxic and flammable gasses within the smoke.
For this purpose widespread smoke exams conducted on cable insulation materials in massive 3 meter3 chambers with loads of air can provide deceptive smoke figures because full burning will often launch significantly less smoke than partial incomplete burning which is in all probability going in practice. Simply specifying IEC 61034 with a defined obscuration value then thinking this will present a low smoke surroundings throughout fireplace may sadly be little of assist for the folks really concerned.
Halogens, Toxicity, Fuel Element, Oxygen Depletion and Temperature Rise
It is concerning that Europe and different international locations undertake the idea of halogen free materials without properly addressing the subject of toxicity. Halogens released during combustion are extremely poisonous however so too is carbon monoxide and this is not a halogen gas. It is common to call for halogen free cables and then enable the use of Polyethylene as a end result of it’s halogen free. Burning Polyethylene (which could be seen from the desk above has the very best MJ gasoline load per Kg of all insulations) will generate nearly three occasions extra heat than an equal PVC cable. This means is that burning polyethylene won’t solely generate nearly three occasions more heat but also devour almost three instances extra oxygen and produce considerably more carbon monoxide. Given carbon monoxide is answerable for most toxicity deaths in fires this situation is at greatest alarming!
The gasoline elements shown in the table above point out the amount of warmth which shall be generated by burning 1kg of the frequent cable insulations tabled. Certainly this heat will accelerate the burning of different adjoining supplies and may assist unfold the fireplace in a building but importantly, to be able to generate the heat power, oxygen must be consumed. The larger the heat of combustion the more oxygen is required, so by selecting insulations with high fuel parts is adding considerably to a minimal of four of the first risks of fires: Temperature Rise, Oxygen Depletion, Flame Spread and Carbon Monoxide Release.
Perhaps it’s best to install polymeric cables inside steel conduits. This will certainly assist flame unfold and minimize smoke as a result of contained in the conduit oxygen is proscribed; nonetheless this isn’t a solution. As mentioned beforehand, lots of the gasses from the decomposing polymeric insulations inside the conduits are extremely flammable and poisonous. These gases will migrate alongside the conduits to junction bins, switch panels, distribution boards, motor control centers, lamps, switches, and so on. On entering the gases can ignite or explode with any arcing such as the make/break of a circuit breaker, contactor, swap or relay causing the hearth to spread to a different location.
The popularity of “Halogen Free” while ignoring the opposite toxic elements of fireplace is a transparent admission we don’t understand the subject well nor can we simply define the hazards of combined toxic parts or human physiological response to them. It is important however, that we do not proceed to design with solely half an understanding of the issue. While no excellent solution exists for natural primarily based cables, we are able to certainly reduce these critically essential effects of fire danger:
One option possibly to choose on cable insulations and jacket materials that are halogen free and have a low gas factor, then install them in metal conduit or maybe the American method is healthier: to use highly halogenated insulations so that in case of fire any flame spread is minimized.
For most energy, management, communication and data circuits there could be one complete answer out there for all the problems raised in this paper. It is an answer which has been used reliably for over eighty years. MICC cables can provide a complete and full reply to all the problems related to 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 haven’t any organic 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 can’t generate any halogen or toxic gasses at all together with CO.
Unfortunately many common cable fireplace test methods used right now might inadvertently mislead people into believing the polymeric versatile cable merchandise they buy and use will perform as expected in all fireplace conditions. As outlined in this paper, sadly this may not be correct.
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