Fire Performance of Electric Cables

Often the most effective flame retardant cables are halogenated because both the insulation and outer Jacket are flame retardant but when we want Halogen Free cables we find it’s typically only the outer jacket which is flame retardant and the inside insulation is not.
This has significance as a end result of whereas cables with a flame retardant outer jacket will usually move flame retardance tests with exterior flame, the same cables when subjected to high overload or prolonged quick circuits have proved in university exams to be highly flammable and might even start a hearth. This impact is thought and revealed (8th International Conference on Insulated Power Cables (Jicable’11 – June 2011) held in Versailles, France) so it is maybe shocking that there aren’t any widespread test protocols for this seemingly common occasion and one cited by both authorities and media as cause of constructing fires.
Further, in Flame Retardant check methods corresponding to IEC60332 components 1 & three which employ an external flame source, the cable samples aren’t pre-conditioned to regular working temperature however tested at room temperature. This oversight is important particularly for power circuits as a 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 simply it’s going to propagate hearth.
It would seem that a need exists to re-evaluate present cable flame retardance test methods as these are generally understood by consultants and customers alike to offer a reliable indication of a cables capacity to retard the propagation of fire.
If we can’t trust the Standards what will we do?
In the USA many constructing requirements don’t require halogen free cables. Certainly this isn’t as a result of Americans aren’t wisely informed of the risks; quite the strategy taken is that: “It is healthier to have extremely flame retardant cables which don’t propagate fire than minimally flame retardant cables which may unfold a fire” – (a small fire with some halogen could also be higher than a big hearth with out halogens). One of the best ways to make a cable insulation and cable jacket highly flame retardant is by utilizing halogens.
Europe and lots of international locations around the world undertake a different 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 much less stringent than a few of the flame propagation tests for cables in USA resulting in the conclusion that widespread checks in UK and Europe might merely be tests the cables can pass rather than tests the cables should cross.
Conclusion
For most versatile polymeric cables the selection remains at present between excessive flame propagation performance with halogens or reduced flame propagation efficiency without halogens.
Enclosing cables in metal conduit will scale back propagation at the level of fire however hydrocarbon based mostly combustion gasses from decomposing polymers are doubtless propagate through the conduits to switchboards, distribution boards and junction packing containers in other parts of the building. Any spark such as the opening or closing of circuit breakers, or contactors is likely to ignite the combustible gasses leading to explosion and spreading the hearth to another location.
While MICC (Mineral Insulated Metal Sheathed) cables would offer an answer, there’s often no singe excellent reply for each installation so designers want to evaluate the required efficiency on a “project-by-project” foundation to resolve which expertise is perfect.
The major importance of fireside load
Inside all buildings and tasks electrical cables present the connectivity which keeps lights on, air-conditioning working and the lifts working. It powers computer systems, workplace tools and supplies the connection for our phone and computers. Even our mobile phones need to connect with wireless or GSM antennas which are connected to the telecom network 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, fire sprinkler pumps, smoke and warmth detectors, and so many other features of a modern Building Management System.
Where public security is necessary we often request cables to have added security options similar to flame retardance to ensure the cables don’t simply spread fire, circuit integrity during hearth so that essential fire-fighting and life security gear keep working. Sometimes we might acknowledge 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 special properties the cables we buy and install might be safer
Because cables are put in by many different trades for various applications 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 symbolize one of the biggest fire hundreds in the building. This point is definitely value thinking more about.
PVC, XLPE, EPR, CSP, LSOH (Low Smoke Zero Halogen) and even HFFR (Halogen Free Flame Retardant) cable supplies are largely primarily based on hydrocarbon polymers. These base supplies usually are not usually flame retardant and naturally have a high fire load. Cable producers make them flame retardant by including compounds and chemicals. Certainly this improves the volatility of burning but the fuel content of the base polymers remains.
Tables 1 and 2 above examine the fire load in MJ/Kg for frequent cable insulating supplies against some common fuels. The Heat Release Rate and volatility in air for these materials will differ however the gas added to a fire per kilogram and the consequential quantity of heat generated and oxygen consumed is relative.
The volume in kilometers and tons of cables put in in our buildings and the associated fireplace load of the insulations is appreciable. This is particularly essential in projects with lengthy egress occasions like high rise, public buildings, tunnels and underground environments, airports, hospitals and so on.
When contemplating fireplace security we must first perceive crucial components. Fire experts tell us most fire associated deaths in buildings are brought on by smoke inhalation, temperature rise and oxygen depletion or by trauma brought on by jumping in attempting to escape these effects.
Smoke
The first and most essential side of smoke is how a lot smoke? Typically the larger the hearth the extra smoke is generated so something we are able to do to reduce the unfold of fireside may also correspondingly scale back the quantity of smoke.
Smoke will comprise particulates of carbon, ash and different solids, liquids and gasses, many are toxic and combustible. 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 amounts of smoke and poisonous byproducts including CO and CO2. Presence of halogenated supplies will launch toxic Halides like Hydrogen Chloride together with many different toxic and flammable gasses within the smoke.
For this purpose common smoke checks performed on cable insulation supplies in giant 3 meter3 chambers with plenty of air can present misleading smoke figures because full burning will typically launch significantly less smoke than partial incomplete burning which is in all probability going in follow. Simply specifying IEC 61034 with an outlined obscuration worth then thinking it will provide a low smoke surroundings throughout fire could unfortunately be little of assist for the folks truly concerned.
Halogens, Toxicity, Fuel Element, Oxygen Depletion and Temperature Rise
It is regarding that Europe and other nations undertake the concept of halogen free supplies with out properly addressing the topic of toxicity. Halogens released throughout combustion are extraordinarily poisonous but so too is carbon monoxide and this is not a halogen fuel. It is common to name for halogen free cables after which allow using Polyethylene because it is halogen free. Burning Polyethylene (which could be seen from the table above has the very best MJ gas load per Kg of all insulations) will generate almost three times more warmth than an equivalent PVC cable. This means is that burning polyethylene will not only generate virtually 3 instances more warmth but additionally eat almost 3 instances extra oxygen and produce considerably extra carbon monoxide. Given carbon monoxide is responsible for most toxicity deaths in fires this example is at finest alarming!
The gasoline parts proven in the desk above point out the quantity of warmth which shall be generated by burning 1kg of the frequent cable insulations tabled. Certainly this heat will speed up the burning of different adjoining supplies and should assist spread the fireplace in a building however importantly, to find a way to generate the warmth energy, oxygen needs to be consumed. The higher the warmth of combustion the extra oxygen is required, so by selecting insulations with excessive fuel parts is including significantly to a minimum of 4 of the primary risks of fires: Temperature Rise, Oxygen Depletion, Flame Spread and Carbon Monoxide Release.
Perhaps it is best to put in polymeric cables inside steel conduits. This will certainly help flame unfold and minimize smoke as a end result of inside the conduit oxygen is restricted; nonetheless this isn’t an answer. As mentioned previously, 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 boxes, switch panels, distribution boards, motor management centers, lamps, switches, etc. On coming into the gases can ignite or explode with any arcing such because the make/break of a circuit breaker, contactor, swap or relay causing the hearth to unfold to another location.
Conclusion
The reputation of “Halogen Free” whereas ignoring the opposite toxic components of fireplace is a transparent admission we do not perceive the topic well nor can we simply define the dangers of mixed poisonous elements or human physiological response to them. It is necessary nonetheless, that we do not continue to design with only half an understanding of the issue. While เกจวัดแรงกด exists for natural primarily based cables, we can certainly minimize these critically essential effects of fire danger:
One option perhaps to decide on cable insulations and jacket materials which are halogen free and have a low gasoline component, then install them in steel conduit or maybe the American method is best: to use highly halogenated insulations so that in case of fireplace any flame spread is minimized.
For most energy, management, communication and data circuits there is one full answer obtainable for all the problems raised on this paper. It is an answer which has been used reliably for over eighty years. MICC cables can present a total and complete reply to all the problems related to the fireplace security of organic polymer cables.
The copper jacket, magnesium oxide insulation and copper conductors of MICC ensure the cable is effectively hearth proof. MICC cables haven’t any natural content material so simply cannot propagate flame or generate any smoke. The zero gas load ensures no heat is added and no oxygen is consumed.
Being inorganic MICC cables cannot generate any halogen or toxic gasses in any respect together with CO.
Unfortunately many common cable fireplace take a look at methods used today could inadvertently mislead folks into believing the polymeric flexible cable products they buy and use will carry out as anticipated in all fire situations. As outlined on this paper, sadly this may not be right.
For more data, go to www.temperature-house.com
Share

Leave a Comment