Fire Performance of Electric Cables

Often the best flame retardant cables are halogenated because each the insulation and outer Jacket are flame retardant however when we want Halogen Free cables we find it’s usually solely the outer jacket which is flame retardant and the inner insulation is not.
This has significance as a end result of while 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 prolonged brief circuits have proved in university checks to be highly flammable and can even begin a fire. This effect is thought and revealed (8th International Conference on Insulated Power Cables (Jicable’11 – June 2011) held in Versailles, France) so it is maybe stunning that there are no widespread test protocols for this seemingly frequent occasion and one cited by both authorities and media as explanation for building 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 usually are not pre-conditioned to regular working temperature however tested at room temperature. This oversight is important especially for energy 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) might be significantly affected by its beginning temperature i.e.: The hotter the cable is, the extra easily it will propagate fire.
It would seem that a want exists to re-evaluate present cable flame retardance take a look at strategies as these are commonly understood by consultants and customers alike to provide a dependable indication of a cables ability to retard the propagation of fireplace.
If we can’t belief the Standards what can we do?
In the USA many building requirements do not require halogen free cables. Certainly this is not as a result of Americans are not correctly knowledgeable of the dangers; quite the approach taken is that: “It is better to have extremely flame retardant cables which do not propagate fire than minimally flame retardant cables which can spread a fire” – (a small hearth with some halogen could also be better than a big hearth without halogens). One of the best ways to make a cable insulation and cable jacket highly flame retardant is through the use of halogens.
Europe and heaps of countries around the world undertake a special mentality: Halogen Free and Flame Retardant. Whilst this is an admirable mandate the reality is somewhat completely different: Flame propagation tests for cables as adopted in UK and Europe can arguably be stated to be less stringent than some of the flame propagation exams for cables in USA resulting in the conclusion that widespread tests in UK and Europe might merely be tests the cables can pass rather than tests the cables ought to move.
For most flexible polymeric cables the selection remains today between high flame propagation performance with halogens or reduced flame propagation performance without halogens.
Enclosing cables in metal conduit will reduce propagation at the level of fireplace but hydrocarbon based mostly combustion gasses from decomposing polymers are doubtless propagate through the conduits to switchboards, distribution boards and junction bins in different elements of the constructing. Any spark such because the opening or closing of circuit breakers, or contactors is likely to ignite the combustible gasses resulting in explosion and spreading the fire to a different location.
While MICC (Mineral Insulated Metal Sheathed) cables would offer an answer, there is usually no singe perfect reply for every set up so designers need to judge the required performance on a “project-by-project” foundation to decide which expertise is optimal.
The primary significance of fire load
Inside all buildings and tasks electric cables present the connectivity which retains lights on, air-conditioning working and the lifts operating. It powers computers, workplace equipment and offers the connection for our telephone and computer systems. Even our mobile phones want to connect with wi-fi or GSM antennas that are linked to the telecom community by fiber optic or copper cables. Cables guarantee our security by connecting
hearth alarms, emergency voice communication, CCTV, smoke shutters, air pressurization fans, emergency lighting, hearth sprinkler pumps, smoke and heat detectors, and so many other options of a contemporary Building Management System.
Where public security is necessary we frequently request cables to have added safety features such as flame retardance to ensure the cables don’t simply spread fireplace, circuit integrity throughout hearth so that essential fire-fighting and life security tools keep working. Sometimes we may acknowledge that the combustion of electrical cables produces smoke and this can be toxic 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 purchase and set up might be safer
Because cables are put in by many alternative trades for different functions and are principally hidden or embedded in our constructions, what is often not realized is that the many miles of cables and tons of plastic polymers which make up the cables can symbolize one of the greatest fireplace hundreds within the building. This level is actually price thinking extra about.
PVC, XLPE, EPR, CSP, LSOH (Low Smoke Zero Halogen) and even HFFR (Halogen Free Flame Retardant) cable materials are largely based mostly on hydrocarbon polymers. These base supplies usually are not usually flame retardant and naturally have a excessive fireplace load. Cable manufacturers make them flame retardant by including compounds and chemicals. Certainly this improves the volatility of burning but the gas content of the bottom polymers remains.
Tables 1 and 2 above compare the fire load in MJ/Kg for widespread cable insulating supplies against some widespread fuels. The Heat Release Rate and volatility in air for these materials will differ however the gasoline added to a fireplace per kilogram and the consequential volume of heat generated and oxygen consumed is relative.
The quantity in kilometers and tons of cables put in in our buildings and the related fire load of the insulations is considerable. This is especially necessary in projects with lengthy egress occasions like high rise, public buildings, tunnels and underground environments, airports, hospitals and so forth.
When contemplating hearth security we must first understand the most important factors. Fire specialists inform us most hearth related deaths in buildings are brought on by smoke inhalation, temperature rise and oxygen depletion or by trauma attributable to leaping in making an attempt to escape these results.
The first and most important aspect of smoke is how much smoke? Typically the bigger the fire the extra smoke is generated so something we will do to reduce the spread of fireside will also correspondingly reduce the amount of smoke.
Smoke will comprise particulates of carbon, ash and different solids, liquids and gasses, many are toxic and combustible. In specific, fires in confined areas like buildings, tunnels and underground environments trigger oxygen levels to drop, this contributes to incomplete burning and smoldering which produces elevated quantities of smoke and poisonous byproducts including CO and CO2. Presence of halogenated supplies will launch poisonous Halides like Hydrogen Chloride along with many other poisonous and flammable gasses in the smoke.
For this purpose common smoke checks performed on cable insulation supplies in massive three meter3 chambers with plenty of air can provide misleading smoke figures as a result of full burning will usually 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 considering this will provide a low smoke environment during fireplace may sadly be little of assist for the people truly concerned.
Halogens, Toxicity, Fuel Element, Oxygen Depletion and Temperature Rise
It is regarding that Europe and other international locations adopt the idea of halogen free materials with out correctly addressing the topic of toxicity. Halogens launched during combustion are extraordinarily toxic however so too is carbon monoxide and this is not a halogen gas. Protected 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 best MJ gas load per Kg of all insulations) will generate almost three instances more warmth than an equal PVC cable. This means is that burning polyethylene is not going to solely generate almost three occasions extra heat but additionally eat almost 3 times more oxygen and produce considerably more carbon monoxide. Given carbon monoxide is liable for most toxicity deaths in fires this case is at best alarming!
The fuel elements proven within the desk above indicate the amount of warmth which might be generated by burning 1kg of the widespread cable insulations tabled. Certainly this heat will accelerate the burning of different adjacent supplies and will assist spread the fire in a constructing but importantly, to have the ability to generate the heat vitality, oxygen needs to be consumed. The larger the heat of combustion the more oxygen is required, so by selecting insulations with high gas components is including significantly to a minimal of four of the first dangers of fires: Temperature Rise, Oxygen Depletion, Flame Spread and Carbon Monoxide Release.
Perhaps it is best to install polymeric cables inside metallic conduits. This will certainly assist flame spread and reduce smoke because contained in the conduit oxygen is limited; however this isn’t a solution. As stated previously, many of the gasses from the decomposing polymeric insulations inside the conduits are highly flammable and toxic. These gases will migrate along the conduits to junction boxes, switch panels, distribution boards, motor control facilities, lamps, switches, etc. On entering the gases can ignite or explode with any arcing such as the make/break of a circuit breaker, contactor, swap or relay inflicting the hearth to unfold to another location.
The recognition of “Halogen Free” whereas ignoring the other toxic elements of fireplace is a clear admission we do not understand the topic properly nor can we simply define the risks of combined poisonous parts or human physiological response to them. It is necessary nevertheless, that we do not proceed to design with solely half an understanding of the problem. While no excellent resolution exists for natural based mostly cables, we will actually minimize these critically essential results of fireplace threat:
One choice possibly to choose cable insulations and jacket supplies which are halogen free and have a low gasoline factor, then install them in metal conduit or maybe the American approach is best: to use highly halogenated insulations in order that in case of fireside any flame unfold is minimized.
For most power, control, communication and information circuits there’s one complete solution available 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 complete and complete answer to all the problems associated with the hearth safety of natural polymer cables.
The copper jacket, magnesium oxide insulation and copper conductors of MICC make certain the cable is successfully hearth proof. MICC cables have no organic content material so simply can not 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 widespread cable hearth test strategies used right now could inadvertently mislead people into believing the polymeric flexible cable merchandise they purchase and use will perform as expected in all fireplace situations. As outlined on this paper, sadly this will not be correct.
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