Fire performance of Electric Cables
Electrical cables are frequently blamed by the media and fire authorities as the cause of building fires however it is often not the failure of the cable which starts a fire but the misuse of the cable by frayed or damaged insulation, overloading due to incorrect or insufficient circuit protection, short circuit or over voltage. These situations can cause high temperatures in the cable conductors or electrical arcing which may heat the cable insulation and any surrounding combustible materials to start a fire.
Cable manufacturers generally endeavour to manufacture electric cables which under the above situations, or in cases where a fire is started by another unrelated cause, will not burn or at least will not help spread a fire through the building.
Today there are various cable flame retardance test standards written by technical standards committees in Europe and USA. These common standards propose test methods intended to determine if the electric cables or materials they are made from are self-extinguishing (Flame Retardant). These test methods may also be embedded by Authorities into mandatory building design codes.
This article takes a look at the common test methods and questions if the test protocols employed do in fact provide the implied level of flame retardance performance when cables are installed and used in buildings.
Making flexible electric cables
Most common flexible cables are made from hydrocarbon based polymers. These base polymers are not usually flame retardant and have a high calorific value so chemicals are added to make them more suited to electrical cable use. Halogens like Chlorine are particularly good additives which help retard flame propagation and don’t significantly impact the dielectric properties of the polymer so Halogens can be used in both cable insulations and in cable sheaths. These halogenated polymers (example: PVC & CSP) also have a negative side effect that in fire they can release the halogens which are extremely toxic and when combined with the moisture in eyes, mouth and lungs are very irritant.
For cables which need to be ‘Halogen Free’ and ‘Flame Retardant’ other non-halogen flame retarding elements like alumina-trihydrate (ATH) can be used instead of Halogens, but while effective in retarding flame propagation these fillers often negatively affect the polymer by reducing dielectric performance or affecting mechanical and water resistance. For this reason additives like ATH are mostly used only in cable jackets. Halogen Free Flame Retardant cables most often use a more pure polymer like PE, XLPE or EPR for the insulation which has good dielectric and mechanical properties but may not be very flame retardant.
Electric Cables: Propagation performance in fire
Often the best flame retardant cables are halogenated because both the insulation and outer Jacket are flame retardant but when we need Halogen Free cables we find it is often only the outer jacket which is flame retardant and the inner insulation is not.
This has significance because while cables with a flame retardant outer jacket will often pass flame retardance tests with external flame, the same cables when subjected to high overload or prolonged short circuits have proved in university tests to be highly flammable and can even start a fire. This effect is known and published (8th International Conference on Insulated Power Cables (Jicable’11 – June 2011) held in Versailles, France) so it is perhaps surprising that there are no common test protocols for this seemingly common event and one cited by both authorities and media as cause of building fires.
Further, in Flame Retardant test methods such as IEC60332 parts 1 & 3 which employ an external flame source, the cable samples are not pre-conditioned to normal operating temperature but tested at room temperature. This oversight is important especially for power circuits because the temperature index of the cable (the temperature at which the cable material will self-support combustion in normal air) will be significantly affected by its starting temperature i.e.: The hotter the cable is, the more easily it will propagate fire.
It would seem that a need exists to re-evaluate current cable flame retardance test methods as these are commonly understood by consultants and consumers alike to provide a reliable indication of a cables ability to retard the propagation of fire.
If we can’t trust the Standards what do we do?
In the USA many building standards do not require halogen free cables. Certainly this is not because Americans are not wisely 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 may spread a fire” – (a small fire with some halogen may be better than a large fire without halogens). One of the best ways to make a cable insulation and cable jacket highly flame retardant is by using halogens.
Europe and many countries around the world adopt a different mentality: Halogen Free and Flame Retardant. Whilst this is an admirable mandate the reality is rather different: Flame propagation tests for cables as adopted in UK and Europe can arguably be said to be less stringent than some of the flame propagation tests for cables in USA leading to the conclusion that common tests in UK and Europe may simply be tests the cables can pass rather than tests the cables should pass.
For most flexible polymeric cables the choice remains today between high flame propagation performance with halogens or reduced flame propagation performance without halogens.
Enclosing cables in steel conduit will reduce propagation at the point of fire but hydrocarbon based combustion gasses from decomposing polymers are likely propagate through the conduits to switchboards, distribution boards and junction boxes 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 fire to another location.
While MICC (Mineral Insulated Metal Sheathed) cables would provide a solution, there is often no singe perfect answer for every installation so designers need to evaluate the required performance on a “project-by-project” basis to decide which technology is optimal.
The primary importance of fire load
Inside all buildings and projects electric cables provide the connectivity which keeps lights on, air-conditioning working and the lifts running. It powers computers, office equipment and provides the connection for our telephone 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 ensure our safety by connecting
fire alarms, emergency voice communication, CCTV, smoke shutters, air pressurization fans, emergency lighting, fire sprinkler pumps, smoke and heat detectors, and so many other features of a modern Building Management System.
Where public safety is important we often request cables to have added safety features such as flame retardance to ensure the cables do not easily spread fire, circuit integrity during fire so that essential fire-fighting and life safety equipment keep working. Sometimes we may 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 special properties the cables we buy and install will be safer
Because cables are installed by many different trades for different applications and are mostly 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 represent one of the biggest fire loads in the building. This point is certainly worth thinking more 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 materials are not generally flame retardant and naturally have a high fire load. Cable manufacturers make them flame retardant by adding compounds and chemicals. Certainly this improves the volatility of burning but the fuel content of the base polymers remains.
Tables 1 and 2 above compare the fire load in MJ/Kg for common cable insulating materials against some common fuels. The Heat Release Rate and volatility in air for these materials will differ but the fuel added to a fire per kilogram and the consequential volume of heat generated and oxygen consumed is relative.
The volume in kilometers and tons of cables installed in our buildings and the associated fire load of the insulations is considerable. This is particularly important in projects with long egress times like high rise, public buildings, tunnels and underground environments, airports, hospitals etc.
When considering fire safety we must first understand the most important factors. Fire experts tell us most fire related deaths in buildings are caused by smoke inhalation, temperature rise and oxygen depletion or by trauma caused by jumping in trying to escape these effects.
The first and most important aspect of smoke is how much smoke? Typically the larger the fire the more smoke is generated so anything we can do to reduce the spread of fire will also correspondingly reduce the amount of smoke.
Smoke will contain particulates of carbon, ash and other solids, liquids and gasses, many are toxic and combustible. In particular, fires in confined areas like buildings, tunnels and underground environments cause oxygen levels to drop, this contributes to incomplete burning and smoldering which produces increased amounts of smoke and toxic byproducts including CO and CO2. Presence of halogenated materials will release toxic Halides like Hydrogen Chloride together with many other toxic and flammable gasses in the smoke.
For this reason common smoke tests conducted on cable insulation materials in large 3 meter3 chambers with plenty of air can provide misleading smoke figures because complete burning will often release significantly less smoke than partial incomplete burning which is likely in practice. Simply specifying IEC 61034 with a defined obscuration value then thinking this will provide a low smoke environment during fire may unfortunately be little of help for the people actually involved.
Halogens, Toxicity, Fuel Element, Oxygen Depletion and Temperature Rise
It is concerning that Europe and other countries adopt the concept of halogen free materials without properly addressing the subject of toxicity. Halogens released during combustion are extremely toxic but so too is carbon monoxide and this is not a halogen gas. It is common to call for halogen free cables and then allow the use of Polyethylene because it is halogen free. Burning Polyethylene (which can be seen from the table above has the highest MJ fuel load per Kg of all insulations) will generate almost 3 times more heat than an equivalent PVC cable. This means is that burning polyethylene will not only generate almost 3 times more heat but also consume almost 3 times more oxygen and produce significantly more carbon monoxide. Given carbon monoxide is responsible for most toxicity deaths in fires this situation is at best alarming!
The fuel elements shown in the table above indicate the amount of heat which will be generated by burning 1kg of the common cable insulations tabled. Certainly this heat will accelerate the burning of other adjacent materials and may help spread the fire in a building but importantly, in order to generate the heat energy, oxygen needs to be consumed. The higher the heat of combustion the more oxygen is needed, so by choosing insulations with high fuel elements is adding significantly to at least four 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 metal conduits. This will certainly help flame spread and minimize smoke because inside the conduit oxygen is limited; however this is not a solution. As said 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 centers, lamps, switches, etc. On entering the gases can ignite or explode with any arcing such as the make/break of a circuit breaker, contactor, switch or relay causing the fire to spread to another location.
The popularity of “Halogen Free” while ignoring the other toxic elements of fire is a clear admission we do not understand the subject well nor can we easily define the dangers of combined toxic elements or human physiological response to them. It is important however, that we do not continue to design with only half an understanding of the problem. While no perfect solution exists for organic based cables, we can certainly minimize these critically important effects of fire risk:
One option maybe to choose cable insulations and jacket materials which are halogen free and have a low fuel element, then install them in steel conduit or maybe the American approach is better: to use highly halogenated insulations so that in case of fire any flame spread is minimized.
For most power, control, communication and data circuits there is one complete solution available 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 associated with 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 have no organic content so simply cannot 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 toxic gasses at all including CO.
Unfortunately many common cable fire test methods used today may inadvertently mislead people into believing the polymeric flexible cable products they buy and use will perform as expected in all fire situations. As outlined in this paper, sadly this may not be correct.
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