How Flame-Retardant Cable Insulations Work
Choosing the correct flame-retardant insulation for your application depends on the electrical and physical requirements of the application. It’s important to evaluate these requirements carefully since flame-retardant compounds can affect critical properties, such as flexibility, abrasion resistance, installation, temperature requirements, electrical properties, life expectancy, pulling lubes to be used and more.
Components of Fire
To better understand flame retardants, it helps to know what causes fire. There are three main elements necessary to create fire: oxygen, fuel and ignition. A fourth factor that can cause fire is heat, such as when overheated wires from a short circuit degrade insulation to the point that it creates combustible materials in both gas and liquid form. These materials combine with oxygen and provide ignition, which releases more heat and creates more combustible material to ignite. This cycle will continue until at least one of the three elements is eliminated.
How Flame Retardants Work
Flame retardants work by interfering with one or more of these three fire essentials by:
1. Eliminating the fuel
2. Cutting out the availability of oxygen by chemically reacting with the fuel
3. Reducing the amount of heat available to degrade the insulation
Halogenated Flame Retardants
Flame retardants used in wire and cable insulations are either halogenated or non-halogenated. Halogens are elements from group 17 of the periodic table and reduce flammability by free-radical reduction. A free radical is an atom or group of atoms with a single unpaired electron, usually produced by breaking a covalent bond.
Halogenated compounds, especially bromine-based types, are the most commonly used flame retardants. They act to halt exothermic processes—how much heat is released by the cable—by trapping high-energy free radicals after they form. This cools the system by greatly reducing or limiting the supply of flammable gases. Brominated flame retardants either prevent a fire from starting in the first place or significantly slow a fire down. They are a popular choice because they do not significantly affect the extrusion process and are inexpensive. Other examples of halogens used in flame retardants include fluorine, chlorine and iodine.
Non-Halogenated Flame Retardants
Unlike halogenated flame retardants, which limit exothermic processes, non-halogenated flame retardants work by limiting endothermic processes, or how much heat is absorbed by the cable. Metal hydroxides are the most commonly used halogen-free flame retardants and work by employing three methods of flame retardancy: forming water vapor during combustion, reducing available organic fuel load or forming a protective layer of char.
The advantage of non-halogenated cable constructions is that they generally emit less smoke and acidic gas than their halogenated counterparts, improving visibility and breathability during a fire and making it easier for people to find their way out to safety. For this reason, non-halogenated insulated cable should be used in enclosed areas where human life is at risk. They also have low levels of hydrogen chloride, a compound which is damaging to electronic circuits, making them the preferred cable construction in expensive electronic equipment rooms, such as telecommunications rooms and shipboard applications.
The disadvantage of non-halogenated insulations is the considerable loadings of inorganics required. Loadings can be as high as 65 percent in the final product, which will affect the mechanical properties of the polymer. They also tend to be more expensive and may have inferior physical property performance compared to their halogen-based counterparts.
As you can see, there are a few things that must be taken into consideration when selecting the correct insulation system for an application. Need help deciding which type of flame-retardant cable insulation is best for your application? Visit anixter.com/contact to get advice from one of our experts.