Preventing Electrical Fires
Shockingly, almost 90 percent of all fire accidents originate from “electrical” causes, and behind a vast majority of these fires are low-quality wires and cables, that is, either low-quality insulation or inferior conductor or both. The article provides a snapshot of how consumers, consultants, specifiers and decision-makers can avoid dreadful consequences by choosing the right kind of wires and cables.
- Pratap M Anam
The past several decades have been a grim reminder of countless human lives lost in fire accidents resulting from electrical fires. The fatal fire of Delhi’s Uphaar Cinema is just one of endless examples of electrical fire accidents that occur routinely year after year, taking toll of precious human life, property and data.
Yet when it comes to give a serious thought to choosing the right kind of wire, many decision makers just shrug off this vital link between purchase decisions and electrical safety while specifying their requirements, thus leaving a gaping hole in their safety strategy and endangering their own life as well as the lives of others. The basic problem of such irrational buying decisions is a general lack of awareness or insensitivity to safety issues until such a calamity strikes.
Conductors are classified under different categories as given below.
Class 1: This category is characterized by a solid conductor, which is used mainly for power application cables that are laid without undergoing much of bending and are restricted up to 10 sq. mm. Copper as well as aluminium conductor is used.
Class 2: In this category, the conductors are made of a number of strands - 7, 14, 19 - depending on various sizes. The bunched strands offer greater flexibility than a solid conductor. Wires with bunched conductor is normally used in building wires as well as power cables.
Class 5: The cable is made with a higher number of strands than in Class-2, offering still-higher flexibility and is typically used for panel wiring, industrial wiring, as the number of bends that the cable undergoes during installation is very high and also there is also space constraint in these cases.
Class 6: The cable construction is designed to offer very small cross sections of strands, often as low as 0.05 mm in diameter, while using conductor strands in large numbers. This type of cables are necessary typically in moving applications such as drag chain, drum reeling, and robotics. In these applications, the cable is constantly moving and also needs to bend in very tight radii. Higher number of strands with very small diameter offers far greater higher flexibility.
An example of this case type using a 2.5 sq mm cable is given, for which the following constructions are possible:
Class 1: A solid wire of 2.5 sq mm
Class 2: A cable conductor made of 7 strands of 0.67 mm diameter or 19 strands of 0.41 mm diameter
Class 5: 50 Strands of 0.25 mm diameter
Class 6: 140 strands of 0.15 mm diameter,or
512 strands of 0.1 mm diameter, or
651 strands of 0.07 mm diameter, or
1280 strands of 0.05 mm diameter.
Properties of Conducting Material
Ideally, copper is the material of choice, as it is a far superior conducting material. However, for greater economy, aluminium is often preferred. But apart from this, there are other things to consider also:
- Efficiency of electrical conductivity of an electrical cable depends on the purity of the conducting metal. ‘Electrolyte-grade’ copper provides the best option for highest conductivity while greatly minimizing the heating up of the conductor and resistive losses.
- It also provides better ductility, which reduces internal physical stresses inside the conductor strands, thereby minimizing the possibility of abrupt rupture during cable installation.
- Better malleability imparts higher flexibility to the conductor.
Bunching of Conducting Strands
Unilay-type Bunching: Typically, the conductor of an electrical cable is made of several strands laid out in a traditional linear manner and encapsulated within the insulation. This kind of strand arrangement causes uneven spacing of conducting strands inside the cable, resulting in air gaps, which in turn can cause overheating of the conductor. Better to avoid these traditional wires if you desire highest fire safety. Unilay-type conductor, on the other hand, provides a sophisticated alternative. The bunching of this type of conductor is carried out by a high-precision process, in which the strands of the conductor form a highly uniform and compact, precision-twisted bunch that makes any physical deformation impossible under all normal operating and installation procedure. Thus, unilay-type conductor is a significant advantage over traditionally bunched conductor, for long, safe service life of the cable and to avoid overheating.
Unilay - type precision conductor
What’s more, even the contractor is also happy because the pre-twisted precision unilay conductor makes insulation-stripping easier and also does not require manual twisting of conductor ends to make connection or while terminating a lug on the conductor.
PVC Cable Fire Hazard
Polyvinylchloride (PVC) insulation is traditionally used as insulation in electrical wires and cables. However, PVC cable insulation catches fire easily and continues to burn rapidly, spreading the flame all along its entire length. During a fire situation, the burning PVC insulation produces hydrochloric acid gas, which causes severe suffocation and even death. In fact, a majority of fire-accident victims die of toxic suffocation rather than by the flames. To add to the calamity, the dark dense fumes reduce visibility drastically, making rescue operation extremely difficult. In pitch darkness and amid intensely toxic fumes that cause breathing impossible, saving victims becomes ever more challenging. If enough time is lost in fumbling and searching, many lives are lost even due to toxicity of the PVC flames before the actual fire can reach the fire-accident victims. On the other hand, halogen-free cables do not catch fire readily nor spread it. When a burning candle is applied at one end of a piece of cable, a PVC-insulated cable will spontaneously catch fire and will continue to burn all along its length even after the candle is removed. There will be dark intense smoke coming out of the burning cable. In comparison, a halogen-free cable does not catch flame. In fact, only some charring (blackening) of the wire tip occurs, but it does not readily burn, neither does it spread fire. Unbelievable but true! Needless to say that choosing halogen-free insulated wire saves lives.
PVC cable insulation catches fire spontaneously and continues to burn rapidly, spreading the flame all along. On the other hand cables do not burn readily nor spread fire, thereby saving lives.
Some Insulation Types at a Glance
- Polyvinylchloride (PVC) - suitable for moderate temperature of up to 70OC
- Cross-linked Polyethylene (XLPE) - Suitable for up to 90OC - for outdoor applications
- Cross-linked Polyolefin (XLPO) - Suitable for up to 105OC
- Electron Beam Cured XLPE- Suitable for 150OC – For special applications
- Silicon (Si) - Suitable for up to 180OC
- Polyethylene (PE)
- Thermoplastic Elastomer (TPE)
- Polypropylene (PP)
- Polyurethane (PUR) - High abrasion resistance
- Rubber (EPR) - Resistant to weathering effects like temperature, moisture, oil, water, gasses, chemicals, wind pressure etc.
International Certifications: Last but not the least, for choosing the finest and the safest wires and cables, one should specifically look for reputed international certifications like UL, UN, VDE, BS, ISO, DIN, DEMKO, CSA, PSB, SABS, etc. in addition to ISI.
Author is a writer and media consultant.