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NETWORK / 15 / OCTOBER 2017 in the vicinity. One of the primary risks is burns. Another risk is that material such as copper conductor or plastic is superheated and vaporised by the arc flash explosion and becomes gaseous. Alan O'Kelly of Premium Power, a specialist electrical safety consultancy, explains: "A natural human reaction when there is an explosion is to breathe in. We know of a case where a worker was near an arc flash event and subsequent to the event appeared not be badly burnt. He later died in hospital due to lung damage because he had breathed in superheated and toxic material in gaseous form." The arc flash phenomenon may be thought of as similar to arc welding, where there "is a very bright light and source of heat", explains Paul Hopton, principal electrical consultant at Electrical Safety UK. In an industrial environment, this will generate ultraviolet light, infrared radiation and heat – at 25,000°C, an arc flash is hotter than the surface of the sun. There can also be a blast wave caused by the fact that copper that vaporises will mas- sively expand, to 50,000 times its original size. "Because of the heat the copper goes from a solid state to a vapour," explains Hopton. "This tends to cause a shockwave." O'Kelly says arc flashes are "explosions of light and heat that are an extremely nasty hazard". As well as external and internal burns, the noise of an arc flash can be dev- astating, he points out. Noise six feet away from the source of an arc can be as great as 140 to 165 DB, which is enough to cause deafness or permanent hearing loss. Hands and faces are frequently close to the source of the arc and can thus suffer the most damage. According to the US National Safety Council, there are 30,000 arc flashes per year, with 7,000 burns a year, and 2,000 hospitalisations because of arc flashes. Demystifying the arc Arc flashes can develop quickly and in any three-phase electrical system. It is a myth that higher voltage systems are more sus- ceptible to arc flashes than low or medium voltage systems, O'Kelly points out. This is in part because work at lower voltage equip- ment is more common. It is also possible that personnel treat higher voltage systems with greater caution and respect. Many arc flashes – up to 70% – are caused by human interaction, such as dropping a spanner onto a conductor or busbar. Arc flash studies and modelling by safety experts will determine how much energy might be given off at any point on an electri- cal system if there were an arc. Since the severity of an arc flash is partly dependent on its duration, protection equipment such as circuit breakers can be programmed to trip and shut down the system more quickly. This has to be balanced against the require- ment for reliability in a plant. Changes to fuse sizes can be made, and sometimes electrical systems can be rede- signed. If it is not practical to reduce the level of incident energy, the client is advised to ensure staff wear protective equipment [see box]. "The PPE needs to provide protection against the thermal effects of the incident energy," Hopton says. Having a clean envi- ronment in the industrial facility also helps, as moisture and particulates in the air can contribute to arc flashes. Other measures to prevent arc flashes might include relatively simple ones, such as using insulated tools. Energy release The severity, or potential severity, of an arc flash can be measured in calories per cen- timetre, or the amount of energy in a given area. If an event does occur, the distance personnel are from the source of the arc flash will determine how severe injuries are. Energy from arc flashes varies approxi- mately according to the inverse square law. If you have doubled your distance from the arc flash, you experience a quarter of the energy. Conversely, if you halve the distance between yourself and the source of the arc flash, you can experience four times the energy. "Distance is a huge contributor to how severe the arc flash is to the person who experiences it," says O'Kelly. PPE must take into account the potential for energy release. "For example, if we have done a calculation for switchgear to find out how much instant energy would be given off in the event of an arc, and it turns out to be 20 calories per centimetre squared, then the PPE needs to be rated at 20 calories per centimetre squared, or more," says Hopton. In this scenario, the incident energy must be reduced, or another means of pro- tecting personnel found, such as increasing the working distance from the potential arc flash. "Ultimately, the output from an arc flash study should be used as a starting point in ensuring electrical safety," O'Kelly says. Calculating prospective incident energy is a vital input to risk assessment, taking into account work activity, type and condition of electrical equipment, and so on. "The incident energy analysis must explore the opportunity to reduce the risk at source through protection coordination," he says. "Finally, the effective use of risk as- sessment to control the remaining hazards is essential." O'Kelly uses an analogy drawn from the cinema: "It is like Mr Miyagi says in The Karate Kid: the best block, Daniel, is not to be there." PPE: the last line of defence Much of the PPE for arc flash will protect against a thermal hazard. Many people have been burnt during arc flashes because their clothing has caught fire. "The necessity for arc flash rated clothing is huge," says Alan O'Kelly of Premium Power. "If somebody is experiencing an arc flash, the last thing you want is for their clothing to catch fire, because that is where many burns and injuries come from." Human factors come into play with PPE. PPE that is comfortable and well-designed should be used. This will help guard against the tendency for workers to remove uncomfortable cloth- ing during the task at hand. Workers should also wear ear protection when it comes to mitigat- ing the risk of injury or worse from an arc flash. O'Kelly says: "You have to risk assess: you can't just prescribe PPE. There is no 'one size fits all' solution. "And prevention is better than protection – for any hazard." How to prevent arc flash injuries 1. Assess the workplace to identify employ- ees exposed to hazards from flames or from electric arcs. 2. Calculate the prospective arc incident energy according to IEEE 1584. Ensure that employees exposed to hazards from flames or electric arcs do not wear cloth- ing that could melt onto their skin, or that could ignite and continue to burn when exposed to flames or the estimated heat energy. 3. Ensure that the outer layer of clothing worn by an employee is flame-resistant under all conditions. 4. Ensure that employees exposed to haz- ards from electric arcs wear protective clothing and other protective equipment with an arc rating greater than or equal to the estimated heat energy.