FEATURE ARC FLASH
Arc flash risk in low voltage electrical installations
Arc flash is an underestimated hazard in many electrical installations, especially low voltage applications, says Jeremy Gadd, GSE Systems
distribution transformer can produce a short circuit fault current on the secondary side, typically in the order of 20-30kA, a substantial value when considering arc flash hazards. It should also be noted that such low
O
perators of high voltage electrical switchgear are well aware of the
risks of arc flash incidents, the occurrence of a rapid release of electrical energy through the air that can cause an explosion, projecting high temperature plasma, debris and molten metal over a distance of several metres. Personnel exposed to arc flash can suffer severe burns, lung damage, vision loss, eardrum ruptures, barotrauma and even death. Less well understood is that arc flash hazards are often present in low voltage (LV) installations as well. This can include power supply equipment used in factories, data centres and commercial buildings. In an analysis of more than 6,000 lower voltage distribution and switchboards, we found that almost a quarter presented incident energy hazards of NFPA 70E Category 3 or higher. Furthermore, 6.5 per cent were in the dangerous category where PPE alone will not provide adequate protection to personnel. There is a common misconception that system impedances, such as transformers and similar, can reduce arc flash incident energy values on LV systems, to a low level. However, even a small 1MVA
Arc flash hazards can be present in low voltage installations such as power supply equipment used in factories, data centres and commercial buildings
voltage installations can often be accessed and operated by persons with minimal training, and who are less familiar with the potential of an arc flash incident than those who are qualified to operate high voltage equipment, where access is more controlled. Consequently, such personnel could be considered at greater risk than those who regularly operate high voltage equipment. Equipment owners must undertake adequate consideration of arc flash hazards as part of their responsibilities under health and safety legislation. Relevant standards and regulations include the NFPA70E ‘Standard for Electrical Safety in the Workplace’, the UK Management of Health & Safety at Work Regulations (1999) and the Electricity at Work Regulations (1989). In any system, the likelihood of an
incident depends on the condition of the equipment, and the way it is operated. This leads to the concept of a risk assessment, an area of study which is coming more to the fore in terms of both HV and LV equipment. Calculating arc flash incident energy
values is a specialist task. The potential severity of an arc flash event depends on the configuration of the entire electrical system at a site, along with knowledge of system fault currents and protection clearance times. Specialist software is usually required to model full electrical systems at a site, and conduct incident energy calculations in accordance with IEEE 1584. The output of this assessment is a full understanding of the incident energy levels across the distribution system, allowing appropriate mitigation measures, labelling, hazard boundaries and PPE requirements, to be identified. Mitigation measures may include
changes to the system design, and the adjustments to the settings of overcurrent protection devices, to reduce
22 DECEMBER/JANUARY 2017 | ELECTRICAL ENGINEERING
potential arc flash energy without sacrificing selectivity. This is so the portion of a power system that is disconnected in response to an event is minimised. In some cases, the installation of additional equipment may be recommended, including arc fault detection relays and other equipment and methodologies designed to protect operators from arc flash energy. There are also measures, which can be taken to reduce the impact of fire within electrical panels, including suppression systems, which are able to detect high temperature and locally extinguish using non-conductive liquids or gases. Such systems help to minimise equipment damage, although hazard prevention and personnel protection is the ultimate aim of arc flash assessment and mitigation. Once appropriate mitigation strategies
are in place, an equipment operator must understand the residual risk, and take appropriate steps to manage them. Where arc flash hazards are present, equipment should be clearly marked with a label indicating the required PPE Category and the Arc Flash boundary, in accordance with the NFPA 70E standard for electrical safety in the workplace. All personnel involved with the operation and maintenance of switchgear must receive appropriate training to ensure awareness of arc flash risks, and the processes and procedures to implement in order to minimise those risks. Owners must ensure that all personnel involved in operation and maintenance activities have a suitable level of competence for those tasks. Staff working on, or in close proximity to equipment where arc flash hazards are present, must use appropriate protective equipment in accordance with the level of the incident energy present. Depending on the level of residual risk, this equipment can include one or more layers of arc resistant clothing, gloves and suitable eye and face protection.
GSE Systems T: 01642 613622
www.gses.com
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