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The Risk of Arc Flash: How to Design for a Safer Workplace
August 22, 2017 - Contact FacilitiesNet Editorial Staff »
A new dimension to electrical safety has been recognized over the past 15-20 years as the dangers of arc flash hazards have been considered alongside the more traditional concerns of shock and electrocution. During this time, improved arc flash safety practices have been incorporated into electrical safety standards, most notably, NFPA 70E: Standard for Electrical Safety in the Workplace. To comply with the arc flash protection requirements of NFPA 70E, employers must:
- Develop and audit an Electrical Safe Work Practices policy.
- Conduct an arc flash risk assessment to determine the present degree of arc flash hazards and apply associated equipment labeling.
- Ensure appropriate personal protective equipment (PPE) and proper tools are available for workers.
- Conduct regularly-scheduled safety training and audits for all electrical workers.
- Maintain all electrical distribution system components.
Many employers are proactively taking things further by adding a ‘sixth step’ to the above requirements and implementing mitigation strategies to reduce the likelihood and severity of an arc flash event.
Arc flash mitigation has been a rapidly developing area of research and development. ANSI Z10 contains a hierarchy of mitigation controls that can be applied to arc flash protection. In order from most effective to least effective, these are:
- Engineering controls
- Administrative controls
- Personal protective equipment (PPE)
The most effective controls, elimination (removing the hazard) and substitution (replacing the hazard), are difficult to implement in electrical systems. After all, when electrical loads are present, an electrical distribution system must be present to feed them. Warnings and administrative controls include important aspects of a safety program like worker training and safe work practices, but still require workers to do the right thing when exposed to hazardous energy.
PPE is often viewed as ‘the’ solution to protect electrical workers. However, NFPA 70E only makes the claim that injuries sustained during an arc ﬂash event would be reduced and survivable due to the mitigating effects of arc-rated PPE. It is considered the least effective solution because PPE is intended to be the “last line of defense,” not the primary protection method.
This leaves engineering controls — the application of product or design solutions to help mitigate the hazard. While they may not completely eliminate the risk of an arc flash event, engineering controls may help reduce the severity of the arc flash. Solutions could include fast-acting fuses, circuit breakers, or relays being applied to reduce the available arc flash incident energy level. Other types of engineering controls can lower risk by reducing the likelihood that a worker is exposed to an arc flash hazard for certain work tasks/conditions. Though such solutions may be easier to implement when a project is still in the design stage, many are available for either new installations or for retrofit into existing facilities.
Arc Flash Mitigation Strategies
Mitigation strategies that lower arc flash energy by reducing the available arc flash energy level include:
- Optimization of device settings. An overcurrent protective device (OCPD) study is often performed for facilities to develop circuit breaker and relay settings; if it also considers arc flash levels, the study may be able to recommend settings that result in increased arc flash protection. The speed of operation of the OCPD determines the duration of an arc flash event.
- Specialized relaying such as light sensing technology. Strategically-placed light sensors in switchgear compartments makes it possible to sense the arc within a millisecond. Modern relays can sense this condition and trip the appropriate circuit breaker.
- Virtual main relay. Upgrading switchgear with a virtual main relay adds over-current sensing to the low-voltage side of the service transformer and is designed to trip an existing upstream fault-breaking device.
It is important to note that arc flash reduction systems do not eliminate the electric shock hazard of working on or inside energized equipment. Personal protective equipment (PPE) is required when an arc flash energy reduction system is employed, but the level of PPE may be reduced. The amount of arc flash energy reduction will be determined by the engineering study.
Other arc flash mitigation strategies to employ include those that remove workers from harm’s way, such as:
- Infrared (IR) viewing windows. Having IR windows permanently installed into electrical equipment panels enables IR scans to be performed without exposing the worker to hazardous energy.
- Online temperature monitoring. Online temperature monitoring, via wireless sensors (installed during a planned outage) provides continual access to critical connection points.
- Remote racking system. A remote racking system (RRS) removes the operator from manual contact with the circuit breaker since racking operations are performed via a control panel located away from the cell.
Workplace safety is a multi-faceted and never-ending process. Planning for safety in the initial design process of a facility can ensure that safety goals will be met once the facility is commissioned. Once the facility is in service, complying with the requirements of NFPA 70E provides a good foundation for building a safer workplace. Companies can go further to protect employees and reduce financial risk by implementing arc flash mitigation strategies, such as those discussed in this article.
Antony Parsons, Ph.D., P.E. is a technical consultant in Schneider Electric’s Power Systems Engineering group. He is responsible for providing power system analysis, troubleshooting, and design consulting services for Schneider Electric’s customers, as well as engineering support for Schneider Electric’s field services operations. Dr. Parsons is a member of the IEEE 1584 working group on Arc Flash Calculations and represents Schneider Electric as a member of the Technical Advisory Committee to the IEEE/NFPA Arc Flash Collaborative Research Project.
This Quick Read was submitted by Naomi Millán, senior editor, Building Operating Manangement, firstname.lastname@example.org. To learn how workplace culture can impact arc flash hazard, go to www.facilitiesnet.com/15537bom.