Mitigating arc flash hazards

Engineers should know about selecting the appropriate risk-reducing strategies to help their clients ensure compliance with NEC, NFPA 70E, and OSHA.

09/18/2014


This article has been peer-reviewed.

Learning objectives

  • Know the codes and standards that govern arc flash energy calculations
  • Know how to perform an arc flash hazard analysis.
  • Know the arc flash mitigating design strategies and how to implement them.

Electricians and maintenance staff often willingly work on energized electrical equipment to perform routine maintenance, take measurements, or eliminate downtime of critical loads in the system. Unfortunately, this creates a very dangerous work environment that is prone to arc flash incidents, which can result in serious injury or even death.

While consulting engineers are not responsible for determining the process for achieving an electrically safe work condition as defined by the National Fire Protection Association (NFPA), they can assist in determining the appropriate personal protective equipment (PPE) by performing arc-flash calculations. While the appropriate PPE should always be worn by contractors or maintenance staff while working on live equipment, the reality of the built environment is that there are some situations when the appropriate minimum PPE may not be enough to prevent serious injury, or when contractors and maintenance staff are not wearing the appropriate PPE identified for the task.

Although the most effective way to prevent injuries is to deenergize and ground equipment, there are various ways consulting engineers can reduce arc flash hazards by implementing mitigating strategies.

Figure 1: This is a typical arc flash label applied to distribution equipment identifying the hazard category or danger level associated with working on that equipment while it is energized. Courtesy: Environmental Systems DesignCalculating arc flash energy

An arc flash occurs when the energy that is normally channeled into magnetic and heating forces for a bolted fault is released into the atmosphere in the form of intense heat, pressure, and light, which are incredibly dangerous and can result in the destruction of equipment, fire, and serious injury to electrical workers and bystanders. The event can result from contamination, water or condensation coming in contact with the system, deterioration, or even faulty installations. However, electrical equipment that has been installed, inspected, operated, and maintained in accordance with the National Electrical Code (NEC) and the manufacturer’s specifications is not likely to pose an arc flash hazard under normal operating conditions. Unfortunately, violent arc flash incidents are commonly results of human error, such as dropping a tool into the system or pulling on loose connections. Therefore, the purpose of an arc-flash hazard analysis is to quantify the worst-case potential risk to individuals working on live electrical equipment so that the minimum proper PPE can be selected to protect the workers from thermal burns.

In recent years, the increased awareness of the dangers associated with working on live electrical equipment has prompted our national consensus standards and government agencies to invoke more stringent laws to ensure worker safety. NFPA 70: National Electrical Code Section 110.16 states that all electrical equipment that may require work to be performed while energized, be field or factory marked to warn qualified persons of potential electric arc flash hazards. OSHA mandates compliance with the NEC when implementing electrical regulations that address the employer and employee in the workplace. OSHA, however, does not simply require a label that indicates the existence of a potential risk, but further requires an employer to assess the workplace to determine if arc flash and shock hazards are present, inform its employees of the potential risks, and select and provide the appropriate PPE required to protect the affected employees from the hazards identified in the assessment. As part of this assessment, OSHA recommends that employers consult consensus standards such as NFPA 70E: Standard for Electrical Safety in the Workplace as a guide for hazard analyses. While OSHA does not specifically enforce the contents of NFPA 70E, the standard can be used by OSHA as evidence that a hazard exists or that there is a means of remediating the risk.

Engineering firms are often contracted to perform an arc flash hazard analysis to help their clients ensure compliance with NFPA 70E and OSHA. The goal of the analysis is to provide warning or danger labels that indicate the minimum PPE required at that particular system location (i.e., switchboard, panelboard, motor control center, disconnect switch, etc.) (see Figure 1). The engineer will typically use a power system analysis software tool to calculate the incident energy: that which would be released during an arc flash event. Depending on the type of facility, voltage class of the system, and frequency of work to be performed on energized equipment, engineering firms may be called upon not only to report the findings of the arc flash hazard analysis, but also to provide potential solutions for minimizing the risk and reducing the available incident energy.

Mitigating design strategies

The calculated incident energy is proportional to the arcing current and the time or duration an individual is exposed to the arc, and inversely proportional to the distance of the worker to the arc. Therefore, solutions for minimizing the arc flash hazard focus on reducing the arcing current, removing the individual from direct contact with the source (increasing the distance from the arc), and decreasing the time it takes for the overcurrent protection to clear the anticipated fault. Consulting engineers can influence these variables through the power distribution scheme they choose, the electrical distribution equipment they specify, and the relays/overcurrent protection devices they select.


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