Electric shock hazards, including electrocution, have been studied since the introduction of electricity. More recently, however, the dangers of arc flash have come to the forefront of electrical safety programs and procedures. This is because arc flash hazards pose an everyday risk in plants across the world, possessing the potential to destroy equipment and seriously injure, or kill, employees.
In order to accurately determine the arc flash hazard at each electrical assembly in an industrial or commercial facility — and thus determine how best to protect people and equipment — it is first necessary to conduct a short-circuit study, coordination study, and then an arc flash hazard analysis of the entire power distribution system. Let's take a closer look at what's involved in this process.
Arc flash hazard analysis
Collection of data is the first task at hand. The data collected includes the arrangement of components on a one-line drawing with nameplate specifications of every device. Also required are details of the lengths and cross-sectional area of all cables. The electric utility should be contacted for information, including the minimum and maximum fault currents that can be expected at the electrical service entrance to the facility.
Once the data has been collected, a short-circuit analysis followed by a coordination study should be performed. The resultant data can then be fed into the equations described by either NFPA 70E-2004 or IEEE Standard 1584-2002 (see Arc Flash Regulations and Standards). These equations produce the necessary flash protection boundary distances and incident energy levels expected at various points on the system. Such boundary distances and incident energy levels are then used to determine the minimum personal protective equipment (PPE) required to maintain personnel safety.
These equations or tables will most likely yield a requirement for Hazard Risk Category 2 PPE while working in downstream electrical panels. Category 2 PPE is designed to protect workers from the onset of a second-degree burn for exposures up to 8 calories per centimeter-squared. As a reference point, the heat exposure to the skin of 1 calorie per centimeter squared is roughly equivalent to holding a lighted match 1 inch from your finger for 1 second. Typically, Category 4 PPE (40 calories per centimeter-squared) is required only at the main cubicle of a 480V unit substation and for medium-voltage circuits.
Category 4 PPE is very unwieldy and can be costly in terms of time taken to perform work. It can also introduce added safety risks, such as heat stress. In addition, personnel are more prone to make mistakes working in restrictive safety clothing. For these reasons, successful completion of an arc flash hazard study represents only the beginning of the work.
The initial data collected and calculated results can then be used to perform a sensitivity study to obtain breaker/fuse characteristics, which lower the PPE requirement. To achieve this goal, existing circuit protective devices may need to be replaced, generally by more modern counterparts. It is expected that the outcome of this sensitivity study, when implemented, will result in many Category 4 PPE requirements being decreased to Category 1 or 2.
Let's take a look at a case study to gain a more specific understanding of the steps involved in implementing an arc flash hazard compliance program.
Case in point
Founded in 1900 and currently employing approximately 41,000 people, Weyerhaeuser is an international forest products business with annual sales of approximately $20 billion. A few years back, the company began implementing an arc flash compliance program for all packaging facilities in the United States and Mexico. These facilities are some of Weyerhaeuser's smaller manufacturing sites, with electrical distribution systems typically rated at 480V, 3-phase with 1,500kVA to 2,500kVA total load.
Weyerhauser looked to Eaton for assistance in developing a strategic approach to arc flash safety. The objective of the program focused on several areas: to first identify the hazard by modeling every packaging plant; complete the required coordination, short-circuit, and arc flash studies; label existing electrical equipment to identify the hazard/required PPE; and implement a process to reduce or eliminate the need for Category 4 PPE mentioned above. In most cases, the company's goal was to identify areas where its plants could upgrade fuses, circuit breakers, and other electrical components to reduce the potential arc flash energy to a level in the lower energy category, thus eliminating the need for Category 4 PPE.
Four pilot sites were selected for the initial effort — Pulaski, Tenn., Memphis, Tenn., Bowling Green North, Ky., and Bowling Green South, Ky. In many cases, current information of the existing electrical systems was either incomplete or not available, so the first step involved identifying the location of each electrical panel. Data from circuit breaker nameplates, including existing settings, fuses, and conductor sizes/lengths, was collected for the entire facility. Standard templates were used to record the data, and each plant required a two- to three-day period for all data collection. Because collection of data required opening all electrical panels and exposing personnel to potential arc flash hazards, the field engineer assigned to gather the data was deployed with the appropriate PPE. Much of the data gathering effort was completed during an electrical outage to assure the safety of all involved.
Following the data-gathering effort, a power systems engineer conducted short-circuit, coordination, and arc flash studies. The company established a common electronic file for all site studies, including existing calculated hazards and system documentation for all completed work. This information needed to be kept current after system enhancements designed to reduce the hazard were implemented, as well as any time other electrical system changes were made.
Completed studies were delivered to the pilot plants including arc flash warning labels unique to each electrical panel in the facility. These labels clearly identified the electrocution (shock) hazard, arc flash hazard, and the appropriate PPE necessary before personnel could work within the energized panel.
A final visit to each site was made to deliver the labels, review the new facility one-line diagram and system studies with plant personnel, and outline the recommendations for system changes necessary to reduce arc flash hazards to a lower level. The pilot site efforts were completed in the fall of 2004 and continued with the balance of just more than 100 packaging plants completed in late 2005.
Weyerhaeuser's technical education center in Bowling Green, Ky., was used to complete a two-day structured training course for selected facility personnel in conjunction with this project. Two facility champions were selected from each packaging plant that traveled to the facility to learn more about NFPA 70E requirements and arc flash hazards. Training experts were on-hand to deliver the course, which was followed by a hands-on session required by OSHA for qualified personnel. The champions then returned to their local facilities and used the training materials, including instructor-led presentations, video, and Web-based tools, to educate other personnel in the facilities on the new safety initiatives within Weyerhaeuser Packaging.
Wilhite is equipment support lead for Weyerhaeuser in Bowling Green, Ky., and Durocher is forest products industry director for Eaton Corp. in Wilsonville, Ore.
Sidebar: Arc Flash Regulations and Standards
The National Fire Protection Association (NFPA), Institute of Electrical and Electronics Engineers (IEEE), and Occupational Safety and Health Administration (OSHA) work together to develop regulations and standards that best protect personnel and equipment against electrical hazards, including arc flash. Four separate industry standards focus on the prevention of arc flash incidents:
- NFPA 70-2005, National Electrical Code (NEC)
- NFPA 70E-2004, Standard for Electrical Safety in the Workplace
- IEEE Standard 1584-2002, Guide for Performing Arc Flash Hazard Calculations
- OSHA 29 Code of Federal Regulations (CFR) Part 1910 Subpart S
Recently, NFPA standard 70E has had an increased profile, thanks to changes in the NEC and OSHA. Both organizations are now referring to it in their documents. Companies not complying with these standards will most likely be cited and fined for any arc flash-related incident.