Arc-Flash Hazard Assessment: Just the Start
By Ken Cybart

Arc flash is the most serious type of electrical accident, but OSHA expects employers to address a much broader scope of electrical safety issues.

More than 80 percent of the electrical accidents and fatalities among qualified electrical workers are the result of arc flash, and up to 10 arc-flash accidents occur in the United States everyday. As a result, OSHA expects companies to perform an arc-flash hazard assessment to eliminate or minimize the hazards and determine personal protective equipment (PPE) needed to protect workers that may be exposed to the hazards.

As important as that is, OSHA expects more of employers. Minimizing arc-flash risks is only one part of a comprehensive electrical hazards risk assessment and safety program. OSHA also establishes the following employer responsibilities:

• Hazard identification and risk analysis
• Electrical worker training requirements
• Policies and procedures manuals
• Work practices and procedures
• Proper tools
• PPE and clothing
• Equipment labeling
• Protection boundaries and safety barriers
  

Fortunately, many of the steps required for an arc-flash assessment, such as electrical one-line drawings and analysis, are also the same needed for a comprehensive electrical hazard assessment. Therefore, much of the preparation work must be done for a more complete analysis. Not only is this the best practice, it also helps ensure compliance with OSHA, National Fire Protection (NFPA), and National Electrical Code (NEC) electrical safety standards. The following sections describe the basic elements of a complete electrical hazards assessment and safety program.

Audit and Update One-Line Drawings

Plant environments are dynamic. Over the course of time, many changes to the equipment can occur, as well as changes in the electrical services supplied to the plant. One-line electrical system drawings should be updated annually or when new equipment is installed or equipment is moved. Prior to an electrical hazard assessment, these changes must be audited and documented. An up-to-date set of one-line drawings is essential for a successful electrical system safety program.

Figure 1. Portion of a one-line electrical system drawing used in hazard assessments.

One-line drawings trace the electrical path from the transformer outside the building down to each piece of equipment (Figure 1) and are used for short circuit calculations, coordination studies, and arc-flash calculations. The drawings must document the location and description of all power sources, size and length of cable runs, and all electrical components and equipment. The aim is to provide users with a clear and precise understanding of the installed equipment, reveal power flow, allow power demand analysis, and direct attention to potential hazards. For example, Figure 1 illustrates the impact that longer cable runs can have on potential arc-flash hazards.

Audit Circuit Protection Devices

Armed with an accurate set of electrical drawings, a qualified engineer can evaluate a plant's electrical system. A circuit protection audit helps manufacturing and commercial facilities improve employee safety while optimizing efficiency and reliability. Often, electrical safety can be dramatically improved by simply upgrading circuit breakers and fuses to the current-limiting type.

UL-listed current-limiting fuses or current-limiting circuit breakers must open within the first half cycle of an AC fault (8.3msec). According to studies done by the Institute of Electrical and Electronics Engineers Inc. and other organizations, most standard non-current-limiting circuit breakers can take up to six AC cycles (0.1 second) to open under arc-flash conditions. Although this is relatively fast, it is a minimum of 12 times longer than a typical current-limiting fuse or current-limiting circuit breaker.

Current-limiting protective devices dramatically reduce the destructive energy of an arc-flash. If non-current-limiting devices are used, the instantaneous peak current during the first half cycle of a fault can reach as high as 2.3 times the available RMS bolted fault current at the equipment. For example, if the available fault current at an industrial control panel is 100,000A, the maximum possible instantaneous peak current could reach 233,000A. If a 30A rated current-limiting fuse were used, the maximum instantaneous peak let-through current during the first half cycle would be no more than 6000A. Limiting the instantaneous peak let-through current during an arc-flash will reduce incident energy. A difference of one-half cycle clearing time can make a big difference in the amount of energy released during an arc-flash accident.

In addition to analyzing the installed inventory of over-current protective devices, a circuit protection audit should result in detailed summary reports that describe the following areas:

• How to reduce dangerous energy levels with current-limiting fuses
• Unsafe or inadequate equipment to be replaced
• Opportunities for downtime reductions
• Changes that can help meet selective coordination standards
• A storage system with bin labels for implementing fuse upgrades
  

Short Circuit Fault Current Analysis

Using the information gathered above, engineers can identify any part of the system that may not have adequate short circuit protection. One-line drawings and commercial software are used to build a computer model of short circuit current flow, and identify how to provide selective coordination.

To do this effectively, the firm or engineer doing the analysis must have an appropriate technical library and software for modeling short circuit characteristics. For equipment types not included in commercial software, the engineer needs to possess the knowledge and ability to evaluate the system using other methods. In general, this requires experience with one or more of the most widely used electrical data management and analysis software packages.

Selective Coordination Circuit Protection Study

The NEC defines selective coordination as “localization of an over-current condition to restrict outages to the circuit or equipment affected, accomplished by the choice of over-current protective devices and their ratings or settings.” The NEC requires selective coordination for elevator feeders, emergency systems, and legally required standby systems, among others. For example, emergency systems supply power to emergency lighting and fire pumps, while legally required standby systems supply power to equipment that aids in firefighting, rescue and control of health hazards. If these systems are not selectively coordinated, a fault in any branch circuit could cause the entire system to shut down, endangering people and disrupting essential services. Most state and local building codes require compliance with NEC selective coordination requirements.

The aim of a selective coordination study is to properly size individual circuit protection devices in relationship to the circuit protection devices upstream. A properly sized current-limiting fuse near the load will open before a circuit breaker or fuse located upstream.

Beyond human safety issues, selective coordination in a plant can reduce unplanned work stoppages and speed troubleshooting. Also, current-limiting fuses can increase the short circuit current rating (SCCR) of industrial control panels, which according to the 2005 edition of NEC Article 409, must be clearly marked on the panels.

The engineer performing the study needs to understand the various degrees of current-limitation. For example, UL Class RK1 fuses are more current-limiting than UL Class RK5 fuses. And UL Class J, T, and CC fuses are more current-limiting than Class RK5 or Class RK1 fuses. In many cases, replacing a non-current-limiting fuse or one fuse with a more current-limiting fuse will reduce arc-flash hazards and increase the SCCR of equipment. To maintain selectivity in circuit breaker systems that provide power to two or more circuits, care should be taken to apply current-limiting fuses on the load-side of circuit breakers rather than on their line side. Under high fault currents, current-limiting fuses will open before most common circuit breakers.

Detailed Electrical Hazard Analysis

Having completed the previous steps in the electrical hazard assessment program, a qualified engineer is now in a position to analyze the data that was collected. For each piece of equipment, the following steps must be taken:

• Perform shock hazard analysis
• Establish shock protection boundaries
• Perform flash hazard analysis
• Calculate incident energies
• Determine hazard risk categories
• Establish flash protection boundaries

Shock hazards and protection boundaries are clearly spelled out in OSHA and NFPA guidelines. Arc-flash analysis is a bit more complicated.

Arc-Flash Incident Energy Calculations

As part of NFPA 70E and OSHA 1910 compliance, employers are required to identify arc-flash hazards and provide a safe workplace. As indicated earlier, current-limiting devices can help to minimize hazards.

The principal requirement in arc-flash analysis is calculating the level of potential incident energy that could be released at each piece of equipment if an arc-flash occurs. NFPA 70E-2004 references IEEE 1584 “Guide for Performing Arc-Flash Hazard Calculations” as one method of calculating the hazards. These calculations involve determining the length and size of the wire, the available fault current at the equipment being analyzed, and the type and coordination of circuit protection devices. The potential arc-flash energy found from these calculations determines the equipment's hazard risk category, which determines the type of protective clothing workers need to safely approach energized equipment.

Again, appropriate expertise is required in conducting these calculations and subsequent analyses. For example, some plant engineers mistakenly assume that when equipment is determined to be a hazard risk category 0, equipment fed downstream is also hazard risk category 0. The severity of an arc-flash depends on the available fault current at the location being analyzed and the reaction time of circuit protection devices. Because available fault current is reduced by impedance, a fuse or circuit breaker may open more slowly in equipment located downstream. This can actually increase the amount of potential incident energy and the hazard risk category. (See PPE categories in Figure 1.)

Arc-Flash Protection Boundaries and Barriers

Based on the system voltage and potential incident energy from the arc-flash analysis, NFPA 70E prescribes safety boundaries (Figure 2). Within these boundaries, workers must wear a certain level of protective gear indicated by the hazard risk category. The boundaries also indicate safe distances for non-qualified workers to observe. Safety barriers must be used to protect un-qualified workers from entering the protection boundaries.

Figure 2. Electrical shock and arc-flash boundaries.

Shock and arc-flash analyses are frequently misinterpreted, resulting in hazard assessments that fall short of OSHA compliance. For instance, some engineers misinterpret IEEE 1584 to mean that there is no need to assess equipment operating below 240 volts for arc-flash hazards. Arc-flash incident energy can be dangerous at 240 volts and lower voltages if they are supplied from a transformer rated greater than 125 kVA or if the available fault current is greater than 7kA. OSHA and NFPA standards require all equipment operating at 50 volts and higher to be assessed for potential shock and arc-flash hazards.

Label Equipment Per NEC Specifications

OSHA and NFPA 70E recognizes NEC 110.16, which details the requirements for arc-flash warning labels. These are particularly important for equipment that may be worked on while energized. In addition to the warning, labels can identify the shock protection boundaries, flash protection boundary, hazard risk category, and required PPE (Figure 3.). Some of this information can be gleaned from one-line drawings (see Figure 1), or from NFPA 70E energized work permits.

Figure 3. Typical electrical hazard warning label.

When there is a lot of equipment to label, the organization performing the hazard assessment should be capable of producing labels in volume. To avoid errors and minimize liability, the same organization should install or supervise the installation of the labels.

Develop an Ongoing Electrical Safety Program

After a company has analyzed and evaluated its electrical system, corrected deficiencies, labeled equipment and issued reports, there is still work to be done. An ongoing electrical safety program must be established. The general requirements for this program involve three major elements:

• Establish the program scope and philosophy
• Define standards and procedures
• Publish an electrical safety policy and manual

The scope of an electrical safety program depends on the type of facility involved and the work that goes on there. The philosophy of any safety program should be to prevent injuries to personnel and increase the safety and reliability of equipment.

Work standards, procedures and associated job descriptions are typically covered in separate manuals that are referenced in the electrical safety manual. Safe work practices and proper use of tools are usually covered in the safety manual but may also be covered in a procedure manual. Among other things, these practices include proper lock-out/tag-out procedures, and live-work permits that must justify when the electrical equipment may be worked on while energized during servicing or maintenance. The electrical safety manual should also cover the use of personal protective equipment and clothing based on the hazard risk categories defined in NFPA 70E.

An overriding concern in any successful safety program must be employee training. This needs to be audited, reviewed and documented annually by experienced trainers. Training topics should include the following:

• The requirements of standards that govern electrical work, including OSHA, NFPA and NEC
• Required and recommended maintenance and safety inspections
• Safe work practices, including lockout/tagout procedures per 29 CFR 1910 and the use of key interlocking systems
• The consequences of poor electrical safety practices to people and equipment
• The difference between “qualified” and “unqualified” workers, and work limitations for unqualified workers
• Rules for authorized “hot work" and use of live-work permits and job briefings
• Identification of the type and level of hazards, including electrical shock and arc-flash
• The proper use of test equipment to identify energized components and conductors and their voltage levels
• Interpretation of hazard warning labels
• Safe approach distances to exposed electrical conductors
• PPE requirements, including selection, proper use and care

Conclusions

Much attention has been given to arc-flash safety in recent years. Sometimes this overshadows the need for a broader electrical hazard assessment and safety program. Implementation of such a program requires professional engineers that understand OSHA, NFPA and NEC standards and can perform or oversee the entire scope of work required. This includes data collection, one-line drawings, detailed short-circuit current analysis, coordination studies, arc-flash calculations, electrical hazard analysis, safety manuals and training.

For more information and a detailed discussion on this topic, listen to the Audio Conference archived at www.workplacemagazine.com, “Are Your Electrical Workers Qualified?”

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Ken Cybart is Senior Technical Sales Engineer for Littelfuse Inc. He has more than 20 years of experience teaching and training engineers, managers and electrical workers on safe electrical design, electrical safety, NFPA and OSHA regulations. He has worked closely with engineers and safety managers, Federal and State OSHA investigators and compliance officers, Underwriters Laboratories and the OSHA National Training Institute. He may be reached at KCybart@Littelfuse.com.