Electrical Safety / Electrical Systems

Best practices for conducting an arc flash risk assessment

Elevate safety with heightened risk awareness.

By Dee Jones, P.E., AVO Engineering Division Manager

An arc flash occurs when electric current passes through the air instead of along its intended path. The result is extremely high heat that can cause severe burns, blinding light, and an explosion that can result in hearing damage and potentially fatal injury. Multiple arc flash incidents occur every day in workplaces across the United States.

There’s urgency to completing arc flash risk assessments and shock risk assessments: According to changes made for NFPA 70E 2015, these assessments must be conducted before any person is exposed to electrical hazards. The risk of an arc flash explosion occurring at your facility is not negligible, and the trend of increased power use combined with aging electrical infrastructure across the U.S. heightens the danger. The Electrical Power Research Institute (EPRI) estimates direct and indirect costs to an employer from a fatal electrical accident in the millions of dollars.

Navigating all of the requirements, conditions, and exceptions that result from these assessments requires a great familiarity with the new standard. Specifically, NFPA 70E 2015 Section 130.4 requires that a shock risk assessment be performed before beginning energized work, and NFPA 70E 2015 Section 130.5 requires that an arc flash risk assessment be performed to:

1. Determine whether an arc flash hazard exists
2. If a hazard exists
    A. Determine appropriate safety-related work practices
    B. The arc flash boundary
    C. The PPE to be used

In the end, OSHA will enforce compliance, including the requirement that an arc flash fisk assessment be performed. The costs of an electrical accident would vary by location, but without exception, it would be far more expensive to allow one arc flash accident to occur than it would be to prevent it. The following steps will help ensure that your arc flash risk assessments going forward will comply with both OSHA requirements and NFPA 70E 2015.

Preparation and bidding

Developing, documenting, training, and implementing safety programs and procedures can be expedited with the aid of third-party experts in electrical safety and associated regulations and standards. An arc flash incident energy analysis is a complex project, and you may need to solicit bids from multiple providers. Note when you’re putting together a request for proposal that it’s important for all bidders to bid on the same deliverables. Lower-cost bids may have items added after the study is done to complete a thorough analysis.

An arc flash incident energy analysis is still the foundation upon which an accurate risk assessment is built. Once you have the incident energy analysis, you can complete your risk assessment and provide proper PPE and work practices for your workers. It is always a good idea to request samples of an arc flash incident energy analysis report from each potential service provider. While this report is based on technical data, you need to ensure that the report uses language and formatting that relevant personnel will easily understand.

Also, ensure that the company performing the analysis adheres to a standardized process in performing every arc flash incident energy analysis. This correlates with IEEE Standard 1584, Chapter 4 and IEEE 1584.1. The entire project also should be performed under the supervision of a registered professional engineer (PE).

A standard analysis will apply to three-phase equipment rated 240 volts or greater and three-phase equipment rated lower than 240 volts when served from a transformer 125 kva and larger. You will need to determine whether you want an expanded scope that includes 208 volt three-phase equipment served from a smaller-than-125-kva transformer or DC equipment rated 50 volts or higher.

Data collection

Before beginning the actual study, hold a project meeting (via conference call or on-site) with all personnel who will be involved to establish roles, responsibilities, and the plan for data gathering. Qualified staff must gather data from all applicable electrical equipment. Required information includes:

  • Data from the utility, including available fault current, operating voltage, and specifics regarding the utility’s protective equipment at the point of service
  • Specifics for each protective device in the electrical system, including manufacturer, model, available time/current settings, and short-circuit interrupting rating
  • Transformer impedance, tap settings, and ratings
  • Conductor specifics, including lengths, sizes, and types of all overhead lines, bus ducts, and cables

Power system modeling

One-line diagrams must be developed or updated to show the current configuration and modes of operation for the power system. Accurate electrical system drawings are necessary to identify power sources, voltage levels, electrical equipment, and protective devices. If you already have one-line diagrams, update the data and work from them, if possible. SKM Power Tools for Windows®, ETAP®, and other available engineering software is commonly used in the modeling.

Short-circuit study

A short-circuit study is required to determine the magnitude of current flowing through the power system at critical points at various time intervals after a “fault” occurs. These calculations will be used to determine the bolted fault current, which is essential for the calculation of incident energy and interrupting ratings of your equipment. Comparison of equipment ratings with calculated short circuit and operating conditions will identify underrated equipment.

Protective device coordination

A protective device coordination should be performed to ensure that selection and arrangement of protective devices limits the effects of an overcurrent situation to the smallest area. Results will be used to make suggestions for mitigation of arc flash hazards. Although this is an optional study, arc flash mitigation cannot be performed without completing this step. It is recommended that this study be performed in accordance with IEEE Std. 242-2001 (i.e., the Buff Book).

Arc flash calculations

These calculations are based on available short-circuit current, protective device clearing time, and distance from the arc. Calculations of incident energy levels and flash protection boundaries will be completed for all relevant equipment busses and system configurations. The magnitude of arc hazards is determined using methods from NFPA 70E, IEEE 1584, or NESC Tables 410-1 and 410-2, as applicable.

Reporting

Upon completion of the calculations, generate and review the arc flash report. Your analysis provider will supply this report to you for a short review period during which your team can assess mitigation recommendations. At this point, hold a management summary meeting to interpret the report results. Upon approval, a final report and full-size one-line diagrams stamped by a registered PE are required. It’s a good idea to get this report in a digital format.

Label installation

Arc flash warning labels should be generated. These labels identify incident energy and working distance, nominal system voltage, and the arc flash boundary. In addition to standard requirements, make sure the labels also include limited and restricted approach boundaries, date, upstream protective device, and recommended personal protection equipment. Bolted fault current may be included if desired. Make sure your labels comply with NFPA 70E 130.5(C), NEC 110.16 and ANSI Z535.