Debunking the myths around arc flash safety, prevention

Steps on how to prevent arc flash and arc blast hazards and dispelling some popular myths about how they occur

06/11/2013


According to the Electrical Safety Foundation, Int'l, 2,000 workers each year are admitted to burn centers for treatment of severe arc flash burns. While the threat of shock and electrocution from inadvertent contact with energized parts has long been recognized, arc flash and arc blast hazards have only recently been incorporated into the electrical safety standards.

This may be why the myth exists that arc flash incidents are rare. The National Fire Protection Association (NFPA) 70E: Standard for Electrical Safety in the Workplace is the document most often referenced for electrical safety, including arc flash safety. OSHA enforces electrical workplace safety standards outlined in NFPA 70E. Enforcement may take place following an electrical accident or during the agency’s normal on-site inspection process.

Companies that have not met the requirements of NFPA 70E should immediately take steps to begin the compliance process not only for worker safety, but also for equipment productivity. An arc flash accident can render equipment unusable and place the facility in a costly downtime mode, which could last hours or days.

Here are some of the other myths around arc flash safety and prevention:

Myth: Having an arc flash analysis performed deems a company’s workplace NFPA 70E compliant.

Basic compliance with the 2012 edition of NFPA 70E is actually established with a five-step process, which includes an arc flash analysis. The five steps are:

  1. Develop and audit on a regularly scheduled basis an electrical safe work practice policy.
  2. Conduct an electrical system study to determine the present degree of arc flash hazards and apply associated equipment labeling.
  3. Ensure adequate supplies of personal protective equipment (PPE) and proper tools for electrical workers.
  4. Conduct regularly scheduled safety training and audits for all electrical workers.
  5. Maintain all electrical distribution system equipment and components per manufacturers’ recommendations.  

Myth: There is no difference between the NFPA 70E Hazard/Risk Category Tables and the IEEE 1584 method for calculating arc energy levels to determine appropriate levels of PPE.

NFPA 70E provides Hazard/Risk Category tables, which highlight specific personal protective equipment to be used on various electrical distribution equipment. However, these tables are based on fundamental assumptions about the available fault current and the overcurrent device clearing time. In order to use the tables, the person in charge must verify that the available fault current and the overcurrent protective device (OCPD) tripping time are both equal to or lower than the values assumed for developing the tables.

The simplistic approach of selecting the arc flash hazard category based on equipment type, voltage, and the task being performed could subject the electrical workers to either too little PPE (risk of injury) or too much PPE (risk of reduced mobility). Therefore, in order to properly apply the tables, some degree of electrical calculations must be performed to calculate the fault current and to establish the overcurrent protective device tripping time. The tables could be subjected to misuse if they are applied without knowledge of the necessary calculations. The NFPA 70E committee recognized the importance of these parameters and moved them from the endnotes to the body of the table for the 2012 edition in order to draw attention to this important information.

In contrast, the IEEE 1584 standard has been the de facto standard for calculating the arc energy levels at different points in the electrical power system. The analysis procedure begins with a complete data collection from the power system. Characteristics of the power source and the power system components, such as transformers and cables, as well as the tripping characteristics of overcurrent protective devices are identified and entered into a digital computer program. Calculations include the:

  • Bolted 3-phase short circuit current at each bus of concern in the system
  • Arc fault current at each location
  • Clearing time of the protective device protecting the bus (under arc-fault condition)
  • Working distance at each bus
  • Incident energy for each bus
  • Incident energy is the amount of energy impressed on the face and body of the electrical worker. One of the units used to measure incident energy is calories/cm2.
  • Flash protection boundary based on an incident energy of 1.2 calories/cm2

This is the generally accepted energy level that causes the onset of a second-degree burn.

It is after the above calculations have been performed that the hazard risk category and the worker’s protective clothing system for each bus under consideration are determined. 

Myth: De-energizing equipment absolves a company from having an arc flash analysis performed.

Both OSHA and NFPA have basic rules that prohibit energized work. In order to establish that a circuit is de-energized, the circuit must be approached to verify that it has been de-energized. Until the verification testing is completed, the circuit must be considered energized per NFPA 70E. Therefore, the worker who approaches the circuit for verification testing must wear full PPE.

Hiring electrical contractors to perform work does not absolve a company from assuring compliance to safe work practices. The facility owner is ultimately responsible for safety at its site and must communicate known hazards covered by NFPA 70E to the contracted worker(s). 

Conclusion

The continued focus on arc flash and arc blast hazards will play a critical role in reducing the frequency and severity of electrical accidents over time. Meeting the requirements of NFPA 70E will enhance workplace safety for employees and reduce the financial risk for your company. 

Reza Tajali, a registered electrical engineer in several states, is a manager of engineering for Schneider Electric Engineering Services in Nashville, Tenn. 

See articles below on electrical safety and GFCIs.



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