Criteria for selecting arc flash protection techniques, Part 1

Engineers must be aware of the causes of arc flash incidents and relevant safety standards before determining the best arc flash mitigation method for a particular facility.


Learning Objectives:

  1. Understand the causes and consequences of arc flash incidents.
  2. Properly categorize different types of arc flash mitigation techniques.
  3. Learn the pros and cons of different types of arc flash mitigation techniques.

Safety is the most important concern for most industrial management and facilities engineers. Arc flash incidents are a primary safety worry in any industrial facility where electrical power is used. There are many options for protecting both personnel and equipment from arc flash hazards. The choice of which methods to use should be made during the facility design phase, but many options are available as retrofit applications. Many aspects of the facility, electrical equipment, and their use should be taken into account when choosing proper arc flash protection methods, including available resources and funding, available fault energy, standards to be met, desired recovery time, and targets of protection (personnel, equipment, or both).

This is the first installment of a three-part series on selecting arc flash protection techniques. Part 2 will explore work safety practices and the importance of arc flash hazard studies; part 3 will discuss arc flash mitigation systems and active arc flash resistant switchgear, as well as the benefits and disadvantages of each.

Arc flash characteristics

An arc flash event occurs when dielectric resistance breaks down between two phases or between a phase and a ground potential object. Current leaves its intended path and travels through the air. The air is ionized as electrons jump between air molecules. The air heats and expands rapidly. Temperatures at the site of the incident can reach as high as 35,000 F (19,400 C). This extreme release of energy can create light bursts that are 2,000 times stronger than normal office lighting. Pressure waves that reach hundreds of pounds of force per square inch can form from the rapidly expanding air. The extreme heat can cause busbars to melt, vaporize, and expand rapidly into molten projectiles.

All these very dangerous phenomena can quickly result in great physical injury to nearby personnel. Injuries can include hearing damage, lung damage due to vaporized metal and hazardous gases, eyesight loss from intense light, broken bones, internal injuries, severe burns, concussion, and psychological trauma. Even with proper PPE according to NFPA 70E: Standard for Electrical Safety in the Workplace, most of these injuries are still possible. Without proper PPE the risk of death and injury increases dramatically.

Arc flash causes

An arc flash event can be initiated by either equipment damage or human error. Human error causes include, but are not limited to, mishandling tools, leaving tools behind after working on the equipment, or coming in contact with live parts. Company operating procedures can go a long way in reducing these typical causes. Some companies' best practices require that employees replace all personal clothing and remove all jewelry prior to the start of the workday. Other requirements include a complete inventory of tools when entering or exiting the work site. Operational procedures like these reduce the occurrence of left-behind tools and help prevent potential hazards.

Equipment damage can be attributed to one or more of the following:

  • Damaged insulation
  • Airborne particles and contaminants
  • Improper installation
  • Voltage spikes
  • Overcurrent events
  • Oxidation on terminal points
  • Animal entry into equipment (i.e., snakes, rats, squirrels)
  • Moisture entering live part areas
  • Loose electrical connections
  • Chemical vapors.

These risks can be significantly reduced by instituting a preventative maintenance program for all electrical equipment. The investment in equipment maintenance pays off threefold: decreased unexpected downtime due to mechanical failure, reduced probability of arc flash events, and the extension of the equipment's useful life.

Arc flash safety standards

Each of the following standards/codes listed below provides workplace safety guidelines and requirements for employers:

  • OSHA 29 CFR-1910, Subpart S, provides the legal requirements for employers to guard against arc flash hazards. It sets general requirements for safe work practices, PPE, and hazard analysis.
  • NFPA 70, called the National Electric Code ® (NEC®), provides a requirement for arc flash labels.
  • NFPA 70E: Standard for Electrical Safety in the Workplace®, outlines the specific procedures and practices to be followed for OSHA compliance and safety when working on electrical equipment.
  • IEEE 1584: Guide to Performing Arc Flash Calculations, provides the formulas necessary for analyzing arc flash hazards.

Additionally, an internal arc test guide for medium-voltage arc resistant equipment, IEEE C37.20.7, establishes criteria for both arc flash containment and accessibility types for equipment while running under normal conditions. This test guide is not intended to provide protection to those individuals who are working on, in, above, or below the equipment. The best method to totally eliminate the incident of arc flash is to not work on energized equipment. When doing so is not practical, then workplace practices are necessary to lower the possibility of an arc flash event.

The next installment of this three-part series will explore work safety practices and the importance of arc flash hazard studies.

Ken Joye is staff marketing specialist at Schneider Electric. He has worked for Schneider Electric for 39 years, specializing in medium-voltage equipment applications for more than 20 years. Joe Richard is senior marketing specialist at Schneider Electric. He graduated with a BSEE from the Georgia Institute of Technology in 2007. He has worked for Schneider Electric for 6 years, specializing in medium-voltage switchgear and applications.

wayne , NH, United States, 04/24/14 07:00 PM:

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