The Hidden Danger of Arc Flash
June 1, 2009
Arc flash is a ball of fire produced by an electrical fault. These faults can be characterized as “an unintentional connection between current carrying conductors to each other or to ground.” Since the invention of electricity, arc flash has caused untold equipment damage, injury, and even death, but the past several years have seen a new emphasis on protecting against this hidden danger. As a result, new safety standards have been developed to protect people and facilities.
Arc flash safety concerns have also changed the way electrical equipment and systems are designed. New designs will have to incorporate the ability to de-energize and secure portions of electrical systems in order to perform work safely as well as the ability to withstand internal faults and associated arc flash plus the ability to deflect arc flashes up and out, minimizing the danger to individuals in the immediate area.
There are also two basic types of faults:
- The bolted fault is a solidly connected fault path. Bolted faults have a higher current than arcing faults; most of the energy flows through the fault.
- The arcing fault occurs when current flows through ionized air. Arcing faults release most of their energy into the surrounding environment. Arcing faults result in arc flash.
Here are a few facts to keep in mind.
- Arc-flash incidents have caused serious workplace injuries and fatalities ever since electrical energy was first generated and distributed.
- Arc-flash incidents that result in serious injury or fatality take place five to 10 times a day in the U.S.
- The temperature of an arcing fault can reach 35,000 °Fahrenheit (The sun reaches 10,000°F)
- The face of the fireball is a pressure wave that can exert 300-500 pounds of force per square inch
- The sound pressure level of an arc flash is 160 decibels (dB) (Rock concerts max at 115 dB)
- Metal fragments and molten metal droplets from arc flash move at 5000 feet/second
- Copper vapor will destroy lungs if inhaled
Knowledge is the weapon of choice. The key is the arc-flash analysis.
Arc-flash analysis is a systematic study designed to quantify the available energy at all points in an electrical distribution system and the amount of incident energy generated during an arc event in calories/ square centimeter (cal/cm2).
BoundariesIncident energy is the energy impressed on a surface generated during an electrical arc event at a certain distance from the source and is expressed in calories per square centimeter (cal/cm2) at 18 inches from the source of the arc (the typical length of a person’s forearm). The farther you are from the source, the lower the cal/cm2. 1.2 cal/cm2 is the amount of heat required to just produce the onset of a second-degree burn to unprotected skin.
Based on this analysis it is possible to identify key boundaries in the work area. These invisible lines identify the personal protective equipment (PPE) required to be worn by those involved in the activity.
The flash protection boundary is the closest approach allowed by qualified or unqualified persons without the use of arc protection PPE. No unqualified person may approach any closer than the limited approach boundary identified.
The restricted approach boundary is the line that qualified persons are not typically permitted to approach exposed, energized conductors, unless they are wearing appropriate PPE and have a written and approved plan for the work they are performing. These individuals must break the restricted boundary only to the extent that is absolutely required to perform their work.
When qualified personnel cross the prohibited approach boundary, it is considered to be the same as actually contacting the exposed part. In addition to the requirements set forth for the restricted boundary approach, personnel must perform a risk assessment prior to crossing the prohibited boundary. Figure 1 demonstrates the concept of these boundaries.
Once the amount of incident energy available at the work site has been measured, the Hazard/Risk Category can be determined. NFPA 70E lists six hazard categories:
Category 0 – Up to 1.2 cal/cm2
Category 1 – 1.2 to 4 cal/cm2
Category 2 – 4.1 to 8 cal/cm2
Category 3 – 8.1 to 25 cal/cm2
Category 4 – 25.1 to 40 cal/cm2
Unacceptable Risk - Over 40 cal/cm2
OSHA and NFPA spell out what protective clothing is required based upon the Hazard/Risk category. As a result, each piece of gear should have a warning label affixed. See figure 2.
Normally, the building owner or manager is responsible for performing the arc-flash analysis and enforcing associated safety measures. It is also the responsibility of those working on the gear or in the area to note the hazards involved based on the available information. If no label is present, work should not be commenced and the owner should be informed of the information required to protect workers in accordance with statutory agencies and industry standards.
OSHA mandates that all services to electrical equipment be done in a de-energized state.
Working on energized gear can only be done under special circumstances. If it is necessary to work live (> 50 volts to ground), the regulations outlined in NFPA 70E, Article 130 should be used as a tool to comply with OSHA mandates found in Subpart S part 1910.333(a)(1).
Now that the potential hazards have been identified and boundaries set, we’re ready to begin work. Not so fast, what about lock out/tag out (LOTO)? LOTO is a strategy intended to work in concert with the steps above.
Each person working on the system shall apply their lock and tag to the lockout device. Each lock will have a unique key, be readily identified, and used only for this purpose. Only the person applying the lock shall remove it once work is complete. In addition, each person applying a lock shall apply his or her personal tag identifying who they are, the date, and other required information.
The workplace and job sites are full of hazards. It is our combined responsibility to protect ourselves, our employees, others in the area, and the customer’s equipment and facility.
Learn more about electrical safety in the workplace by reading the following industry standards and regulations.
- National Electrical Code (NEC)
- Occupational Safety and Health Administration (OSHA) 29 CFR 1910
- National Fire Protection Association (NFPA) 70E - 2009
- Institute of Electrical and Electronics Engineers (IEEE) 1584
- National Electrical Safety Code (NESC)