For every action, there is an equal and opposite reaction. In cases of arc flash incidents, the consequence for each action can be far beyond anyone’s expectations. Employee safety should always be considered the top priority in every facility. Often times, safety measures are ignored until a catastrophic event occurs. The results from such event can mean significant fines, jail time, lawsuits, lost revenues or even death. The Occupational Health & Safety Administration (OSHA) places the responsibility on the employers to ensure employee safety including working on or near energized equipment. Although arc flash awareness has increased over the past decade, further progress must be made to educate the electrical community. This discussion will provide a brief overview on arc flash safety and how to minimize the risks associated with it.

In a nutshell, arc flash is short circuit through air. An electrical arc flash occurs when electric current passes between two conducting metals through an ionized gas or vapor, usually air. When this phenomenon happens, a large amount of heat (incident energy) is released that can severely burn human skin and even set clothing on fire. The temperature of an arc can reach more than 5000 F. Besides intense heat, electrical arc flash also produces high-pressure blast with molten metal and fragments accompanied by high deaf defying sounds.

In many cases, electrical arcing faults occur within electrical equipment as a result of insulation failure or human error. However, electrical arcs can be initiated by any of the following:

• Dust, impurities, and corrosion at contact surfaces producing heat, contact and creating sparks;

• Sparks produced during racking of breakers, replacement of fuses, breakers/closing into faulted lines;

• Snapping of leads at connections due to force – human, rodents or birds; and

• Accidental touching and dropping of tools, nuts-bolts, or metal parts.

In order to adequately protect employees, a thorough knowledge surrounding the causes of explosive arc flashes is essential. The National Fire Protection Association (NFPA 70E) and the Institute of Electrical and Electronics Engineers (IEEE 1584) are the two widely used standards for electrical arc flash analysis and safety. A joint committee from both organizations is currently underway to conduct further arc flash testing and a revised standard is scheduled to be released in 2008. Both these standards provide methods to assist in calculating the incident energy that may be exposed to the worker as well as establishing the required Personal Protection Equipment (PPE) worn by the worker.

In the United States, hundreds of cases of electrical burns are reported each year – many of them avoidable with proper training and precautions. Exposure to high incident energy can result in severe burns, loss of eyesight, coma and even death. Incident energy is the measure of released energy on the surface at a specific distance from an electrical fault. Factors affecting the amount of incident energy include the available short-circuit current, arc current and the tripping time of the protective devices. Incident energy is also a function of the system voltage.

NFPA 70E has developed a table (NFPA70E-2004, Table 130.7(C)(11)) to determine the arc flash hazard risk category and proper Personal Protective Equipment (PPE) to be worn when a worker is working in an environment that may experience certain arc flash incident energy levels. Based on the calculated incident energy, this table can be used to determine the hazard risk category and proper PPE. This table is commonly known as PPE Table.

To properly determine the incident energy and required PPE, an arc flash hazard analysis is necessary. One important concept during the analysis consists of determining both the minimum and maximum arcing faults. In the time-current curve below, it is observed that a maximum arcing fault will invoke the device in the instantaneous region at 0.07 seconds with an incident energy of 1.22 J/cm2. The lower fault current will trip the device in 1.05 seconds and produce an incident energy of 11.9 J/cm2. The maximum arcing fault will require a Class 0 Fire Retardant Clothing Class while a Class 1 Fire Retardant Clothing Class will be required for the minimum arcing fault. In this situation, calculations based on the largest fault current would not adequately safeguard the worker. In addition, PPE equipment should never be solely relied upon since it should always be considered the last line of defense. It is also advised that correct level of PPE should be selected and worn for each activity. Wearing Category 4 PPE clothing for a lower rated situation can limit mobility and cause inadvertent injuries.

A longer trip time from the lower fault current will produce higher incident energy.

After an arc flash analysis has been completed, arc flash labels must be generated and applied to all serviceable equipment, such as electrical panels. For maximum safety, several key information should be printed on the labels:

• Flash hazard boundary;
• Flash hazard at a specified distance (18 inches);
• PPE category;
• Shock hazard;
• Glove class; and
• Approach boundaries.

Several methods can be utilized to minimize arc flash exposures. The most apparent option would be to de-energize live equipment. Aside from corporate red-tape, this option is seldom feasible as loss in significant revenues can result from facility downtime. In such cases, electricians should work on the equipment as far away as possible. Remote racking or switching away from the source can be beneficial as it allows the worker to be out of the flash boundary. Current-limiting protective devices that can interrupt a fault within the first half-cycle may also be effective in limiting the amount of incident energy expelled to the environment. Other methods include the use of arc resistant switchgear, bus differential protection, zone-selective interlocking or other optical devices. These devices carry the ability to detect the arcing while tripping the protective device instantly.

The development of electricity in the history of mankind has helped build the foundation for the world that we live in today. As the ever-growing need for electricity increases, new safety risks are imposed on workers everyday. Aside from liability concerns and government regulations, proper training and protection is crucial to the power industry outlook as we forge ahead into the 21 st century.