The term “working distance” appears 20 times in the 2012 Edition of NFPA 70E, the Standard for Electrical Safety in the Workplace. It appears 12 more times in the annexes. The working distance is an important component of the arc flash hazard analysis and is frequently listed on arc flash warning labels and in the arc flash report.
IEEE 1584—IEEE Guide for Performing Arc Flash Hazard Calculations 2002 defines the working distance as “the dimension between the possible arc point and the head and body of the worker positioned in place to perform the assigned task.”
When incident energy is calculated as part of an analysis, it corresponds to a specific distance from the prospective arc flash source, making the working distance a very important piece of information. If a person is located closer than the working distance, the incident energy increases dramatically.
The table above illustrates how incident energy varies with distance. Calculations were performed for distances ranging from 6 to 36 inches for a 480-volt panel. Depending on which distance is used, the incident energy varies from 2.6 calories per square centimeter (cal/cm2) to 48.5 cal/cm2. If the working distance was not included, a worker would not know that 2.6 cal/cm2 is based on being located a minimum of 36 inches away.
Five common (and dangerous) working distance mistakes
Even though working distance seems to be a simple concept, it is not always properly considered when performing energized work. The following are five common mistakes:
1 Working too close—Even though the working distance may be listed on the arc flash label, there is sometimes a tendency to move closer for various reasons. This could spell trouble in the event of an arc flash, since the incident energy would be greater. The electrical safety training and job briefing should always emphasize the importance of maintaining a proper working distance and include a discussion of what could happen if this distance is reduced.
2 Parts of the body closer than the working distance—Even if the correct working distance is maintained, it is possible that other parts of the body, such as hands and arms, may be closer to the arc flash source. NFPA 70E requires additional protection to be used for any parts of the body that are closer than the distance used in determining the incident energy.
3 Incorrect input data—When performing arc flash studies using commercially available software, care must be taken to select an appropriate working distance. IEEE 1584 suggests working distance ranging from 18 to 36 inches, depending on the type of equipment and voltage level. Many times, I have reviewed arc flash studies only to find the distance selected is not what IEEE suggests for the equipment. If a larger than normal working distance is used, it can give a false sense of safety by suggesting the incident energy is much less than it could be.
4 Unexpected energized parts upfront—Working distance appears to be a simple concept until you attempt to determine the location of the possible source of the arc. Is the equipment’s energized bus located toward the rear of the enclosure as is usually the case? Or are there also energized conductors located closer to the front? How would you know before opening the enclosure door? The working distance generally assumes the energized conductors are located near the rear of the equipment, but that may not always be true.
5 Rear accessible equipment—Electrical equipment that can be accessed from the rear might result in a big surprise when it is opened. Instead of the energized bus being located at the back, you are now at the back, and the working distance can be very short. Any live work performed at the rear must take this greatly reduced working distance into consideration. Normally, this situation requires a second set of calculations and labels based on the short working distance.
Distance is everything when it comes to an arc flash. Workers should never be closer than the working distance associated with the incident energy for which they are protected. If a different working distance becomes necessary, new calculations will be required to determine the increase in incident energy and the requirements for possible increased protection. Getting too close could be catastrophic.
PHILLIPS, founder of www.brainfiller.com and www.ArcFlashForum.com, is an internationally known educator on electrical power systems and author of “Complete Guide to Arc Flash Hazard Calculation Studies.” His experience includes industrial, commercial and utility systems, and he is a member of the IEEE 1584 Arc Flash Working Group. Reach him at email@example.com.