This article is the fifth and final part in a series that provides a step-by-step approach for performing arc flash hazard calculations. The previous parts appeared in the January, March, May and July 2016 issues of ELECTRICAL CONTRACTOR and are on www.ecmag.com.
So far, this series has demonstrated how to calculate the arcing short-circuit current, incident-energy and arc flash boundaries using a series of examples and worksheets. However, once the calculations have been completed, what do you do with all of the results?
The primary use for the results is to comply with the various arc-flash risk-assessment requirements in NFPA 70E, Standard for Electrical Safety in the Workplace. Incident energy calculations are one of the methods listed in NFPA 70E that can be used for selecting arc-rated personal protective equipment (PPE) and clothing. The arc flash boundary is the distance where the incident energy is 1.2 calories per centimeter squared (cal/cm2) and must also be known as part of the risk assessment. Both of these values can be used to comply with the arc-flash label requirements.
There is more to the calculations than just boundaries, PPE and labeling. Results can be used to analyze the effectiveness of using a risk control hierarchy. This hierarchy was first introduced in the 2015 NFPA 70E and includes six safety controls for the risk associated with electrical hazards such as arc flash. Controls are ranked in order from the most to least effective (see my May 2015 article, “Every Line of Defense”).
1. Elimination: The most effective risk control is to eliminate the hazard. This means establishing and verifying an electrically safe working condition as defined in NFPA 70E.
However, if the arc flash hazard is eliminated, why would the calculations matter? They matter because, as part of the many steps in the process, the absence of voltage must be verified. Until all steps have been completed, the circuit is assumed to be energized, and the arc flash hazard is assumed to be present. Knowing the incident energy enables a person to select the appropriate arc flash PPE to be used.
2. Substitution: Using substitution as a safety control may be an effective method to reduce the risk to a more acceptable level. The incident-energy calculations can be used to verify the extent to which the hazard and risk are reduced.
3. Engineering controls: There are several methods that can be used to reduce the risk associated with the hazard by using engineering controls. These often involve reducing the arc flash duration with an alternate or additional protection scheme. A few of these schemes may include differential protection, zone selective interlocking or arc flash relays, or alternate temporary relay settings.
Regardless of the methods selected, the incident-energy calculations can be used to evaluate the hazard before the controls are implemented. The same calculation methods can be used with new data that represents the system after the engineering controls would be implemented to see how the hazard and risk could be been reduced.
4. Awareness: NFPA 70E 130.5(D) has specific requirements for warning labels on equipment. As part of these requirements, the arc flash boundary must be included. NFPA 70E provides several options that can be used on the label for selecting the PPE, such as listing the calculated incident energy and working distance, minimum arc rating of the protection and site-specific PPE. Each of these requires knowing the calculated prospective incident energy.
5. Administrative controls: NFPA 70E Chapter 1 provides quite a bit of information about administrative controls. These controls could include procedures, training, risk-assessment job briefings and more. For many of these controls, the incident energy and arc flash boundary must be known.
6. PPE: This is often the first thing that comes to mind when reviewing the incident-energy calculations. However, it is considered the least effective in the risk control hierarchy. When all else fails, PPE is the last resort.
PPE and clothing have an arc rating in terms of cal/cm2. The PPE and clothing should be selected with a rating sufficient for the calculated incident energy. Keep in mind, the calculations are for a specific working distance, and, if a worker gets closer for whatever reason, the incident energy could increase dramatically, rendering the selected protection insufficient.
Do you think you are done? Sorry, this is only a temporary pause. NFPA 70E 130.5(2) states that the arc flash risk assessment must be updated when a major modification or renovation takes place and reviewed periodically at intervals not exceeding five years. So, the clock is ticking!
About The Author
PHILLIPS, P.E., is founder of brainfiller.com and provides training globally. He is Vice-Chair of IEEE 1584 Arc Flash Working Group, International Chair of IEC TC78 Live Working Standards and Technical Committee Member of NFPA 70E. He can be reached at [email protected].