Incident energy analysis

Responding is Ryan Downey, principal engineer, AVO Training Institute, Dallas

This is one of the most common questions I am asked. The NFPA 70E 2015 standard states in Section 130.5 that an arc flash risk assessment should be performed and that either of two methods – but not both – should be used for the selection of personal protective equipment:

• Incident energy analysis method
• Arc flash PPE categories method

It is definitely easier to turn to the tables in Section 130.7(C)(15) and 130.7(C)(16), which are based on the type of equipment you are working on and task at hand, to determine what PPE you should wear. However, this is where a lot of the confusion and misunderstanding can come into play. How do you know for certain you can use these tables?

Often overlooked is that NFPA 70E requires the available fault current and clearing time of the protective devices to be known, which is typically not the case. NFPA 70E specifically states that an incident energy analysis should be required for the following:

• Tasks not listed in Table 130.7(C)(15)(A)(a)
• Power systems with greater than the estimated maximum available short-circuit current
• Power systems with longer than the maximum fault clearing times
• Tasks with less than the minimum working distance

The PPE tables were created as a conservative approach to giving guidance to electricians working on electrical equipment without proper signage or that has not had an arc flash study performed. This is typically the case (unfortunately) for contracted workers. The results of improperly using the PPE tables can result in non-compliance. NFPA 70E also states that the owner of the electrical equipment is responsible for having an incident energy analysis performed. In the case of a study not having been performed, the tables should be used on an interim basis until one is completed.

When the NFPA 70E tables are used instead of an incident energy analysis, some things to consider are:

• Notes in the tables that have specific requirements for the PPE are generally ignored.
• The short-circuit current is assumed.
• The protective device clearing time is assumed.

Also, maintenance of the protective devices is not considered, which can definitely affect the incident energy in the event a sticky breaker or such is not opening when it should, so the clearing time of the device would be inaccurate.

It is also important to understand that the tables and the incident energy calculations are not intended to work together. This is why NFPA 70E has done away with the PPE values and identifies PPE with actual incident energy values for the analysis. Arc flash protective clothing is rated in arc thermal performance value (or ATPV), which is also expressed in cal/cm². Essentially, you need to be certain that the cal/cm² rating of the PPE you are wearing is greater than the calculated incident energy (or cal/cm²) of the equipment you are working on. NFPA 70E is setting the tone for the industry to start selecting the PPE based on a cal/cm² value instead of a PPE category number.

Editor's note: This article represents the independent views of the author and should not be construed as a National Safety Council endorsement.