In addition, an arc flash study should include a discussion of the operating scenarios considered, including normal utility operation and alternative sources of power such as diesel standby generators. Incident energy numbers calculated in the arc flash study also determine what kind of personal protective equipment (PPE) is required.

For employee safety, OSHA requires that this information be based on the most current version of NFPA 70E, documented, and up to date.

In the past, the industry focused on arc flash mitigation at areas that had “high” incident energy or in other words, where standard PPE would not be enough to protect a worker from an arc flash.

Now, NFPA 70®, National Electric Code® (NEC) has a prescriptive approach to reducing incident energy.

The scope of the NEC Arc Energy Reduction requirements applies to all fusible (Section 240.67) and circuit breaker (Section 240.87) devices 1200 A and above.

For fusible devices, the fuse must have a clearing time of 0.07 seconds or less at the available arcing current, or one of the below approved methods for arc energy reduction must be implemented:

• Differential relaying
• Energy-reducing maintenance switching with a local-status indicator
• Energy-reducing active arc flash mitigation system
• Current-limiting electronically actuated fuses
• An approved equivalent means

For circuit breaker devices, there are seven approved methods for arc energy reduction that facilities may employ:

• Zone Selective Interlocking (ZSI)
• Differential relaying
• Energy-reducing maintenance switching with a local-status indicator
• Energy-reducing active arc flash mitigation system
• An instantaneous trip setting. Temporary adjustment of the instantaneous trip setting to achieve arc energy reduction shall not be permitted.
• An instantaneous override
• An approved equivalent means

In addition, both sections require:

• Documentation that identifies the location of the interrupting device(s) and the method chosen for the reduction must be available to demonstrate that clearing time is set to operate at a value below the available arcing current
• Performance testing of the arc energy reduction protection system by a qualified individual via primary current injection testing or another approved method when first installed on site

Once your arc flash solution has been implemented, it is important to update the arc flash labels to reflect the reduced incident energy levels. Typically, per NFPA 70E, labels are required on equipment that is likely to need service or maintenance while energized, such as:

• Industrial control panels
• Panelboards and switchboards
• Switchgear
• Meter socket enclosures
• Motor control centers

These labels require specific information be prominently displayed:

• Nominal system voltage
• Arc flash boundary
• At least one of the following:
  • Available incident energy and the corresponding working distance
  • Minimum arc rating of clothing
  • Required level of PPE
  • Highest Hazard/Risk Category (HRC) for the equipment

While not required, adding the date the available fault current was calculated on the label will be an indicator to update the calculation if changes are made to the system.

More on arc energy reduction