Decoded: Arc Flash in the National Electric Code
Credit to Author: Steve Hinton| Date: Fri, 17 May 2019 08:13:57 +0000
Installation standards and work practices complement each other. Before any arc flash discussion can be made about the National Electric Code (NEC), it’s important to realize that NFPA 70E, Standard for Electrical Safety in the Workplace provides necessary guidance to protect personnel by requiring electrical safety programs to include shock and arc flash risk assessments per article 110.1(H). Elements of a risk assessment include the following:
- Identifying hazards
- Assessing risks
- Implementation of risk control methods.
The hierarchy of risk control in NFPA 70E requires the following six risk control methods to be implemented: Elimination, substitution, engineering controls, awareness, administrative controls, and PPE. Article 130.5 goes on to describe how to perform an arc flash risk assessment and recognizes that documented additional protective measures are not only permitted, but required depending on the type of arc flash mitigation being used.
Recent risk-control mechanisms added to the 2017 NEC include additional signage requirements and two sections that address the severity of the arc flash hazard by decreasing clearing time (for implied arc flash reduction) for circuits rated 1200 A or greater protected by circuit breakers or fuses. What do these specific risk-control mechanisms intend to achieve? And how can electrical workers and their employers ensure that the NEC is properly enforced to minimize the level of hazard and/or the risk associated with an arc flash event?
NEC 240.87: Circuit Breakers & NEC 240.67: Fuses
The NEC mainly focuses on the installation of equipment and has included stringent requirements for arc flash-related labeling for several code cycles. These rules aim to protect personnel during planned energized work. The 2017 NEC addresses required minimum arc energy reduction for low-voltage fuses and circuit breakers rated 1,200 A and higher.
For fuses, Section 240.67 takes effect January 1, 2020 while the circuit breaker section 240.87 has already been implemented. Both sections require documentation accessible to those authorized to design, install, operate, or inspect the installation as to the location of the circuit breaker or fuse. A detailed description of each section can be found here (link to Tables 1 & 2 in the white paper/article or include within post as images).
While several methods can be used to reduce clearing time with the intent to mitigate arc flash incident energy values, they are very different solutions that require a clear understanding of their intents and applications. Differences between the methods can make it challenging for a designer to know if the specified mechanism provides the desired reduction of arc flash incident energy (AFIE). It is important to have a deep understanding of each because adding a mechanism to reduce clearing time does not ensure effective mitigation unless the added method operates as intended. To comply with the intent of the code, the ability for the control measures to work at the expected available arcing current is the key operating parameter that must be determined, even if not explicitly stated as such in the NEC.
Applying the NEC
To ensure that the NEC is properly enforced, authorized personnel must be able to verify the requirements and take steps to minimize the level of hazard and/or the risk associated with an arc flash event. This requires expert knowledge of the applications detailed in the code and an understanding of the selected method. It’s also critical that electrical inspectors carefully examine coordination and arc flash risk assessment study documentation to confirm that arc flash incident energy mitigation has been suitably implemented. A more practical approach may be to require authorized engineering personnel to complete a report listing what methods have been applied to the installation to document that the devices provide a reasonable limited level of incident energy. This could be reported as a calories/cm2 target or noting that all devices above a certain size are operating with no intentional delay at or below the expected minimum arcing current level calculated.
Arc flash hazards carry significant safety and financial risks for electrical workers and their employers. Understanding the latest NEC requirements is an important, but often overlooked step in a robust arc flash mitigation plan.
To learn more about the arc flash in the NEC and the specific risk-control methods outlined above, read this whitepaper.
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