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The potential magnitude of an arc flash is characterized as arc energy, or “incident energy” and is quantified by the arc flash heat energy found at the worker’s body and is measured in cal/cm2 or J/cm2. Incident energy increases as available fault current and clearing time increase. The higher the incident energy the more severe the arc flash event can be. Therein lies the importance of arc energy reduction.
To support this, the National Electrical Code (NEC) introduced and has continued to evolve arc energy reduction Sections 240.67 for fuse applications and Section 240.87 for circuit breaker applications. Incident energy reduction in accordance with the NEC offers enhanced safety to personnel and the public by proper calculation and design.
Reduce the incident energy = reduce the severity
Arc energy is directly proportional to clearing time – the longer it takes to clear, the more intense the energy can become and the more devastating the incident energy can be. So, a clearing time of 67 milliseconds (about four cycles in the case of power circuit breakers) can be enough time to allow a significant arc flash event to occur. Learn about the compliant solution that clears in 4 milliseconds or less.
No matter where you are in your arc flash safety journey, it may at times seem confusing and overwhelming. This handbook provides the information you need to know about NFPA 70E workplace standard, arc flash studies, labels, boundaries, PPE, training and mitigation solutions.
The purpose of an arc flash study is to quantify how much energy could be emitted in the event of an arc flash, as calculated per IEEE Std 1584-2018 and based on a number of factors including:
That quantity is called “incident energy” and it’s expressed in calories per centimeters squared (cal/cm2).
The incident energy is used to determine what personal protective equipment (PPE) should be worn when working with energized equipment and where the arc flash boundary is, or the distance from the equipment where no special arc flash PPE is required. When incident energy numbers are derived from an arc flash study, facilities can make informed decisions on how to protect employees and mitigate risks.
Personal protective equipment is only part of the story. The National Electrical Code (NEC) introduced articles 240.67 (for fusible equipment) and 240.87 (for circuit breaker equipment) requiring an approved method for reducing arc energy by reducing clearing times.
The scope of the NEC Arc Energy Reduction requirements applies to all fusible and circuit breaker devices 1200 A and above. Achieving compliance with NEC Arc Energy Reduction requirements starts with understanding arcing current levels around your facility and how they relate to incident energy. Once you have a thorough understanding of the risks, you can take the proper steps to reduce them.
Significantly high levels of arc-flash incident energy can be extremely hazardous. It is imperative to incorporate incident energy reduction methods into the design and installation of electrical equipment to lower these levels and provide for a safer installation for the qualified electrical workers.
The NEC covers arc energy reduction under NEC Section 240.67 for fuses and NEC Section 240.87 for circuit breaker applications.
Learn about the 7 approved methods of NEC 240.87 for circuit breaker applications
Is your current fuse application in compliance with NEC 240.67?
Need help determining the best mitigation plan for your facility? Eaton can help.
