Designing Safety

In the electrical industry, a new method of protecting workers from arc energy is gaining popularity: prevention through design. Simply speaking, the design and installation of equipment or systems incorporates inherent safety features that protect workers from serious arc-flash injuries or death. The 2011 National Electrical Code (NEC) added Section 240.87, Non-instantaneous Trip, achieving safety measures through a reduction of arc energy on the load side of all circuit breakers that were not equipped with an instantaneous trip function (non-instantaneous trip types).

Note that, in the 2011 NEC, there was no ampere (A) value to trigger this requirement. The rule required all circuit breakers not equipped with an instantaneous-trip function to provide a means of reducing energy on the device’s load side by use of zone-selective
interlocking, differential relaying or an energy-reducing maintenance switch with local status indicator. By requiring this type of equipment, a level of energy reduction can be achieved for any justified energized work that may be performed on the breaker’s load side. The device’s clearing time is directly proportional to the amount of energy and is reduced to allow fast operation, thus reducing the amount of arc energy in the event of a ground fault or short circuit. This is fairly new ground for the NEC in that the design and installation would provide inherent arc flash and arc blast protection for workers on the load side of non-instantaneous trip breakers installed in the distribution systems.

Fast forward to the 2014 NEC. This new requirement was significantly revised to clarify its application in systems, and it was expanded to include other acceptable methods of arc flash mitigation. The first revision is the title of 240.87, which was changed from “Non-instantaneous Trip” to “Arc-Energy Reduction.” This change is appropriate for two reasons. First, it provides significant clarity. Second, arc-energy reduction can be achieved by equipment other than non-instantaneous trip circuit breakers. Another important revision was to introduce an ampere-size threshold for the breaker. The rule now indicates that, where the highest continuous current trip setting is rated at or can be adjusted to 1,200A or higher, the requirements in 240.87 apply.

Additionally, other technologies effectively reduce energy through an active arc-flash mitigation system. In other words, these systems incorporate multiple components that work together during an arcing event and quickly open the circuit, thus reducing the arc-energy level on the equipment load side. Note that a new list item (5) in 240.87(B) recognizes an approved equivalent means, allowing the authority having jurisdiction to approve equal and effective methods of providing the same protection. This is important, as technologies that can provide this type of protection continue to be developed.

This arc-energy-reduction requirement provides protection on the load side of 1,200A and larger circuit breakers by simply requiring a means to reduce the amount of arc energy should a ground fault or short circuit occur while justified energized work is being performed on load-side equipment. By having arc-energy-reduction means ahead of the circuit, workers are protected to a great degree because the equipment-clearing time is significantly reduced. It should be understood that, in the hierarchy of risk-control methods (NFPA 70E, 110.1(G), Informational Note), hazard elimination by establishing an electrically safe work condition is always the first choice, and personal protective equipment is the last.

An important operational aspect of the arc-energy-reduction methods specified in 240.87 is that all but one method is automatic, meaning no human intervention is necessary to ensure the energy-reduction process would occur automatically if a ground fault or short circuit were to occur during justified energized work. The one energy-reduction method mentioned in this rule that requires human intervention is in 240.87(3). The energy-reducing maintenance switching equipment must be activated by the user so as to place the equipment in the instantaneous mode without intentional delay, thus reducing the clearing time and energy on its load side. Ideally, the equipment on the load side of such a device should be clearly labeled to trigger the human intervention necessary to activate the energy-reducing maintenance switch. Currently, there is no such labeling requirement. This is an opportunity for NEC users to revise the Code and address the apparent need for marking.

By including requirements for arc-energy-reduction methods in the NEC, protection for workers against dangerous levels of arc flash is achieved through design and installation. Recognized terms for this type of injury prevention are “safety by design” and “prevention through design.” Looking ahead, a new Section 240.67 has been proposed and will require fusible switches at 1,200A and greater ratings to incorporate arc-reduction means, similar to those provided in 240.87. This is new ground for the NEC that increases safety for workers that must perform justified energized work on larger equipment.

About the Author

Michael Johnston

Executive Director of Standards and Safety, NECA

Michael Johnston is NECA’s executive director of standards and safety. He is a member of the NEC Correlating Committee, NFPA Standards Council, IBEW, UL Electrical Council and NFPA’s Electrical Section. Reach him at

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