Top 10

By Jim Phillips | Mar 15, 2011




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You’re thinking, “I’m glad that’s finally over. Never again!” Yes, an arc flash calculation study can be quite overwhelming, especially your first one. But now the labels are on the equipment, the report has been filed away and this never-ending project is quickly becoming a faded memory. You’re finished.

Not so fast! The electric power system is evolving, which means the arc flash study results may also evolve. The arc flash study is based on information, such as the available short-circuit current, protective-device settings and much more. If any of this data changes, the study results—and the requirements for arc-rated flame-resistant clothing and personal protective equipment—may also change.

The National Fire Protection Association (NFPA) 70E, the Standard for Electrical Safety in the Workplace, addresses this concern in Article 130.3 Arc Flash Hazard Analysis: “The arc flash hazard analysis shall be updated when a major modification or renovation takes place. It shall be reviewed periodically, not to exceed five years, to account for changes in the electrical distribution system that could affect the results of the arc flash hazard analysis.”

What is a major modification or renovation? It is actually more a matter of what changes could affect the study results. The following is a list of 10 reasons to take another look at your completed arc flash study.

• Utility short-circuit current in-creases: Electric utilities are continually adding new transmission and distribution lines, substations, and similar equipment to accommodate load growth. Each of these additions creates new electrical paths to an area. This can increase the available short-circuit current, affecting the incident energy.
• Utility short-circuit current de-creases: What goes up can also come down. When utility lines and equipment are taken out of service, the short-circuit current can decrease. The lower current could cause protective devices to operate slower when responding to an arc flash, resulting in increased incident energy.
• Change in transformer size: Short-circuit current is dependent on the size, voltage and impedance of upstream transformers. If a transformer is replaced, the study should be revised to account for the new data.
• Unknown protective-device setting changes: Although not correct or advisable, it happens. If a protective device trips, a common “solution” is to increase the device size or setting rather than determine why it tripped. Sometimes this is done without anyone being told. Unfortunately, this type of “stealth” change will often increase the arc flash clearing time and result in an increase in incident energy.
• New settings: The arc flash study should include a coordination study. However, if you perform a new coordination study after the original study, new device settings, sizes or ratings are often part of the recommendations. These changes need to be included in a revised arc flash study.
• New equipment additions: Any new equipment that was added after the original study also needs to be added to the arc flash study.
• Breaker upgrade: An upgrade of a circuit breaker’s trip unit can change how it responds to an arc flash. New upgrades will typically introduce different tripping characteristics and require new device settings. The result is a change in the duration of an arc flash.
• Change in fuse size: Properly selected current-limiting fuses can greatly reduce the incident energy. However, changing to a slower or larger fuse can sometimes cause the incident energy to increase under the right circumstances.
• Equipment condition: NFPA 70E Article 205.3 requires that overcurrent--protective devices be maintained. If during testing and/or maintenance it is determined that a device’s operation or condition is questionable, it should no longer be included in the study. Instead, the questionable device should be replaced by a reliable upstream device.
• Relay upgrade: Replacing older electromechanical relays with newer digital relays has many advantages, such as multiple setting groups. In general, digital relays can respond faster and more accurately than older electromechanical devices. However, when an upgrade occurs, a coordination study is also performed to account for the new tripping characteristics. The new relays and settings, including maintenance settings, must be reflected in a revised study.

This is only the beginning. There are certainly more than 10 changes and modifications that could trigger the need to review your study. In the end, the arc flash study is never really finished.

PHILLIPS, founder of and, is an internationally known educator on electrical power systems. His experience includes industrial, commercial and utility systems, and he is a member of the IEEE 1584 Arc Flash Working Group. Reach him at [email protected].

About The Author

PHILLIPS, P.E., is founder of and provides training globally.  He is Vice-Chair of IEEE 1584 Arc Flash Working Group, International Chair of IEC TC78 Live Working Standards and Technical Committee Member of NFPA 70E.  He can be reached at [email protected].


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