Arc Flash Protection Explained

By Bill Brown, P.E., Square D Engineering Services

Arc Flash Protection

With the increased interest in medium voltage power system protection in recent years, the ability of overcurrent protective devices (OCPDs) to provide protection against arcing faults has received much interest. The potential severity of an arc flash event at a given location in a power system depends primarily on the available fault current, the distance of the worker away from the source of the arc, and the time that it takes the upstream OCPD to clear the arcing fault from the system. In many cases, little can be done about the first two factors—the available fault current levels depend on utility system contribution, transformer impedance values, etc.; while the working distance is limited by the fact that a worker working on a piece of equipment must, in most cases, be physically close to the equipment.

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Proper selection and application of OCPDs to improve arc flash protection can have a great deal of impact on the fault clearing time. Clearing the fault more quickly can provide a great deal of protection for workers, as the available incident energy is directly proportional to the duration of the arcing fault—i.e., the incident energy can be cut in half if the fault can be cleared twice as quickly. Equations appearing in IEEE Standard 1584-2002 [5] provide the present “state-ofthe- art” methods for determining the arc-flash hazard levels in a system and for evaluating the impact of potential arc-flash mitigation options. For low-voltage systems, which OCPDs provide the best protection against arc flash?

• Circuit breakers, with adjustable trip units that can be set to strike a balance between providing selective coordination and arc-flash protection?

• Current-limiting fuses, which can clear high-level faults very quickly and minimize damage to both equipment and personnel?

Unfortunately, there is no simple answer to this question, despite claims made by manufacturers of both types of OCPDs. In some cases, both circuit breakers and fuses provide excellent protection. There are situations when circuit breakers can perform better than fuses, and there are situations where fuses can perform better than circuit breakers. And there are situations where neither circuit breakers nor fuses provide much arc flash protection at all, requiring either use of other means of personal protective equipment ppe (determined by national fire protection association nfpa) or equipment (alternative system designs, installing systems that allow for remote operation of equipment, etc.) or a total prohibition of work on or near energized parts.

When evaluating OCPDs in terms of the arc flash protection that they may provide, three general principles are important to consider:

• Evaluate specific devices when possible

• Evaluate devices at the actual system fault current levels

• Evaluate adjustable-trip circuit breakers at their chosen settings


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