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By By Rodney West, P.E., and Alan Manche, P.E., Schneider Electric
Electrical distribution equipment in all voltage classes often has ventilation openings to permit the circulation of air by convection within the enclosure. These ventilation openings help to maintain component temperatures inside the enclosure within certain design limits. If these openings are blocked, either inadvertently or intentionally, overheating of the equipment may result. This overheating may even lead to a “thermal runaway” condition resulting in a catastrophic failure and destruction of the equipment.
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Article 100 of the 2011 National Electric Code (NEC), also known as NFPA 70, defines the term “ventilated” as “provided with a means to permit circulation of air sufficient to remove an excess of heat, fumes, or vapors.” NFPA 70E 2009 contains the same definition in Article 100. Both low-voltage and medium-voltage equipment may contain these ventilation openings. For example, some switchgear, switchboards, panelboards, busways, and transformers contain ventilation openings in their enclosures.
Furthermore, the NPFA 70E 2009 Standard for Electrical Safety in the Workplace, and the National Electric Code both define the term “enclosed” in the same way — that is, “surrounded by a case, housing, fence, or wall(s) that prevents persons from accidentally contacting energized parts.”
Ventilated electrical distribution equipment is enclosed. The definitions of “enclosed” in the standards clearly focuses on the design intent of the enclosure, which is to prevent accidental or inadvertent contact by personnel. Being able to visually see energized parts, whether through a chain link fence surrounding an outdoor substation, or through a ventilation opening in an equipment enclosure, is not a factor in determining whether or not the energized parts are suitably isolated to prevent accidental or inadvertent contact.
The design of ventilation openings in electrical equipment is strictly governed by the standards applicable to the particular type of equipment. The National Electrical Manufacturers Assn. (NEMA), Underwriters Laboratories (UL), and the American National Standards Institute (ANSI) are examples of U.S. standards agencies that govern the construction of electrical equipment. For ventilation openings, the standards specify critical details such as the location, sizes, and configurations of these openings in order to improve product performance while preventing access to energized conductors or circuit parts.
Ventilated equipment doesn’t expose users to a hazard. A review of the product safety standards clearly addresses concerns of personnel being accidentally or inadvertently exposed to hazards through equipment ventilation openings. The NEC and NFPA 70E also address this question. In fact, the definitions of “exposed” in the NEC and in the NFPA 70E are nearly identical. The definition in NFPA 70E contains four additional words. The definitions of “exposed” are: “capable of being inadvertently touched or approached nearer than a safe distance by a person. It is applied to (electrical conductors or circuit) parts that are not suitably guarded, isolated, or insulated.”
Once again, the common theme is avoiding inadvertent or accidental contact with energized conductors or circuit parts. This is addressed not only in the product safety standards, but also in the installation code (NEC) and the electrical workplace safety standard NFPA 70E. In addition, the concept of safe approach distances is introduced through the definition of “exposed.” Approach distances, also known as “boundaries,” only exist when there are hazards, such as shock or arc flash hazards, present. The enclosure requirements, including ventilation openings, are addressed by the product safety standards. These clearly address the electrical shock and other hazards. In addition, NFPA 70E provides further guidance on arc flash concerns. NFPA 70E states: “Under normal operating conditions, enclosed energized equipment that has been properly installed and maintained is not likely to pose an arc flash hazard.” In addition to the existing language, the next edition of NFPA 70E (2012) will likely add information further clarifying that normal operation of enclosed equipment is not likely to expose employees to electrical hazards.
The National Safety Council estimates that approximately 30,000 nonfatal electrical shock accidents occur each year. In addition, each year more than 2,000 people are admitted to burn centers with severe arc flash burns. Many of these events happen while people are working on exposed energized conductors, with doors and covers open. While very effective at preventing electrical shock injuries or inadvertent contact, enclosed equipment, whether it is ventilated or non-ventilated, will not necessarily contain an arc flash. Only equipment that is specifically rated as “arc resistant” (ANSI designation) is designed and tested to withstand the internal pressures generated during an arc flash event.
While misinterpretations regarding the safety of ventilation openings in electrical equipment occur from time to time, a careful review of the standards and terminology is an excellent way to truly understand the various construction features used in electrical equipment enclosures and what exposure they create.
Rodney West, P.E., is senior staff engineer at Schneider Electric (www.us.schneider-electric.com). Contact him at email@example.com. Alan Manche, P.E., is director of industry standards at Schneider Electric. Contact him at firstname.lastname@example.org.