Whenever energized electrical equipment is being examined, serviced, maintained or adjusted in any way, there is always the potential for an electrical explosion to occur, resulting in injury to the electrical worker and damage to the equipment. Understanding some of the technical information involving electrical flash hazards should help minimize exposure to the arc and flash potential of certain electrical equipment.
Electrical safety for personnel in the workplace is mandated by the Occupational Safety and Health Act (OSHA) of 1970. National Fire Protection Association (NFPA) 70E, the Standard for Electrical Safety Requirements for Employee Workplaces, provides an electrical safety document that is based upon information extracted from of the National Electrical Code (NEC).
Part II of NFPA 70E covers electrical safety programs, use and types of personal protective clothing and equipment when working on or near energized electrical equipment, and the lockout-tagout of electrical equipment prior to working on the equipment. Part II also contains information on the hazard risk analysis of electrical equipment that essentially quantifies the degree of risk associated with a particular electrical task, such as removing and replacing a circuit breaker in an energized panelboard.
A flash hazard is defined in NFPA 70E as a “dangerous condition associated with the release of energy caused by an electric arc.” A flash hazard analysis must be done before a person is permitted to approach any exposed electrical part that has not been placed in an electrically safe work condition, such as equipment de-energized by lockout and tagout. Once a flash hazard analysis is accomplished, the type of personal protection equipment can be determined based upon the amount of risk that is involved with that particular task.
Part II also contains information on the flash protection boundary for electrical equipment or, in other words, the minimum safe distance away from the electrical equipment in an arc flash. Critical in establishing the arc flash boundary are the amount of arc energy available, the temperature rise on a person’s skin from the arc flash, the amount of arc blast or pressure exerted on the body and the time the person is exposed to the arc flash.
The amount of available fault current at the equipment and the period of time the system is in a “bolted fault condition” are critical in determining the flash boundary for a particular piece of electrical equipment. The higher the fault current level and the longer the fault is permitted to exist, the greater the flash boundary distance that is required. As covered in Section 2-220.127.116.11 of NFPA 70E, systems that are 600 volts or lower must have a flash protection boundary of 4 feet from the equipment so that a burn resulting from an electrical arc should not be greater than second degree (considered a curable burn).
The 4-foot measurement is based on the product of the clearing time of six cycles (0.1 seconds) in a 60-hertz system and the available bolted fault current of 50,000 amps. By multiplying the cycles by the available fault current, a value of 300,000 ampere-cycles is determined. If time is multiplied by the bolted fault current, then a value of 5,000 ampere-seconds can be used. If the bolted fault current is raised, a faster overcurrent protection device can be used so that the combination does not exceed 300,000-ampere cycles.
As can be seen by the method just covered for measuring an electrical arc flash protection boundary on systems rated 600 volt and less, performing the calculations accurately and determining the boundary distance requires a very qualified person. For this and other reasons, the definition for a qualified person in Article 100 of the 2002 NEC has been changed. A qualified person is one who has the skills and knowledge related to the construction and operation of the electrical equipment and installations, and who has received safety training on the hazards involved.
Section 110.16, covering flash protection marking, has been added to the 2002 NEC. It provides a link to NFPA 70E and OSHA for requirements dealing with arc flash hazards. It requires field marking for switchboards, panelboards, industrial control panels and motor control centers. The marking is intended to warn qualified persons of flash hazards where the equipment is likely to be examined, adjusted, serviced or maintained while energized. The marking must be located on the equipment in such a manner as to be clearly visible to qualified personnel before they commence work on the equipment.
Make sure this field marking is applied in a very visible location so when you work on electrical equipment while energized, you are very aware of the hazards involved. Take the necessary safety steps to ensure that no one suffer serious burns or injuries. EC
ODE is staff engineering associate at Underwriters Laboratories, Inc., in Research Triangle Park, N.C. He can be reached at 919.549.1726 or via e-mail at email@example.com.