Approach Boundaries
Jim Phillips, P.E.Nov 7, 2006
What is Arc Flash?
An arc flash is a short circuit that occurs through air producing a tremendous amount of energy that is violently released in the form of heat, pressure, and a blinding flash. The arc flash can produce severe burns from the heat and molten metal which can injure or kill people that are too close. The pressure can damage hearing and throw people with the large forces that result, causing serious injury. The blinding flash can cause permanent eye damage. In the early 1980’s, an IEEE paper written by Ralph Lee titled “The Other Electrical Hazard: Electric Arc Blast Burns” attracted wide spread attention about the seriousness of arc flash event. Today there are several standards that address this topic including:
IEEE Standard 1584 - Guide for Arc Flash Hazard Analysis This standard provides detailed equations and guidelines for performing arc flash calculations of arc flash energy.
NFPA 70E – Standard for Electrical Safety in the Workplace This standard was developed in response to OSHA’s request for the development of a safety standard that could be used to assist OSHA in preparing electrical safety standards.
NFPA 70 – National Electrical Code© In addition to providing an enormous amount of information used for the design of electric power systems, the NEC© addresses labeling requirements for electrical equipment regarding the arc flash hazard.
What is an Arc Flash Study?
An arc flash study is conducted to determine the potential exposure to the arc flash energy by personnel working on energized equipment. The study determines the Flash Protection Boundary as well as other boundaries defined below and the degree of Personal Protective Equipment (PPE) required for people working within the Flash Protection Boundary. This requires the analysis of a power distribution system to determine what level of incident energy could be released if an arc flash occurs. The incident energy is calculated based on calories per square centimeter and is a function of both the magnitude of short circuit current that would flow as well as the time it would take for an overcurrent device to clear the fault.
What are the Various Boundaries?
There are several boundaries defined by NFPA 70E that are used to identify distances from live parts. Calculating and identifying the Limited, Restricted, Prohibited Approach boundaries and the Flash Hazard Boundaries are all a part of the detailed arc flash hazard study. Proper signage can be used to list these boundaries as well as the class of personal protective equipment. The classifications range from Class 0 which is untreated cotton to Class 4 which includes a complete flash suit. The boundaries as defined by NFPA 70E include:
Prohibited Approach Boundary is defined as “An approach limit at distance from an exposed live part within which work is considered the same as making contact with the live part.”
Restricted Approach Boundary is defined as “An approach limit at a distance from an exposed live part within which there is an increased risk of shock, due to electrical arc over combined with inadvertent movement, for personnel working in close proximity to the live part.”
Limited Approach Boundary is defined as “An approach limit at a distance from an exposed live part within which a shock hazard exists.”
Flash Protection Boundary is defined as “An approach limit at a distance from exposed live parts within which a person could receive a second degree burn if an electrical arc flash were to occur.
NFPA 70E lists boundary distances for Prohibited, Restricted and Limited Approach Boundaries by voltage level, but it does not list the Flash Protection Boundary. Instead, NFPA 70E lists the Flash Protection Boundary as 4.0 feet for systems < 600 Volts based on a combination of overcurrent device clearing time and short circuit current of 300 kA cycles. This is based on a fault clearing time of 6 cycles (0.1 Seconds) with a fault current of 50,000 Amps. 6 Cycles X 50,000 Amps = 300,000 Amp-Cycles or 300 kA Cycles.
This approach is a good first step, however this assumes specific current and time values and many variables can greatly influence the actual Flash Protection Boundary. One variable that is significant is short circuit current. If a short circuit current is greater than 50,000 Amps with a 6 cycle clearing time, the incident energy would be greater and the Flash Protection Boundary would increase.
Overcurrent devices with long time delays can be troublesome as well as equipment locations near strong sources with large short circuit currents such as a service entrance. However, there is a more interesting effect that can occur when the short circuit current is significantly lower. The lower a short circuit becomes, due to increased impedances of small transformers, long conductors and weak sources, the longer it usually takes overcurrent devices to respond. It is quite likely that the device operating time can become so slow under this scenario, that the total energy becomes dangerous even with the lower short circuit current.
To learn how to perform a complete arc flash study, sign up for the Arc Flash Hazard Analysis Class from T2G
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