Restricted Approach Boundary- approach by a Qualified Person. Typically, in the UK, switchpanels contain live parts within enclosure and it is only when such enclosures are open and live parts are accessible does an issue exist. In Europe we would know this as an ordinary person, that being a person who is neither electrically skilled nor instructed and is the minimum distance that a non-competent person can approach parts that are energized and dangerous. Limited Approach Boundary- approach by Unqualified Persons. This article looks at the arc flash boundary, however, before moving on it is work discussing the limited and restricted approach boundaries that in fact relate to electric shock protection, rather than arc flash control. Dependant on the remit of any arc flash study carried out, labels fitted to switchgear may make reference to one, or all of the above boundaries. Arc Flash Boundary DistancesĪccording to NFPA 70E, three boundaries exist, these are Limited Approach Boundary, Restricted Approach Boundary and Arc Flash Boundary. The main purpose of PPE is to reduce the level of burn injury to a worker to a level of curable burn.Compliance Director, Andy Linley, discusses Arc Flash Boundary distances, what they are and factors affecting them. There are various types of PPE with distinct levels of thermal protection capabilities termed Arc Thermal Performance Exposure Values (ATPV) rated in cal/cm 2. ARCAD Arc Flash Analytic (AFA) software program for arc flash hazard analysis and labeling calculates arc flash boundary based on 1.2 cal/cm 2 onset energy to second degree burn for bare skin exposure as well as threshold incident energy calculated as a variable function of heat flux. Using 1.2 cal/cm 2 threshold incident energy to 2nd degree burn for exposure times less than one second will result in undervalued arc flash boundaries while resulting in conservative but save arc flash boundaries for exposure times more than one (1) seconds. It was shown in Evaluation of Onset to Second Degree Burn Energy in Arc Flash that using onset to second degree burn energy for bare skin exposure fixed to 1.2 cal/cm 2 in calculating the arc flash boundary for arc durations other than one (1) second is open to dispute and heat flux rate should be factored-in when estimating skin damage imposed by an arc flash. It is assumed that a second-degree burn will be curable and will not result in death. One cal/cm 2 is equivalent to the amount of energy produced by a cigarette lighter in one second at 1 centimeter distance above the flame. The threshold value of incident energy for 2nd degree burn of human skin is generally assumed to be equal to 1.2 cal/cm 2 (5 Joules/cm 2). The incident energy analysis requirement stipulated in NFPA 70E Article 130.3(B)(1) is the basis for performing an arc flash hazard calculation study (AFHCS). The selected PPE arc rating shall be based on the incident energy exposure. Incident energy analysis is required for selecting arc rated Personal Protective Equipment (PPE) when work is performing at or near a potential source of an arc flash. These values are compared and the largest is used in the arc hazard analysis. Two calculations for incident energy are recommended by NFPA 70E-2015 Informative Annex D.5 - one at the predicted arcing current and another at the predicted arcing current reduced by 15%. The incident energy is calculated based on the available bolted fault current, system voltage, the clearing times of the associated over-current protective devices and the individual parameters of the electrical distribution system. The most common distance for which incident energy has been determined in tests is 18 inches. The working distance is the distance from where the worker stands to the flash location. Incident energy is a measure of thermal energy at a working distance from an arc fault.
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