5.2. ERMUSR 03-11-2008 - Laserfiche WebLink

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nited eevices roup~' THREE-PHASE TRANSFORMER ARC FLASH ANALYSIS FOR ELK RIVER MUNICIPAL UTILITIES 2i22io8 Arc flash hazard analysis is important to determine the PPE required for personnel when working with energized equipment. This hazard analysis focuses on three-phase padmounted and substation transformers at various working distances and operational voltages. In addition, time-current curves (TCCs) with the arc flash hazard category superimposed on the TCC are provided for various system fuses and protective devices. The bolted fault current was calculated using an infinite bus calculation and input into the IEEE 1584 arc flash analysis spreadsheet. The resultant arcing fault current was calculated within the spreadsheet through a formula derived from experimental analysis of the relationship between the bolted fault current and the anticipated arcing fault current. Using the calculated arcing fault current value, the total clear time of the transformer high-side fuse or upstream protective device was determined and input into the spreadsheet. The time in which a fault is cleared is by-large the largest factor in the calories emitted during a fault event. In the case of substation transformers, the total clear time was determined using the protection settings proposed in the 2007 Sectionalizing Study. For transformers less than 500 kVA, arc flash hazard analysis was performed for both 208V and 480V bus voltages. For all transformers, working distance of both 381mm (15") and 2440mm (8') were used, which correspond to the distance between the chest/torso and the energized equipment while working with your hands (381 mm) and a hotstick (2440mm). Analysis results include an arc flash hazard risk category classification and arc flash boundary distance, or the distance in which a second degree burn to the face and torso does not occur. Also provided is a table relating transformer secondary current to transformer primary current. This table illustrates that for many transformers a value close to the infinite bus bolted fault current and/or calculated arcing fault current is obtainable given the location of the transformers and the available primary fault current at that location. The following table defines the PPE requirements for each of the arc flash hazard classifications: PPE Ratin Table from NFPA 70E -Table 130.7 C 11 - 2004 Edition Min. Cal/cm Ratin of PPE Max. Cal/cm Ratin of PPE Risk Cate o 0 1.2 0 1.2001 4 1 4.001 8 2 8.001 25 3 25.001 40 4 40.001 No maximum X Exposure to Risk Category X equipment is not recommended. 1 of 3