3/10/2024 0 Comments Arc flash boundary imagesDetermine Protective Device Characteristics And Duration Of Arcs Where Ia = arc fault current in kA K = –0.153 for open-air arcs and –0.097 for enclosed arcs Ibf = 3-phase bolted fault current in kA V = voltage in kV G = conductor gap in mm.ĥ. IEEE 1584 presents two formulas for calculating arc fault currents, one for use with 0.208-1 kV systems, and the other for systems between 1 and 15 kV. Arc fault current calculations are based on voltage, bolted fault current, conductor gap distance, and other factors. The current that flows through an arcing fault is usually significantly less than the bolted fault current, due to greater resistance. The equations shown in 5.2 are incorporated in the programs offered with this standard. For medium voltage applications, the arc current is still a bit lower than the bolted fault current, and it must be calculated. The calculated arc fault current will be lower than the bolted fault current due to arc impedance, especially for applications under 1000 V. The arc fault currents can then be calculated. This will separate fault contributions from normal feeder, alternate feeder, and downstream motors. The bolted fault current in the protective device can be found from the short-circuit study by looking at a one-bus-away run. The arc fault current depends primarily on the bolted fault current. The arc fault current at the point of concern and the portion of that current passing through the first upstream protective device must be found. Lower fault currents often persist longer than higher currents as shown on protective-device time-current curves. It is important to include all cables because to err on the high side does not necessarily increase safety: it may reduce it. For example, connecting transformer secondaries together may not increase fault energy on the primary side. Not every bus needs to be run for every mode because some modes will not significantly impact bolted fault current at some buses. Find the symmetrical root-mean-square (RMS) bolted fault current and X/R ratio at each point of concern-all locations where people could be working-by making each of these points a bus. The simplified calculator included with this standard can determine bolted fault currents for radial systems for up to 600 V (see Figure B.1). Commercially available programs can run thousands of buses and allow easy switching between modes. Input all data from the single-line diagrams and the data collection effort into a short circuit program. Step 3: Determine The Bolted Fault Currents Unit substation with one or two primary feeders.MCC with one or two feeders, one or both energized.Unit substation with two transformers with secondary tie opened or closed.t is important to determine the available short-circuit current for modes of operation that provide both the maximum and the minimum available short-circuit currents. Generators running in parallel with the utility supply or in standby.Utility interface substation secondary bus tie breaker open or closed.One or more utility feeders in service.In a site with a simple radial distribution system there is only one mode of operation-normal-but a more complex system can have many modes. Step 2: Determine The System Modes Of Operation While the data required for this study is similar to data collected for typical short-circuit and protective- device coordination studies, it goes further in that all low-voltage distribution and control equipment plus its feeders and large branch circuits must be included. Regular site employees who are familiar with the site and its safety practices may be able to do this part of the job best. Even for a plant with nominally up-to-date single-line diagrams, time-current curves, and short-circuit study on a computer, the field part of the study will take about half of the effort. The largest effort in an arc-flash hazard study is collecting the field data. Step 1: Collect The System And Installation Data
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