Anomalies and Practical Considerations of Arc Flash Hazard_25Nov2014
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Anomalies and Practical Considerations of Arc Flash Hazard_25Nov2014
- 1. Daniel Lang, EIT November 25th, 2014
- 2. • Brief background of Arc Flash Analysis • How is a study done? • How are the equations derived? • Anomalies (and their consequences) inherent in the equations • Practical Considerations in Energy Calculations • Efficacy of the conventional Arc Flash Hazard Analyses • New Developments Outline
- 3. Simplified Procedure: Acquire the Necessary Data Calculate the Bolted Fault Current Calculate the Estimated Arcing Current Calculate Fault Clearing Time Calculate the Incident Energy How is an AFH analysis done? img. from IEEE Std 141-1993
- 4. • Ralph Lee proposed in a 1982 paper, a method to calculate arcing energy • Assumes Pmax of ½ of available MVA (max power transfer theorem) • Assumes Ia/Ibf = 0.7 𝐸 = 2.142 × 106 𝑉𝐼 𝐵𝐹 𝑡 𝐷2 How The Formulae Are Derived – Theoretical Lee Equations Img. from Practical Solution Guide to Arc Flash Hazards, ESA, Inc., 2003
- 5. • Approximately 300 individual trials were done over the following ranges: • 13.8kV: 5.7kA – 40.8kA • 4.16kA: 5.4kA – 40.4kA • 2.3kV: 2.6kA – 16.6kA • <1kV: 0.7kA – 106kA • Statistical analysis via least squares method • Equation normalized for • 0.2 seconds • 610mm distance. How The Formulae Are Derived – Empirically Derived IEEE 1584-2002 Equations Img. from Calculating Hazards, IEEE IAS, June 2005
- 6. Theoretical Lee Equations: • Three Phase Fault • Over 15kV • No current constraints • Open Air • AC • No gap constraints IEEE 1584 Equations: • Three Phase Fault (or quickly escalate to three phase) • 208V – 15kV • 700A – 106kA • Any grounding method • Many enclosure types • AC The Ranges of the Equations There are currently no equations to analyze single phase or DC arcing events
- 7. • Iarc > IBF • E(air) > E(box) • Unrealistic Clearing Times • Induction and Synchronous Machine Contribution • High Arcing Current Tolerances Some of the possible anomalies and considerations
- 8. • Intuitively IAF < IBF • IBF < 1.489kA effect of gap length in calculating IAF is reversed Result unrealistic arcing current values In practice max at 1.0 pu of IBF Equation Anomalies re Varying Conductor Gap log 𝐼 𝑎𝑟𝑐 = 𝐾𝐴 + 0.662 log 𝐼 𝐵𝐹 0.662 + 0.5588𝑉 − 0.00304𝐺 + 0.0966V + 0.000526G Img. from Calculating Hazards, IEEE IAS, June 2005 ArcingCurrent vs Bolted Fault Current for varying gap size
- 9. • Intuitively E(air) < E(box) • Adjusting via “distance exponent” produces an anomaly. • LV systems, d < 166mm • HV systems, d < 358mm Anomaly is less significant Equation Anomalies re Varying Working Distance Img. from Calculating Hazards, IEEE IAS, June 2005 Ratio of E(air)/E(box) vs. Working Distance
- 10. • Exceedingly long clearing times can lead to unrealistic results • Arc sustainability? • Personnel movement? • Currently no standard to address this Result Widely accepted maximum clearing time of 2 seconds. Considerations for High Clearing Times Max IAF never clears
- 11. • Motor contribution is done by discrete current steps, which overstates the energy. • The IEEE 1584 equations do not take into account asymmetrical (DC) components In Practice? Considerations of Induction and Synchronous Machine Contributions Img. from Cahier Technique No. 158, “Calculation of Short- Circuit Currents”, ECT 158, Sept. 2005. Img. from J.C.Das, “Arc Flash Hazard Analysis and Mitigation”, p. 194, 2012 0 𝑡 𝑖2 𝑡 𝑑𝑡
- 12. • For <1kV systems, at a 95% confidence level, the error can be substantial Considerations of High Arcing Current Tolerances Img. from Practical Solution Guide to Arc Flash Hazards, ESA, Inc., 2003 log 𝐼 𝑎𝑟𝑐 = 𝐾𝐴 + 0.662 log 𝐼 𝐵𝐹 0.662 + 0.5588𝑉 − 0.00304𝐺 + 0.0966V + 0.000526G IE at Ia = 8.0 cal/cm2 IE at 77% Ia = 22.0 cal/cm2
- 13. • Given the above possibilities for error, the formulas still produce good results. • An analysis done in [1] below summarizes 22 arc flash events involving 30 personnel. The article concluded the following : 2 • PPE performs as expected as long as its rating matched the hazard calculation results • When injuries occurred it was because PPE was either used improperly or not used at all Efficacy of conventional arc flash hazard analysis [1] D.R. Doan andT.E. Neal, “Field analysis of arc-flash incidents,” IEEE IndustryApplications Magazine, vol. 16, no. 3, 39-45, June 2010 2 J.C.Das, “Arc Flash Hazard Analysis and Mitigation”, p. 194, 2012 In conclusion: Get an arc hazard risk assessment done and wear the proper PPE!
- 14. • Testing for the new IEEE 1584 standard will have more than 1000 tests conducted • Differing bus orientations • Vertical in cubic box • Vertical in cubic box, terminated • Vertical in open air • Horizontal in cubic box • Horizontal in open air • New research and analysis will be done • Time domain analysis (Ia Time-Variant) • DC arc flash (may) be added • Likely the “208V, 125kVA” rule will be reduced, to examine AFH on smaller systems Result Modeling will become more complex New Developments in Arc Flash Hazard Analysis Img. from Calculating Hazards, IEEE IAS, June 2005
- 15. Questions
Editor's Notes
- The purpose of this presentation is to expose engineers and designers to just a few of the possible errors and inconsistencies one can encounter when conducting arc flash energy calculations.
- If you ask five different people each their method and process for calculating arc flash energy, you may very well get five different answers. For the shown low voltage bus, the following analysis is required. First the equipment parameters must be acquired, so in this case the fuse time current characteristics, the transformer data and the upstream source impedance. The modes of operation need to be considered such as variance in the utility contribution, as well as bus tie scenarios. A short circuit study is then conducted. Using the IEEE arcing fault equations, the estimated arcing current is found. From this arcing current, the upstream protective device clearing time is then found, which is a function of the calculated arcing current. Using additional info such as the grounding and equipment type, the overall worst case incident energy is then calculated.
- In 1982, an engineer named Ralph Lee proposed a method to quantify the energy of an arcing fault. This was done theoretically, and leads to highly conservative results. It would be another 20 years until empirical formulas would become available. -Arcing current of 0.7 bolted fault current is based on the maximum power transfer theorem. As maximum dissipation occurs when source and load impedances are equal
- The empirically derived IEEE 1584 equations were done by laboratory tests. Over 300 individual trials were done. Three conductors would be placed in a vertical arrangement with calorimeters placed at varying distances away. An arc would be initiated vie fuse wires, and fault current, as shown in the specified ranges would flow for 0.2 seconds. Arc current would be measured, as well as the energy dissipated. Statistical analysis would then be done on the raw data to derive the formulas.
- Here is a summary of the ranges of each of the equations. The equations are valid only for three phase faults, and while they statistically make up a small percentage of faults, it provides a conservative estimate. For instances where gap is out of the IEEE 1584 range, such as for substation IPS bus, the Lee equations are then used. At this time there are also no equations which analyze single phase or DC arcing events.
- Intuitively, arcing impedance will always cause the ratio of arcing current to bolted fault current to be less than one. However, at currents less than 1.5kA, the effect of the gap length is reversed, causing higher arcing currents than the possible bolted fault. As a result, it produces unrealistic arcing current values, which as shown in the last slide, drastically increase the overall error in the energy calculation. This can also occur at the opposite end of the equation range. In practice, software packages will max out the arcing current at 1.0pu of the bolted fault.
- -How the experiments are done both in open air and within enclosures -Intuitively, energy in a box is greater than energy in air -Anomaly shows that as you convert from the normalized to
- While not necessarily an anomaly, this slide outlines the greatest source of error within an arc flash energy calculation, the arcing current estimate. The incident energy formula gives a predicted value with a statistical 95% confidence level. This means that there is a numerical 95% confidence that the PPE level will be more than adequate. However, to achieve this confidence level, the predicted arcing current can vary wildly (as shown in the histogram). This is why the IEEE 1584 standard suggests using two energy values, one at the calculated value and one 15% lower.
- These anomalies and errors shown previously may suggest that the formulas do not adequately represent reality. However, the following should be noted: It is the best we have as of now It is the current standard Field events conclude that arc flash analysis does prevent injury