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During last years different procedures have been proposed to calculate electrical arc incident energy, flash boundary limits and provide with flame resistant personal protective equipment recommendations.
The procedures include the IEEE Standard 1584, IEEE Guide for Performing Arc-Flash Hazard Calculations, NFPA 70E Standard for Electrical Safety Requirements for Employee Workplaces 2004 edition as well as program of heat flux calculation from Duke Power.
All methods are based on testing performed and calculations conducted for selected range of prospective fault currents, system voltages, physical configurations etc. The methods caution they are suitable for estimating arc flash hazards only, and actual cases experienced in the field can be expected to vary from the predicted values.
Selecting an appropriate method for incident energy determination is confusing and difficult. Comparison of the above methods shows varying computed values for incident energy (see http://www.atc-trng.com/compareArcFlashCalc.html for more on the comparison of arc flash calculation methods). The IEEE Standards method based on empirical equations developed through multiple tests of varying fault scenarious appears to be the most accurate one. It is applicable for systems with:
- Voltage of 208 to 15,000 volts
- Bolted fault current of 0.700 to 106 KA
- Grounding variations
- Open air or equipment enclosures of commonly available sizes
- Gaps between conductors of 13mm to 152 mm (0.5 to 6 inches)
Still, the procedure is limited to three phase arcing faults and therefore is not applicable for unbalanced arcing fault studies. Duke's heat flux calculator does the job for single phase arc fault in air but requires arc current input. The input arcing current, dependent on the geometry, prospective bolted fault current, voltage, has to be calculated by other means in advance.
The IEEE paper "Predicting Incident Energy to Better Manage the Electric Arc Hazard on 600-V Power Distribution Systems" of the IEEE Transactions on Industry Applications Vol. 36, No. 1, January/February 2000 used for the NFPA 70E calculation method compared the measured three phase results with the calculated results given for single phase arcs. It reported that, "three-phase test values of maximum incident energy for the open arcs were from 2.5 to 3 times the values predicted by the single-phase models. Three-phase test values of maximum incident energy for the arcs in the cubic box were 5.2 to 12.2 times the values predicted by the single-phase models".
Fuses and instantaneous circuit breaker will open in less than quarter of cycle (4 ms) when subjected to high short circuit currents usually observed in 3 phase symmetrical bolted case. The criteria is that the SC current exceeds let through limit specified for a particular protective device. Prospective fault current will never reach there maximum and exersice full incident energy potential when the criteria is met.
The problem with unbalanced fault currents is that they are lower than 3 phase fault currents. Therefore, circuit protection devices may not necessary limit let thru energy and will require more time to clear the fault. Even 3 phase faults may translate into unbalanced fault when one fuse opens before the remaining fuses melt. For safety reason, it makes sence to analyse both three and single phase / unballanced cases and use the highest calculated incident energy value in sizing personnel protective equipment.
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