Electric energy has two major risks: electrocutation and burn injury caused by accidental arc flash.In many cases the burn injury risks are higher and the consequences are more severe than the electrocutation. The arc flash usually happens at unexpected breakdowns or during works carried out on exposed under voltage equipment.
In the United States serious attention is devoted to avoiding the dangerous effects of arc flash caused by operating faults and breakdowns in electric systems. Standards define the arc energy calculation methods, and the personal protective equipment levels to be worn to avoid burn injuries. The minimum goal is to avoid level two or even higher level burn injury when exposed to unexpected arc flash. In the countries of the European Union this topic is not treated as intensively as in the US, there are no standards to be introduced in order to calculate the arc energy level happened during electric short circuits in electric systems.
The topic of my thesis is the completion of arc flash analysis in Energy Department high voltage distribution system and low voltage secondary distribution system in American Alcoa-Köfém Ltd. based on the regulations of IEEE Guide for Performing Arc-Flash Hazard Calculations 1484-2002 and Standard for Electrical Safety in the Workplace NFPA 70E.
At the beginning of my work I thoroughly studied and comprehended the above mentioned standards. Then I defined the parts of the standards concerning the electric systems I was examining. I started on-the-spot data collections from the existing documentations as well as electric systems during repeated inspections with the help of the company’s electric experts.
In order to calculate the arc energy level two main pieces of information are needed: the short circuit currents and the disconnecting time of the protection devices. These pieces of information are not supplied in the electric design documentation, especially in the case of formerly installed electric systems.
So as to determine the short circuit currents first I had to calculate the electric network impedances. For this calculation I specified the length, sizes and type of cables, bus bars during on-the-spot surveys.
The determination of the disconnecting time of the short circuit currents was a complex task. As regards the high voltage circuit breakers I managed to collect this time from the supplied protection protocolls. In the case of the low voltage circuit breakers there were no such protocolls available, therefore I had to read the setting parametres of these circuit breakers on the spot, then determine the corresponding disconnecting time from the manufacturers’ catalogues.