Transient States during Short-circuit in a Line Powered by UPS and its Impact on Earth Fault Loop Impedance Measurement and Power Quality

The short-circuit in a line powered by Uninterruptible Power Supply (UPS) during different UPS operation modes is presented in the article. The transient state condition related to the short-circuit in a line fed from double conversion On-line UPS is discussed in detail. The results of the measurements of earth fault loop impedance (FLI) in circuits powered by UPS with the instruments of various manufacturers are presented. The dispersion of the measurement results, depending on UPS operation mode and on the instrument test current value is observed. The uncertainty of FLI measurement in UPS-powered circuits is pointed out, and its impact on the power quality and the evaluation of effectiveness of protection against electric shock is explained.