UPS: measure impedance without emergency power equipment, e.g. after activating the bypass switch - obtained results will be increased (overstated) by the impedance of the power supply system up to the installed UPS device. While calculating short circuit current take the lowest voltage that occurs after a sudden increase in load. If the calculated fault current is greater than the limited fault current provided by a backup source in the worst operational condition, take the limited current for further calculations.
Motors powered from inverters:
In this case the unknown level of inverter settings may create a problem (resulting in changed fault loop impedance) during the shorting – this prevents determining the fault loop current. Another problem is changing input voltage which prevents correct measurements with instruments for system measurements. In such situation we recommend checking whether during an earth fault the touch voltage on the available conducting part decreases to a value that does not exceed permissible values of prolonged touch voltage in environmental conditions.
Problems during measurements on systems with inverters are described in details in the paper of Lech Danielski PhD Eng. and Ryszard Zacirka PhD Eng. Entitled “Analysing electric shock protection in systems powered by inverters or UPS with frequency converters”, 6th SONEL Technical Conference “Measurements on Protection Instruments and Diagnostics of Systems and Electric Equipment”, Zawiercie 2011.