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Reply To: Protection Riddle No.48 – Earth Fault on motor starting

#1673
Hamid

    In starting situation we front to unbalance and zero sequence secondary currents due to unequal three ct characteristics (CT magnetizing current and ratio error), possible unequal CT core saturation and unequal CTs secondary circuit impedance. Generally there are some recommendations in this regards as following: To provide earth-fault protection, it is recommended the core-balance CT solution whatever type of network earthing is used. This method allows for the detection of small resistive values of earth fault current. Furthermore, an earth fault unit supplied from the residual connection of three line CTs as mentioned above carries the risk of mal-operation during starting. In order to obtain stability the pick-up must not be set below 0.15 to 0.2 In of the CTs, which is often too high compared to the maximum earth-fault current. Most LV systems fall into solidly earthed category, for reasons of personnel safety. Two types of earth fault protection are commonly found