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The relay measures the line current from each of the line CTs and if the current exceeds the set threshold, then relay extends a trip signal after an operating time.The relay's “operating time” is determined by selecting “definite time” or one of the six “inverse time” characteristics.
Input to sensitive earth fault protection can be derived in two ways:Residual current derived from three phase current transformers can be used as an input Out put of core balance current transformer (CBCT) having appropriate characteristics can be used as an input.Input to high impedance restricted earth fault protection will be derived from neutral CT output with external stabilizing resistor.Phase over current, Earth fault, Sensitive earth fault/Restricted earth fault protections features can be made active or inhibit independently. Inhibition of all protection is not possible.