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Current transformer sensitivity

Current transformer sensitivity

Current transformer sensitivity

(OP)
Hi,
1) I've been told that CT (for protection) should not be used to detect a fault current which is less than 10% of its nominal current  because acuracy decreases. Is this true ? if yes Is this applicable for toroid CTs (CBCTs) also ?
2) I have to select a CT (toroid) to install on the neutral link to earth of a HV/LV 2500kVA transformer for earth fault protection. Neutral system is TT (neutal not distributed). What should be the ratio of this CT ? and how sensitive can it be ?    

Thanks for your answers

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