Protection Current Transformer can work well under the current which many times more than the rated and it can also work undermany times more than the rated measurement of transformer. When Protection Relays are powered by Protection Current Transformer, it must make sure that the Protection Relays are keeping their normal electrical characteristics under the several times more than rated current which maybe 5 times, 10 times, 20 times or more. Just as we know ”constraints of security” . So the choose of Protection Current Transformer involve security level and constraints of security. It needs check carefully about relay parameters and the cases of electric circuit such as relay load and aerial wire load.