Protective current transformers are required to operate over a range of current many times the curcuit rating and is frequently subjected to conditions greatly exceeding those which it would be subjected to as a measuring current transformer. When a current transformer is used to energise a protective relay, it must manintain its characteristic ratio up to some multiple ofits rated current.

This multiple, may be5, 10, 20or some even higher value and is know as the "accuracy limit factor "(ALF). Therefore, the selection of proterctive current transformers in relation to accuracy class and ALF require a close examination of relay charactristic and curcuit conditions which inclues the relay burden and the pilot wire lead burden.