In addition to general specification required for CT design, protection CTS require an Accuracy Limit Factor (ALF). This is the multiple of rated current up to which the CT will operate while complying with the accuracy class requirements. In general the following applies:

• Instantaneous over current relays & trip coils: 2.5VA class 10P5
• Thermal inverse time relays :7.5VA Class 10P10
• Low consumption Relay : 2.5VA class 10P10
• Inverse definite min. time relays(IDMT) Over current: 15VA Class 10P10/15
• IDMT earth fault relays with fault stability or accurate time grading required: 15VA 5P10