Protection current transformers for overcurrent protection
Protection CTs differ fundamentally from metering CTs: while metering CTs saturate under overcurrent to protect downstream instruments, protection CTs must deliver a proportional signal even at high fault currents. Classes 5P and 10P with overcurrent factors such as 5P10 or 10P10 define the multiple of rated current at which the transformer stays within its error limits (±5 % or ±10 %). Sizing follows IEC 61869 considering short-circuit current, burden and accuracy limit factor (ALF). Protection CTs are essential for overcurrent, differential and distance protection in medium- and low-voltage systems.