What Is Meant By 5P20 In The Current Transformer?

What Is Meant By 5P20 In The Current Transformer?

“5P20” is a standard designation used in the term of current transformers (CTs) that provides us about the accuracy class & performance characteristics of the CT under certain conditions.


5P - Accuracy Class

The accuracy class of the CT is indicated by the “5P.” This indicates that, under some circumstances, the CT has an error margin of ±1% at its rated current.

5P -Class of Protection

The letter “P” denotes protection. The “P” class of CTs is made for protection; that is, they are meant to function precisely in the event of a fault, supplying protective relays with exact current measurements so that the relays trip or sound an alarm as needed.

The accuracy limit factor (ALF) is represented by the number “20”. This factor shows that, while keeping the stated accuracy, the CT can measure current up to 20 times its rated value accurately. A CT with a 100 A rating may precisely measure up to 2000 A in fault or short circuit conditions.

  • “5P”: Denotes a CT that has a 5% accuracy rating and is of protection class.
  • “20”: Indicates that the CT is accurately capable of handling up to 20 times its rated current.


In protection systems, a CT with a 5P20 rating is usually utilized when high accuracy is required under high fault currents as well as normal operating conditions to ensure proper and dependable protective relay operation.

The number “5P20” basically means that the CT is very reliable when it comes to protection. It can accurately measure current up to 20 times its rated current, which ensures that the electrical system’s protective devices are operating as intended.