Current Transformer Accuracy Class Explained

Two numbers on a rating plate decide whether a reading can be billed. Here is what they guarantee, and where the guarantee stops.

MSQ current transformer rating plate showing accuracy class

On this page

Measuring Classes
Why The S Classes Exist
Protection Classes
Choosing
Class Is Only Valid At The Rated Burden

Measuring Classes

A measuring class such as 0.5 limits the ratio error at rated current. The class is a promise about accuracy near the top of the range, and the error is allowed to grow as the current falls. Standard measuring classes across this range are 3, 1, 0.5, 0.5S, 0.2 and 0.2S.

Why The S Classes Exist

A feeder that idles overnight and peaks in the morning spends most of its life at a small fraction of rated current. An ordinary Class 0.5 transformer only has to hold 0.75 % error at 5 % of rated current. A Class 0.5S has to hold its error down at 1 % of rated current, which is why revenue metering specifications call for the S classes.

Protection Classes

A protection class is written differently: 5P20 means 5 % composite error at an accuracy limit factor of 20 times rated primary current. The design goal is the opposite of a measuring core. A measuring core is meant to saturate above rated current to protect the instrument; a protection core is meant to stay linear so the relay can see the fault.

Choosing

Class 3 for indication only
Class 1 for general panel metering
Class 0.5 for accurate monitoring and check metering
Class 0.5S or 0.2S where the reading is billed
5P10, 5P20, 10P10 or 10P20 for relay inputs, matched to the relay's requirement

Class Is Only Valid At The Rated Burden

Every accuracy class is declared against a rated burden. Change the burden and the class no longer applies, which is why burden and class have to be specified together.