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What are the accuracy levels of current transformers?

Should the secondary current be selected as 1A or 5A?

2025-09-12

current transformer voltage transformer accuracy is specified by an accuracy class, with lower numbers indicating higher accuracy, such as 0.1%, 0.2%, 0.5%, 1%, or 3%. The accuracy level defines the maximum permissible error in current measurement under specific conditions. Some classes, denoted with an "S" (e.g., 0.5S), are special classes with stricter requirements for performance at lower currents, often down to 1% or 5% of rated current, whereas standard classes (e.g., 0.5) may only guarantee accuracy from 5% or 10% of the rated current upwards.

Understanding Accuracy Classes (IEC Standards)

Accuracy Class:
The numerical value (e.g., 0.1, 0.5, 1) indicates the maximum allowable percentage of error in the current measurement.

A Class 0.1 CT has a 0.1% maximum error

A Class 0.5 CT has a 0.5% maximum erro
"S" for Special:
Special classes like 0.2S or 0.5S are designed for higher accuracy at lower currents.

For example, a Class 0.5 CT typically guarantees accuracy between 5% and 120% of the rated current.

A Class 0.5S CT, however, may guarantee accuracy down to 1% or 5% of the rated current.

Key Considerations for Accuracy

Rated Current:

The accuracy class only applies within a specific range of the rated primary current. For example, a Class 0.5 CT's measurement error is guaranteed only above 5% of its rated current.
Burden (Load):

The accuracy also depends on the secondary circuit's burden, which is the load connected to the CT's secondary winding.
Phase Displacement:

In addition to the magnitude of the current, CTs also introduce a phase displacement (lead or lag) between the primary and secondary currents. Accuracy classes specify limits for both ratio error and phase displacement
Application:

High-accuracy CTs are beneficial in variable load applications, such as factories with different shifts, as they generate more revenue by accurately measuring low-load periods.

In summary: When selecting a CT, choose an accuracy class that meets your measurement needs and ensure the rated primary current is close to your expected load to maintain accuracy, especially in applications with low current loads.

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