Accuracy Class for Current Transformers

The accuracy performance of an AC current transformer’s primary current to secondary signal transformation (where the secondary is either a current or voltage) is stated as Accuracy Class. The Accuracy Class for current transformers is measured in accordance with the IEC61869 standard. The IEC61869-2 standard specifies transformation accuracy for current transformers at different percentage levels of Rated Primary Currents. Rated Primary Current is the AC primary current which will result in a secondary signal output equal to the current transformer’s design full scale (for example a model CTSB0816-500A/ 5A Rated Primary Current is 500A with a secondary output at Rated Primary Current of 5A).

IEC61869 Current Transformer Accuracy Class 0.2:

 	
±0.75% ratio error @ 5% of rated primary current,
 	
±0.35% ratio error @ 20% of rated primary current,
 	
±0.20% ratio error @ 100% of rated primary current
 	
±0.20% ratio error @ 120% of rated primary current

IEC61869 Current Transformer Accuracy Class 0.5:

 	
±1.50% ratio error @ 5% of rated primary current,
 	
±0.75% ratio error @ 20% of rated primary current,
 	
±0.50% ratio error @ 100% of rated primary current
 	
±0.50% ratio error @ 120% of rated primary current


The current transformers offered by T.I. Chen Associates are designed for the accurate measurement of AC currents up to 120% of the Rated Primary Current.

The Accuracy Class requirements as applied to a specific model of a current transformer may limit the Rated Primary Current ranges that can meet those accuracy requirements.

Example: CTSB0816 Split-core Current Transformer Accuracy Class 0.5:
The CTSB0816 is a split-core current transformer with an opening of 80mm(3.15″) x 160mm(6.30″. Typical applications would be large primary conductors and BUS bars. Download the CTSB Brochure (pdf 349kb)

Accurate measuring range is 5%-120% of Rated Primary Current, for rated primary currents from 500A-2,000A (e.g. models CTSB0816-500A/ 5A to CTSB0816-2000A/ 5A).

 	
CTBS0816-500A/ 5A: Class 0.5 measurement range would be 25A to 600A.
 	
CTSB0816-2000A/ 5A: Class 0.5 measurement range 100A to 2,400A

Accurate measuring range is 1%-120% of Rated Primary Current, for rated primary currents from 2,000A-5,000A;

 	
CTSB0816-2000A/ 5A: Class 0.5 measurement range would be 20A to 2,400A.
 	
CTSB0816-5000A/ 5A: Class 0.5 measurement range 50A to 6,000A

These examples demonstrate that to achieve the best accuracy over the anticipated primary current operating range, the Rated Primary Current of the current transformer must be carefully considered. So if the application typically measures less than 500A, the CTSB0816-500A/ 5A would be the appropriate selection where 500A is the Rated Primary Current for the current transformer.