The Accuracy Class

The accuracy class nominally demonstrates the expected error of the current transformer taking into account the error of transformation ratio and the error of lag between the primary and secondary currents.

According to the instruments connected to the secondary terminals of the CT, the accuracy classes of this equipment must be as follows:

– For verification and calibration of instruments of laboratory measurements: 0,1

– Meter supply for the purposes of monitoring industrial costs: 0.6;

– Power supply of indicator ammeters, graphic registers, impedance relays, differential relays, directional relays: 1,2.

– Supply of direct – acting relays, for example, applied to primary substation circuit breakers: 3.

Accuracy class 3 has no phase angle error limitation and its percentage ratio correction factor (FCRp) should be between 103 and 97% so that it can be considered within its accuracy class. A TC for measuring service is considered to be within its nominal precision class when the points determined by the percentage ratio correction factors (FCRp) and by the phase angles _ are within the parallelogram of accuracy.