17.1.7.2 Current transformers according to IEC 61869-2, class PX, PXR (andold IEC 60044-6, class TPS and old British Standard, class X) M11623-14 v5CTs according to these classes are specified approximately in the same way by arated knee point e.m.f. Eknee (Ek for class PX and PXR, EkneeBS for class X and thelimiting secondary voltage Ual for TPS). The value of the Eknee is lower than thecorresponding Eal according to IEC 61869-2. It is not possible to give a generalrelation between the Eknee and the Eal but normally the Eknee is approximately 80 %of the Eal. Therefore, the CTs according to class PX, PXR, X and TPS must have arated knee point e.m.f. Eknee that fulfills the following:( )knee k kneeBS al alreqE E E U 0.8 maximum of E» » » > ×EQUATION2100 V2 EN-US (Equation 44)17.1.7.3 Current transformers according to ANSI/IEEE M11623-22 v6Current transformers according to ANSI/IEEE are partly specified in differentways. A rated secondary terminal voltage UANSI is specified for a CT of class C.UANSI is the secondary terminal voltage the CT will deliver to a standard burden at20 times rated secondary current without exceeding 10 % ratio correction. Thereare a number of standardized UANSI values for example, UANSI is 400 V for a C400CT. A corresponding rated equivalent limiting secondary e.m.f. EalANSI can beestimated as follows:alANSI sr ct ANSI sr ct sr bANSIE 20 I R U 20 I R 20 I Z= × × + = × × + × ×EQUATION971 V2 EN-US (Equation 45)where:ZbANSI The impedance (that is, with a complex quantity) of the standard ANSI burden for the specificC class (W)UANSI The secondary terminal voltage for the specific C class (V)The CTs according to class C must have a calculated rated equivalent limitingsecondary e.m.f. EalANSI that fulfils the following:alANSI alreqE maximum of E>EQUATION1384 V2 EN-US (Equation 46)A CT according to ANSI/IEEE is also specified by the knee point voltageUkneeANSI that is graphically defined from an excitation curve. The knee pointvoltage UkneeANSI normally has a lower value than the knee-point e.m.f. accordingto IEC and BS. UkneeANSI can approximately be estimated to 75 % of the1MRK 505 383-UEN A Section 17RequirementsBreaker protection REQ650 2.2 IEC 315Application manual
