according to class P and PR must have a secondary limiting e.m.f. Ealf that fulfills thefollowing:2 max max alreqE E>EQUATION1383 V2 EN (Equation 185)18.1.7.2 Current transformers according to IEC 61869-2, class PX, PXR (and oldIEC 60044-6, class TPS and old British Standard, class X)CTs according to these classes are specified approximately in the same way by a ratedknee point e.m.f. Eknee (Ek for class PX and PXR, EkneeBS for class X and the limitingsecondary voltage Val for TPS). The value of the Eknee is lower than the corresponding Ealaccording to IEC 61869-2. It is not possible to give a general relation between the Ekneeand the Eal but normally the Eknee is approximately 80 % of the Eal. Therefore, the CTsaccording to class PX, PXR, X and TPS must have a rated knee point e.m.f. Eknee thatfulfills the following:(1 )= × + - ×Old CalculatedS TD S TD SEQUATION1893.ANSI V1 EN (Equation 186)18.1.7.3 Current transformers according to ANSI/IEEECurrent transformers according to ANSI/IEEE are partly specified in different ways. Arated secondary terminal voltage VANSI is specified for a CT of class C. VANSI is thesecondary terminal voltage the CT will deliver to a standard burden at 20 times ratedsecondary current without exceeding 10 % ratio correction. There are a number ofstandardized VANSI values for example, VANSI is 400 V for a C400 CT. A correspondingrated equivalent limiting secondary e.m.f. EalANSI can be estimated as follows:a lANS I S N CT ANS I S N CT S N bANS IE 20 I R V 20 I R 20 I Z= × × + = × × + × ×EQUATION1682 V1 EN (Equation 187)where:ZbANSI The impedance (that is, with a complex quantity) of the standard ANSI burden for the specific C class(W)VANSI The secondary terminal voltage for the specific C class (V)1MRK 506 334-UUS A Section 18Requirements497Application manual
