Rct The secondary resistance of the CT (W)RL The resistance of the secondary cable and additional load (W). The loopresistance containing the phase and neutral wires, must be used for faults insolidly grounded systems. The resistance of a single secondary wire should beused for faults in high impedance grounded systems.SR The burden of an IED current input channel (VA). SR=0.020 VA/channel for Ir=1A and SR=0.150 VA/channel for Ir=5 AIndependent of the value of Iop the maximum required Eal is specified according to thefollowing:s n Ra l a lre q ma x CT L 2pn nk ma xI SE E I R RI I³ = × + +æ ö× ç ÷è øEQUATION1680 V1 EN-US (Equation 96)whereIkmax Maximum primary fundamental frequency current for close-in faults (A)21.1.7 Current transformer requirements for CTs according to otherstandards SEMOD53771-1 v1M11623-4 v3All kinds of conventional magnetic core CTs are possible to use with the IEDs if theyfulfill the requirements corresponding to the above specified expressed as the ratedequivalent limiting secondary e.m.f. Eal according to the IEC 61869-2 standard. Fromdifferent standards and available data for relaying applications it is possible toapproximately calculate a secondary e.m.f. of the CT comparable with Eal. Bycomparing this with the required rated equivalent limiting secondary e.m.f. Ealreq it ispossible to judge if the CT fulfills the requirements. The requirements according tosome other standards are specified below.21.1.7.1 Current transformers according to IEC 61869-2, class P, PR M11623-6 v4A CT according to IEC 61869-2 is specified by the secondary limiting e.m.f. EALF. Thevalue of the EALF is approximately equal to the corresponding Eal. Therefore, the CTsaccording to class P and PR must have a secondary limiting e.m.f. EALF that fulfills thefollowing:1MRK 505 370-UUS A Section 21RequirementsBusbar protection REB670 2.2 ANSI 531Application manual
