18.1.6.5 Directional phase and residual overcurrent protectionIf the directional overcurrent function is used the CTs must have a rated equivalentsecondary e.m.f. Eal that is larger than or equal to the required equivalent secondary e.m.f.Ealreq below:s n Ra l a lre q CT L 2pn nk ma xI SE E I R RI I³ = × + +æ ö× ç ÷è øEQUATION1681 V1 EN (Equation 184)where:Ikmax Maximum primary fundamental frequency current for close-in forward and reverse faults (A)Ipn The rated primary CT current (A)Isn The rated secondary CT current (A)In The rated current of the protection IED (A)RCT The secondary resistance of the CT (W)RL The resistance of the secondary cable and additional load (W). The loop resistance containing thephase and neutral wires, must be used for faults in solidly grounded systems. The resistance of asingle secondary wire should be used for faults in high impedance grounded systems.SR The burden of an IED current input channel (VA). Sr=0.010 VA/channel for Ir=1 A and Sr=0.250 VA/channel for Ir=5 A18.1.7 Current transformer requirements for CTs according to otherstandardsAll 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. By comparingthis with the required rated equivalent limiting secondary e.m.f. Ealreq it is possible tojudge if the CT fulfills the requirements. The requirements according to some otherstandards are specified below.18.1.7.1 Current transformers according to IEC 61869-2, class P, PRA 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 CTsSection 18 1MRK 506 334-UUS ARequirements496Application manual
