24.1.6.3 Breaker failure protectionThe CTs must have a rated equivalent limiting secondary e.m.f. Eal that is larger thanor equal to the required rated equivalent limiting secondary e.m.f. Ealreq below:sr Ral alreq op ct L 2pr rI SE E 5 I R RI Iæ ö³ = × × × + +ç ÷è øEQUATION1380 V2 EN (Equation 305)where:Iop The primary operate value (A)Ipr The rated primary CT current (A)Isr The rated secondary CT current (A)Ir 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 containingthe phase and neutral wires, must be used for faults in solidly earthed systems. The resistanceof a single secondary wire should be used for faults in high impedance earthed systems.SR The burden of an IED current input channel (VA). SR=0.020 VA/channel for Ir=1 A and SR=0.150VA/channel for Ir=5 A24.1.6.4 Restricted earth fault protection (low impedance differential)The requirements are specified separately for solidly earthed and impedance earthedtransformers. For impedance earthed transformers the requirements for the phase CTsare depending whether it is three individual CTs connected in parallel or it is a cableCT enclosing all three phases.Neutral CTs and phase CTs for solidly earthed transformersThe neutral CT and the phase CTs must have a rated equivalent limiting secondarye.m.f. Eal that is larger than or equal to the maximum of the required rated equivalentlimiting secondary e.m.f. Ealreq below:230 sr Ral alreq rt ct Lpr rI SE E I R RI Iæ ö³ = × × × + +ç ÷è øEQUATION2237 V2 EN (Equation 306)22 sr Ral alreq etf ct Lpr rI SE E I R RI Iæ ö³ = × × × + +ç ÷è øEQUATION2238 V2 EN (Equation 307)Section 24 1MRK 502 071-UEN -Requirements754 Generator protection REG670 2.2 IEC and Injection equipment REX060, REX061, REX062Application manual
