16.1.6.3 Non-directional inverse time delayed phase and residual overcurrentprotectionThe requirement according to Equation 96 and Equation 97 does not need to be fulfilledif the high set instantaneous or definitive time stage is used. In this case Equation 95 is theonly necessary requirement.If the inverse time delayed function is the only used overcurrent protection function theCTs must have a rated equivalent secondary e.m.f. Eal that is larger than or equal to therequired secondary e.m.f. Ealreq below:s n Ra l a lre q CT L 2pn nopI SE E 20 I R RI I³ = × × + +æ ö× ç ÷è øEQUATION1679 V1 EN (Equation 96)whereIop The primary current set value of the inverse time function (A)Ipn The rated primary CT current (A)Isn The rated secondary CT current (A)In The nominal 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 resistancecontaining the phase and neutral wires, must be used for faults in solidly groundedsystems. The resistance of a single secondary wire should be used for faults in highimpedance grounded systems.SR The burden of an IED current input channel (VA). SR=0.010 VA/channel for Ir=1 Aand SR=0.250 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 (Equation 97)whereIkmax Maximum primary fundamental frequency current for close-in faults (A)Section 16 1MRK 511 286-UUS ARequirements350Application manual
