resistance, the resistance of a single secondary wire, may normally be used in thecalculation for three-phase faults.As the burden can be considerable different for three-phase faults and phase-to-groundfaults it is important to consider both cases. Even in a case where the phase-to-groundfault current is smaller than the three-phase fault current the phase-to-ground fault canbe dimensioning for the CT depending on the higher burden.In isolated or high impedance grounded systems the phase-to-ground fault is not thedimensioning case. Therefore, the resistance of the single secondary wire can alwaysbe used in the calculation for this kind of power systems.24.1.5 General current transformer requirements M11615-3 v3The current transformer ratio is mainly selected based on power system data forexample, maximum load and/or maximum fault current. It should be verified that thecurrent to the protection is higher than the minimum operating value for all faults thatare to be detected with the selected CT ratio. It should also be verified that themaximum possible fault current is within the limits of the IED.The current error of the current transformer can limit the possibility to use a verysensitive setting of a sensitive residual overcurrent protection. If a very sensitivesetting of this function will be used it is recommended that the current transformershould have an accuracy class which have an current error at rated primary current thatis less than ±1% (for example, 5P). If current transformers with less accuracy are usedit is advisable to check the actual unwanted residual current during the commissioning.24.1.6 Rated equivalent secondary e.m.f. requirements SEMOD53723-1 v1M11616-3 v2With regard to saturation of the current transformer all current transformers of highremanence and low remanence type that fulfill the requirements on the rated equivalentlimiting secondary e.m.f. Eal below can be used. The characteristic of the nonremanence type CT (TPZ) is not well defined as far as the phase angle error isconcerned. If no explicit recommendation is given for a specific function we thereforerecommend contacting ABB to confirm that the non remanence type can be used.The CT requirements for the different functions below are specified as a ratedequivalent limiting secondary e.m.f. Eal according to the IEC 61869-2 standard.Requirements for CTs specified according to other classes and standards are given atthe end of this section.1MRK 511 401-UUS A Section 24RequirementsBay control REC670 2.2 ANSI 631Application manual
