The CT accuracy primary limit current describes the highest fault currentmagnitude at which the CT fulfils the specified accuracy. Beyond this level, thesecondary current of the CT is distorted and it might have severe effects on theperformance of the protection IED.In practise, the actual accuracy limit factor (Fa) differs from the rated accuracylimit factor (Fn) and is proportional to the ratio of the rated CT burden and theactual CT burden.The actual accuracy limit factor is calculated using the formula:F F S SS Sa nin nin≈ × ++A071141 V1 ENFn the accuracy limit factor with the nominal external burden SnSin the internal secondary burden of the CTS the actual external burden12.1.1.2 Non-directional overcurrent protectionThe current transformer selectionNon-directional overcurrent protection does not set high requirements on theaccuracy class or on the actual accuracy limit factor (Fa) of the CTs. It is, however,recommended to select a CT with Fa of at least 20.The nominal primary current I1n should be chosen in such a way that the thermaland dynamic strength of the current measuring input of the IED is not exceeded.This is always fulfilled whenI1n > Ikmax / 100,Ikmax is the highest fault current.The saturation of the CT protects the measuring circuit and the current input of theIED. For that reason, in practice, even a few times smaller nominal primary currentcan be used than given by the formula.Recommended start current settingsIf Ikmin is the lowest primary current at which the highest set overcurrent stage is tooperate, the start current should be set using the formula:Current start value < 0.7 x (Ikmin / I1n)I1n is the nominal primary current of the CT.Section 12 1YHT530004D05 DRequirements for measurement transformers844 615 seriesTechnical Manual
