Cahier technique no 195 - Schneider Electric

LexiconI f : maximum through current crossing aprotected area.I s : current threshold setting.k n : nominal accuracy limit factor (ALF) of a CT(associated with its accuracy load).k r : real ALF of a CT associated with its real load.P i : (= R ct I 2 n ). Internal losses of the CT at I n .P n : (= R n I 2 n ). Accuracy power of the CT.P r : (= R r I 2 n). Real load consumption ofthe CT at I n .R ct : CT secondary winding resistance.R L : wiring resistance.R p : protection relay resistance.ALF: accuracy limit factor.CT: current transformer.Overrating of a CT: selection of a CT whoseprimary I n is greater than the I n immediatelygreater than the load I n .Matching, auxiliary or interposing CT:low voltage CTs installed at the secondary of themain CTs for correcting a ratio and/or the currentphase shift.SF: security factor.Cahier Technique Schneider Electric no. 195 / p.2