Editing Current transformer (section)

===Knee-point core-saturation voltage===
The '''knee-point voltage''' of a current transformer is the magnitude of the secondary voltage above which the output current ceases to linearly follow the input current within declared accuracy. In testing, if a voltage is applied across the secondary terminals the [[magnetizing current]] will increase in proportion to the applied voltage, until the knee point is reached.  The knee point is defined as the voltage at which a 10% increase in applied voltage increases the magnetizing current by 50%.<ref>{{Cite web|url=http://www.electropedia.org/iev/iev.nsf/display?openform&ievref=321-02-34|title=IEC 60050 - International Electrotechnical Vocabulary - Details for IEV number 321-02-34: "knee point voltage"|website=www.electropedia.org|access-date=2018-07-12}}</ref> For voltages greater than the knee point, the magnetizing current increases considerably even for small increments in voltage across the secondary terminals. The knee-point voltage is less applicable for metering current transformers as their accuracy is generally much higher but constrained within a very small range of the current transformer rating, typically 1.2 to 1.5 times rated current. However, the concept of knee point voltage is very pertinent to protection current transformers, since they are necessarily exposed to fault currents of 20 to 30 times rated current.<ref name=GEC75>Anon, ''Protective Relays Application Guide Second Edition''The General Electric Company Limited of England, 1975 Section 5.3</ref>