Ct Requirements For High Impedance Restricted Earth Fault Protection - Siemens 7SR224 Technical Manual

3.2 CT Requirements for High Impedance Restricted Earth
Fault Protection
For high impedance schemes it is necessary to establish characteristics of the CT in accordance with Class 'PX'
to IEC 60044. The basic requirements are:
All CT's should, if possible have identical turns ratios.
The knee point voltage of each CT, should be at least 2 x Vs.
The knee point voltage is expressed as the voltage applied to the secondary circuit with the primary
open circuit which when increased by 10% causes the magnetizing current to increase by 50%.
Where the REF function is used then this dictates that the other protection functions are also used with class PX
CTs.
©2010 Siemens Protection Devices Limited
7SR224 Argus Applications
Chapter 7 Page 28 of 50