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7SR11 & 7SR12 Applications Guide
Section 3: CT Requirements
3.1
CT Requirements for Overcurrent and Earth Fault
Protection
3.1.1
Overcurrent Protection CTs
a) For industrial systems with relatively low fault current and no onerous grading requirements - a class
10P10 with VA rating to match the load.
b) For utility distribution networks with relatively high fault current and several grading stages - a class
5P20, with VA rating to match the load.
Note: if an accuracy limit factor is chosen which is much lower than the maximum fault current it will be
necessary to consider any effect on the protection system performance and accuracy e.g. grading margins.
For idmtl applications, because the operating time at high fault current is a definite minimum value, partial
saturation of the CT at values beyond the overcurrent factor has only a minimal effect. However, this must be
taken into account in establishing the appropriate setting to ensure proper grading.
c) For dtl applications utilities as for (b) above - a class 5P10 (or 20), with rated burden to suit the
load.
Note: Overcurrent factors do not need to be high for definite time protection because once the setting is exceeded
magnitude accuracy is not important. Often, however, there is also the need to consider instantaneous HighSet
overcurrent protection as part of the same protection system and the settings would normally be of the order of
10x the CT rating or higher. Where higher settings are to be used then the overcurrent factor must be raised
accordingly, e.g. to P20.
3.1.2
Earth Fault Protection CTs
Considerations and requirements for earth fault protection are the same as for Phase fault. Usually the relay
employs the same CT's e.g. three phase CTs star connected to derive the residual earth fault current.
The accuracy class and overcurrent accuracy limit factors are therefore already determined and for both these
factors the earth fault protection requirements are normally less onerous than for overcurrent.
3.2
CT Requirements for High Impedance Restricted Earth
Fault Protection
For high impedance REF it is recommended that:
Low reactance CTs to IEC Class PX are used, this allows a sensitive current setting to be applied.
All CT's should, if possible have identical turns ratios.
The knee point voltage of the CTs must be greater than 2 x 64H setting voltage Vs.
Where the REF function is used then this dictates that the other protection functions are also used with class PX
CTs.
A full explanation of how to specify CTs for use with REF protection, and set REF relays is available on our
website: www.siemens.com/energy.
Page 34 of 48
©2017 Siemens Protection Devices Limited
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