Section 3: Ct Requirements; Ct Requirements For Overcurrent And Earth Fault Protection; Overcurrent Protection Cts; Earth Fault Protection Cts - Siemens 7SR224 Technical Manual

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 i.d.m.t.l. 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.
Definite Time and Instantaneous Overcurrent
a) For industrial systems with requirements as for i.d.m.t.l. relays item (a) above, a class 10P10 (or 20).
b) For 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.
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.
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Often, however, there is also the need to consider
7SR224 Argus Applications
Chapter 7 Page 27 of 50