Specifications

7SR242 Duobias Applications Guide
Worked Example
Figure 3-1 CT Requirements
Suitability of HV Current Transformers:

126.5V13.5
0.14400101323
1090
4V
3
6
K
u
uuuu
u
ut i.e. less than 250V
Suitability of LV Current Transformers:

V12
0.140010333
1090
4V
3
6
K
9.4494
16
u
uuuu
u
ut i.e. less than 750V
An indication of the suitability of a protection class CT e.g. class 5P to IEC60044 classification can be obtained.
The product of its rated burden expressed in ohms and the secondary current equivalent of its accuracy limit
primary current will give an approximation of the secondary voltage it can produce while operating within the limit
of its stated composite error.
3.2 CT Requirements for High Impedance Restricted Earth
Fault (64H)
For high impedance REF protection:
Low reactance CTs to IEC Class PX must be used, this allows a sensitive current setting to be applied.
All current transformers should have an equal turns ratio.
The knee-point voltage of the CTs must be greater than 2 x 64H Setting Voltage (Vs) – see section 2.4.
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
.
©2010 Siemens Protection Devices Limited Chapter 7 Page 40 of 56