Siemens 7SR45 Argus User Manual page 67

Sample Calculation:
CT Ratio
Rated CT Burden (R )
bn
CT winding resistance (R
Rated Secondary Current (Isr)
Case 1:
Minimum current for relay operation in single phase = 0.2 In (I
R = 28.5 at 0.2 A (Refer graphs)
b
Case 2:
Fault current = 20 A
R = 0.49 at 20 A (Refer graphs)
b
The typical accuracy limit factor would be ALF = 10
The calculation shown above is an example. To select an appropriate CT, similar calculation needs to be done
with the actual parameters.
Recommended CTs:
Considering the CTs are connected to 7SR45 relay only and the distance between CTs and 7SR45 relay
(typically 3 m) is short, the following are the recommended CTs to be used:
·
2.5 VA, 5P10 or 5P20 for In = 1 A
·
2.5 VA, 5P20 for In = 5 A
©2017 Siemens Protection Devices Limited
: 100/1
: 2.5 VA (R
0.5 Ω
) :
ct
: 1 A
= 2.5 Ω for 1A secondary current)
bn
= 0.2A for rated current of 1 A)
s
7SR45 Applications Guide
Chapter 6 Page 6 of 6