7SR45 Applications Guide
©2017 Siemens Protection Devices Limited Chapter 6 Page 6 of 6
Sample Calculation:
CT Ratio : 100/1
Rated CT Burden (R
bn
) : 2.5 VA (R
bn
= 2.5 Ω for 1A secondary current)
CT winding resistance (R
ct
) : 0.5 Ω
Rated Secondary Current (Isr) : 1 A
Case 1:
Minimum current for relay operation in single phase = 0.2 In (I
s
= 0.2A for rated current of 1 A)
R
b
= 28.5 at 0.2 A (Refer graphs)
Case 2:
Fault current = 20 A
R
b
= 0.49 at 20 A (Refer graphs)
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
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