| Example 3:
Assume that secondary burden of a 300:5 class C CT is
5 . The relay setting is 2A and the CT ratio is
 . Using fig 5.7, calculate the primary current required to operate the relay?
Answer:
Secondary burden = 5
Secondary resistance for 300/5 CT = 0.15
Relay setting, 
Vs = 2 x (5 + 0.15) = 10.3V
Corresponding exciting current = 0.04A
Total secondary current = 2 + 0.04 = 2.04A
Note that, strictly, phasor addition of currents has to be done. However, algebraic addition simplifies work and usually leads to conservative estimate of voltages. Hence, this practice is considered acceptable.
Primary current to operate the relay =
 = 122A
Example 4:
A relay is expected to operate for 7000A primary current. The class C CT ratio is  (see fig 5.7). Secondary burden is 3.5 . Will the CT saturate at this burden? Also, comment on the ratio error.
Answer:
Secondary current
Vs = 58.33 (3.5 + 0.31) = 222.25V.
From the excitation curve (fig. 5.7) of 600/5 CT, we can see that the CT will be in deep saturation and % ratio error will exceed the limits.
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