Current Transformer Ratio Modification Techniques.
Change the ratio of any CT.
 
Relatively large changes in a CT ratio may be achieved through the use of "Primary Turns";
1 Primary Turn = 1 pass through the Current Transformer window. More than 1 pass through
results in the electrical ratio modification.
For Example:
CT Ratio
Normal 1 Pass
Number of Primary Turns
Modified Ratio
100:5A
200:5A
300:5A
2
2
2
50:5A
100:5A
150:5A
100:5A
200:5A
300:5A
3
3
3
33.3:5A
66.6:5A
100:5A
100:5A
200:5A
300:5A
4
4
4
25:5A
50:5A
75:5A
 
A "Primary Turn" is the number of times the primary conductor passes through the CTís window.
The main advantage of this ratio modification is you maintain the accuracy and burden capabilities of the higher ratio. The higher the primary rating the better the CT accuracy and burden rating.
You can make smaller ratio modification adjustment by using additive or subtractive
"Secondary Turns". Example: A CT with a ratio of 100:5A, by adding 1 additive "Secondary
Turn" the ratio modification is 105:5A. By adding one "Subtractive "Secondary Turn" the ratio
modification is 95:5A. Subtractive secondary turns are achieved by placing the X1 lead through
the window from the H1 side and out the H2 side.
Additive "Secondary Turns" are achieved by placing the X1 lead through the window fromH2
and out the H1 side. So when there is only 1 primary turn each secondary turn modifies the ratio by 5 amperes. If there is more than 1 primary turn each secondary turn value is changed (i.e. 5A divided by 2 primary turns= 2.5A) The following table illustrates the effects of different combinations of primary and secondary turns.
 
Primary Turns
Secondary Turns
Ratio Adjustment
1
1
1
0
1+
1-
100:5A
105:5A
95:5A
2
2
2
0
1+
2-
50:5A
52.5:5A
45.0:5A
3
3
3
0
1+
1-
33.3:5A
34.97:5A
31.63:5A
In summary with the use of Primary/Secondary turns you can