Letter designation 'C' indicates that the leakeage flux is negligible. Class C CTs are the more accurate bar type CTs. In such CTs, the leakage flux from the core is kept very small. For such CTs, the performance can be evaluated from the standard exciting curves. Also, the ratio error is maintained within  for standard operating conditions. For such CTs, voltage rating on the secondary is specified up to which linear response is guaranteed. For example, a class C CT specification could be as follows: 200:5 C 100. The labeling scheme indicates that we are dealing with a 200:5 class C CT which will provide linear response up to 20 times rated current provided the burden on the secondary is kept below  ohm. Similarly, a corresponding class T CT may be labeled as 200:5 T 100.
For class C CTs, standard chart for
 versus excitation current
 on the secondary side is available. This provides the protection engineer data to do more exact calculations (refer fig 5.7). e.g., in determining relaying sensitivity. |
