Class X Current Transformers In balanced earth fault protection systems, correct operation depends on the accuracy of the current transformers. Class 5P and 10P are suitable where the definite minimum times is greater than 0.2 seconds. For shorter operating time, transient conditions may require current transformers to have cross sectional areas many times greater than those of steady state faults. The following information is required. (a) Turns ratio (b) Knee point voltage (c) Maximum excitation current (d) Secondary circuit resistance